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src/Sdk/DTExpressions2/Expressions2/EvaluationOptions.cs

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+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class EvaluationOptions
+    {
+        public EvaluationOptions()
+        {
+        }
+
+        public EvaluationOptions(EvaluationOptions copy)
+        {
+            if (copy != null)
+            {
+                MaxMemory = copy.MaxMemory;
+            }
+        }
+
+        public Int32 MaxMemory { get; set; }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/EvaluationResult.cs

@@ -0,0 +1,453 @@
+using System;
+using System.ComponentModel;
+using System.Globalization;
+using GitHub.DistributedTask.Expressions2.Sdk;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class EvaluationResult
+    {
+        internal EvaluationResult(
+            EvaluationContext context,
+            Int32 level,
+            Object val,
+            ValueKind kind,
+            Object raw)
+            : this(context, level, val, kind, raw, false)
+        {
+        }
+
+        internal EvaluationResult(
+            EvaluationContext context,
+            Int32 level,
+            Object val,
+            ValueKind kind,
+            Object raw,
+            Boolean omitTracing)
+        {
+            m_level = level;
+            Value = val;
+            Kind = kind;
+            Raw = raw;
+            m_omitTracing = omitTracing;
+
+            if (!omitTracing)
+            {
+                TraceValue(context);
+            }
+        }
+
+        public ValueKind Kind { get; }
+
+        /// <summary>
+        /// When an interface converter is applied to the node result, raw contains the original value
+        /// </summary>
+        public Object Raw { get; }
+
+        public Object Value { get; }
+
+        public Boolean IsFalsy
+        {
+            get
+            {
+                switch (Kind)
+                {
+                    case ValueKind.Null:
+                        return true;
+                    case ValueKind.Boolean:
+                        var boolean = (Boolean)Value;
+                        return !boolean;
+                    case ValueKind.Number:
+                        var number = (Double)Value;
+                        return number == 0d || Double.IsNaN(number);
+                    case ValueKind.String:
+                        var str = (String)Value;
+                        return String.Equals(str, String.Empty, StringComparison.Ordinal);
+                    default:
+                        return false;
+                }
+            }
+        }
+
+        public Boolean IsPrimitive => ExpressionUtility.IsPrimitive(Kind);
+
+        public Boolean IsTruthy => !IsFalsy;
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractEqual(EvaluationResult right)
+        {
+            return AbstractEqual(Value, right.Value);
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractGreaterThan(EvaluationResult right)
+        {
+            return AbstractGreaterThan(Value, right.Value);
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractGreaterThanOrEqual(EvaluationResult right)
+        {
+            return AbstractEqual(Value, right.Value) || AbstractGreaterThan(Value, right.Value);
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractLessThan(EvaluationResult right)
+        {
+            return AbstractLessThan(Value, right.Value);
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractLessThanOrEqual(EvaluationResult right)
+        {
+            return AbstractEqual(Value, right.Value) || AbstractLessThan(Value, right.Value);
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        public Boolean AbstractNotEqual(EvaluationResult right)
+        {
+            return !AbstractEqual(Value, right.Value);
+        }
+
+        public Double ConvertToNumber()
+        {
+            return ConvertToNumber(Value);
+        }
+
+        public String ConvertToString()
+        {
+            switch (Kind)
+            {
+                case ValueKind.Null:
+                    return String.Empty;
+
+                case ValueKind.Boolean:
+                    return ((Boolean)Value) ? ExpressionConstants.True : ExpressionConstants.False;
+
+                case ValueKind.Number:
+                    return ((Double)Value).ToString(ExpressionConstants.NumberFormat, CultureInfo.InvariantCulture);
+
+                case ValueKind.String:
+                    return Value as String;
+
+                default:
+                    return Kind.ToString();
+            }
+        }
+
+        public Boolean TryGetCollectionInterface(out Object collection)
+        {
+            if ((Kind == ValueKind.Object || Kind == ValueKind.Array))
+            {
+                var obj = Value;
+                if (obj is IReadOnlyObject)
+                {
+                    collection = obj;
+                    return true;
+                }
+                else if (obj is IReadOnlyArray)
+                {
+                    collection = obj;
+                    return true;
+                }
+            }
+
+            collection = null;
+            return false;
+        }
+
+        /// <summary>
+        /// Useful for working with values that are not the direct evaluation result of a parameter.
+        /// This allows ExpressionNode authors to leverage the coercion and comparision functions
+        /// for any values.
+        ///
+        /// Also note, the value will be canonicalized (for example numeric types converted to double) and any
+        /// matching interfaces applied.
+        /// </summary>
+        public static EvaluationResult CreateIntermediateResult(
+            EvaluationContext context,
+            Object obj)
+        {
+            var val = ExpressionUtility.ConvertToCanonicalValue(obj, out ValueKind kind, out Object raw);
+            return new EvaluationResult(context, 0, val, kind, raw, omitTracing: true);
+        }
+
+        private void TraceValue(EvaluationContext context)
+        {
+            if (!m_omitTracing)
+            {
+                TraceValue(context, Value, Kind);
+            }
+        }
+
+        private void TraceValue(
+            EvaluationContext context,
+            Object val,
+            ValueKind kind)
+        {
+            if (!m_omitTracing)
+            {
+                TraceVerbose(context, String.Concat("=> ", ExpressionUtility.FormatValue(context?.SecretMasker, val, kind)));
+            }
+        }
+
+        private void TraceVerbose(
+            EvaluationContext context,
+            String message)
+        {
+            if (!m_omitTracing)
+            {
+                context?.Trace.Verbose(String.Empty.PadLeft(m_level * 2, '.') + (message ?? String.Empty));
+            }
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        private static Boolean AbstractEqual(
+            Object canonicalLeftValue,
+            Object canonicalRightValue)
+        {
+            CoerceTypes(ref canonicalLeftValue, ref canonicalRightValue, out var leftKind, out var rightKind);
+
+            // Same kind
+            if (leftKind == rightKind)
+            {
+                switch (leftKind)
+                {
+                    // Null, Null
+                    case ValueKind.Null:
+                        return true;
+
+                    // Number, Number
+                    case ValueKind.Number:
+                        var leftDouble = (Double)canonicalLeftValue;
+                        var rightDouble = (Double)canonicalRightValue;
+                        if (Double.IsNaN(leftDouble) || Double.IsNaN(rightDouble))
+                        {
+                            return false;
+                        }
+                        return leftDouble == rightDouble;
+
+                    // String, String
+                    case ValueKind.String:
+                        var leftString = (String)canonicalLeftValue;
+                        var rightString = (String)canonicalRightValue;
+                        return String.Equals(leftString, rightString, StringComparison.OrdinalIgnoreCase);
+
+                    // Boolean, Boolean
+                    case ValueKind.Boolean:
+                        var leftBoolean = (Boolean)canonicalLeftValue;
+                        var rightBoolean = (Boolean)canonicalRightValue;
+                        return leftBoolean == rightBoolean;
+
+                    // Object, Object
+                    case ValueKind.Object:
+                    case ValueKind.Array:
+                        return Object.ReferenceEquals(canonicalLeftValue, canonicalRightValue);
+                }
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        private static Boolean AbstractGreaterThan(
+            Object canonicalLeftValue,
+            Object canonicalRightValue)
+        {
+            CoerceTypes(ref canonicalLeftValue, ref canonicalRightValue, out var leftKind, out var rightKind);
+
+            // Same kind
+            if (leftKind == rightKind)
+            {
+                switch (leftKind)
+                {
+                    // Number, Number
+                    case ValueKind.Number:
+                        var leftDouble = (Double)canonicalLeftValue;
+                        var rightDouble = (Double)canonicalRightValue;
+                        if (Double.IsNaN(leftDouble) || Double.IsNaN(rightDouble))
+                        {
+                            return false;
+                        }
+                        return leftDouble > rightDouble;
+
+                    // String, String
+                    case ValueKind.String:
+                        var leftString = (String)canonicalLeftValue;
+                        var rightString = (String)canonicalRightValue;
+                        return String.Compare(leftString, rightString, StringComparison.OrdinalIgnoreCase) > 0;
+
+                    // Boolean, Boolean
+                    case ValueKind.Boolean:
+                        var leftBoolean = (Boolean)canonicalLeftValue;
+                        var rightBoolean = (Boolean)canonicalRightValue;
+                        return leftBoolean && !rightBoolean;
+                }
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except string comparison is OrdinalIgnoreCase, and objects are not coerced to primitives.
+        /// </summary>
+        private static Boolean AbstractLessThan(
+            Object canonicalLeftValue,
+            Object canonicalRightValue)
+        {
+            CoerceTypes(ref canonicalLeftValue, ref canonicalRightValue, out var leftKind, out var rightKind);
+
+            // Same kind
+            if (leftKind == rightKind)
+            {
+                switch (leftKind)
+                {
+                    // Number, Number
+                    case ValueKind.Number:
+                        var leftDouble = (Double)canonicalLeftValue;
+                        var rightDouble = (Double)canonicalRightValue;
+                        if (Double.IsNaN(leftDouble) || Double.IsNaN(rightDouble))
+                        {
+                            return false;
+                        }
+                        return leftDouble < rightDouble;
+
+                    // String, String
+                    case ValueKind.String:
+                        var leftString = (String)canonicalLeftValue;
+                        var rightString = (String)canonicalRightValue;
+                        return String.Compare(leftString, rightString, StringComparison.OrdinalIgnoreCase) < 0;
+
+                    // Boolean, Boolean
+                    case ValueKind.Boolean:
+                        var leftBoolean = (Boolean)canonicalLeftValue;
+                        var rightBoolean = (Boolean)canonicalRightValue;
+                        return !leftBoolean && rightBoolean;
+                }
+            }
+
+            return false;
+        }
+
+        /// Similar to the Javascript abstract equality comparison algorithm http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3.
+        /// Except objects are not coerced to primitives.
+        private static void CoerceTypes(
+            ref Object canonicalLeftValue,
+            ref Object canonicalRightValue,
+            out ValueKind leftKind,
+            out ValueKind rightKind)
+        {
+            leftKind = GetKind(canonicalLeftValue);
+            rightKind = GetKind(canonicalRightValue);
+
+            // Same kind
+            if (leftKind == rightKind)
+            {
+            }
+            // Number, String
+            else if (leftKind == ValueKind.Number && rightKind == ValueKind.String)
+            {
+                canonicalRightValue = ConvertToNumber(canonicalRightValue);
+                rightKind = ValueKind.Number;
+            }
+            // String, Number
+            else if (leftKind == ValueKind.String && rightKind == ValueKind.Number)
+            {
+                canonicalLeftValue = ConvertToNumber(canonicalLeftValue);
+                leftKind = ValueKind.Number;
+            }
+            // Boolean|Null, Any
+            else if (leftKind == ValueKind.Boolean || leftKind == ValueKind.Null)
+            {
+                canonicalLeftValue = ConvertToNumber(canonicalLeftValue);
+                CoerceTypes(ref canonicalLeftValue, ref canonicalRightValue, out leftKind, out rightKind);
+            }
+            // Any, Boolean|Null
+            else if (rightKind == ValueKind.Boolean || rightKind == ValueKind.Null)
+            {
+                canonicalRightValue = ConvertToNumber(canonicalRightValue);
+                CoerceTypes(ref canonicalLeftValue, ref canonicalRightValue, out leftKind, out rightKind);
+            }
+        }
+
+        /// <summary>
+        /// For primitives, follows the Javascript rules (the Number function in Javascript). Otherwise NaN.
+        /// </summary>
+        private static Double ConvertToNumber(Object canonicalValue)
+        {
+            var kind = GetKind(canonicalValue);
+            switch (kind)
+            {
+                case ValueKind.Null:
+                    return 0d;
+                case ValueKind.Boolean:
+                    return (Boolean)canonicalValue ? 1d : 0d;
+                case ValueKind.Number:
+                    return (Double)canonicalValue;
+                case ValueKind.String:
+                    return ExpressionUtility.ParseNumber(canonicalValue as String);
+            }
+
+            return Double.NaN;
+        }
+
+        private static ValueKind GetKind(Object canonicalValue)
+        {
+            if (Object.ReferenceEquals(canonicalValue, null))
+            {
+                return ValueKind.Null;
+            }
+            else if (canonicalValue is Boolean)
+            {
+                return ValueKind.Boolean;
+            }
+            else if (canonicalValue is Double)
+            {
+                return ValueKind.Number;
+            }
+            else if (canonicalValue is String)
+            {
+                return ValueKind.String;
+            }
+            else if (canonicalValue is IReadOnlyObject)
+            {
+                return ValueKind.Object;
+            }
+            else if (canonicalValue is IReadOnlyArray)
+            {
+                return ValueKind.Array;
+            }
+
+            return ValueKind.Object;
+        }
+
+        private readonly Int32 m_level;
+        private readonly Boolean m_omitTracing;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ExpressionConstants.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using GitHub.DistributedTask.Expressions2.Sdk;
+using GitHub.DistributedTask.Expressions2.Sdk.Functions;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    internal static class ExpressionConstants
+    {
+        static ExpressionConstants()
+        {
+            AddFunction<Contains>("contains", 2, 2);
+            AddFunction<EndsWith>("endsWith", 2, 2);
+            AddFunction<Format>("format", 1, Byte.MaxValue);
+            AddFunction<Join>("join", 1, 2);
+            AddFunction<StartsWith>("startsWith", 2, 2);
+            AddFunction<ToJson>("toJson", 1, 1);
+            AddFunction<HashFiles>("hashFiles", 1, 1);
+        }
+
+        private static void AddFunction<T>(String name, Int32 minParameters, Int32 maxParameters)
+            where T : Function, new()
+        {
+            WellKnownFunctions.Add(name, new FunctionInfo<T>(name, minParameters, maxParameters));
+        }
+
+        internal static readonly String False = "false";
+        internal static readonly String Infinity = "Infinity";
+        internal static readonly Int32 MaxDepth = 50;
+        internal static readonly Int32 MaxLength = 21000; // Under 85,000 large object heap threshold, even if .NET switches to UTF-32
+        internal static readonly String NaN = "NaN";
+        internal static readonly String NegativeInfinity = "-Infinity";
+        internal static readonly String Null = "null";
+        internal static readonly String NumberFormat = "G15";
+        internal static readonly String True = "true";
+        internal static readonly Dictionary<String, IFunctionInfo> WellKnownFunctions = new Dictionary<String, IFunctionInfo>(StringComparer.OrdinalIgnoreCase);
+
+        // Punctuation
+        internal const Char StartGroup = '(';       // logical grouping
+        internal const Char StartIndex = '[';
+        internal const Char StartParameter = '(';   // function call
+        internal const Char EndGroup = ')';         // logical grouping
+        internal const Char EndIndex = ']';
+        internal const Char EndParameter = ')';     // function calll
+        internal const Char Separator = ',';
+        internal const Char Dereference = '.';
+        internal const Char Wildcard = '*';
+
+        // Operators
+        internal const String Not = "!";
+        internal const String NotEqual = "!=";
+        internal const String GreaterThan = ">";
+        internal const String GreaterThanOrEqual = ">=";
+        internal const String LessThan = "<";
+        internal const String LessThanOrEqual = "<=";
+        internal const String Equal = "==";
+        internal const String And = "&&";
+        internal const String Or = "||";
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ExpressionException.cs

@@ -0,0 +1,24 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Logging;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class ExpressionException : Exception
+    {
+        internal ExpressionException(ISecretMasker secretMasker, String message)
+        {
+            if (secretMasker != null)
+            {
+                message = secretMasker.MaskSecrets(message);
+            }
+
+            m_message = message;
+        }
+
+        public override String Message => m_message;
+
+        private readonly String m_message;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ExpressionParser.cs

@@ -0,0 +1,471 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using GitHub.DistributedTask.Expressions2.Sdk.Operators;
+using GitHub.DistributedTask.Expressions2.Tokens;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    using GitHub.DistributedTask.Expressions2.Sdk;
+    using GitHub.DistributedTask.Expressions2.Sdk.Functions;
+
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class ExpressionParser
+    {
+        public IExpressionNode CreateTree(
+            String expression,
+            ITraceWriter trace,
+            IEnumerable<INamedValueInfo> namedValues,
+            IEnumerable<IFunctionInfo> functions)
+        {
+            var context = new ParseContext(expression, trace, namedValues, functions);
+            context.Trace.Info($"Parsing expression: <{expression}>");
+            return CreateTree(context);
+        }
+
+        public IExpressionNode ValidateSyntax(
+            String expression,
+            ITraceWriter trace)
+        {
+            var context = new ParseContext(expression, trace, namedValues: null, functions: null, allowUnknownKeywords: true);
+            context.Trace.Info($"Validating expression syntax: <{expression}>");
+            return CreateTree(context);
+        }
+
+        private static IExpressionNode CreateTree(ParseContext context)
+        {
+            // Push the tokens
+            while (context.LexicalAnalyzer.TryGetNextToken(ref context.Token))
+            {
+                // Unexpected
+                if (context.Token.Kind == TokenKind.Unexpected)
+                {
+                    throw new ParseException(ParseExceptionKind.UnexpectedSymbol, context.Token, context.Expression);
+                }
+                // Operator
+                else if (context.Token.IsOperator)
+                {
+                    PushOperator(context);
+                }
+                // Operand
+                else
+                {
+                    PushOperand(context);
+                }
+
+                context.LastToken = context.Token;
+            }
+
+            // No tokens
+            if (context.LastToken == null)
+            {
+                return null;
+            }
+
+            // Check unexpected end of expression
+            if (context.Operators.Count > 0)
+            {
+                var unexpectedLastToken = false;
+                switch (context.LastToken.Kind)
+                {
+                    case TokenKind.EndGroup:        // ")" logical grouping
+                    case TokenKind.EndIndex:        // "]"
+                    case TokenKind.EndParameters:   // ")" function call
+                        // Legal
+                        break;
+                    case TokenKind.Function:
+                        // Illegal
+                        unexpectedLastToken = true;
+                        break;
+                    default:
+                        unexpectedLastToken = context.LastToken.IsOperator;
+                        break;
+                }
+
+                if (unexpectedLastToken || context.LexicalAnalyzer.UnclosedTokens.Any())
+                {
+                    throw new ParseException(ParseExceptionKind.UnexpectedEndOfExpression, context.LastToken, context.Expression);
+                }
+            }
+
+            // Flush operators
+            while (context.Operators.Count > 0)
+            {
+                FlushTopOperator(context);
+            }
+
+            // Check max depth
+            var result = context.Operands.Single();
+            CheckMaxDepth(context, result);
+
+            return result;
+        }
+
+        private static void PushOperand(ParseContext context)
+        {
+            // Create the node
+            var node = default(ExpressionNode);
+            switch (context.Token.Kind)
+            {
+                // Function
+                case TokenKind.Function:
+                    var function = context.Token.RawValue;
+                    if (TryGetFunctionInfo(context, function, out var functionInfo))
+                    {
+                        node = functionInfo.CreateNode();
+                        node.Name = function;
+                    }
+                    else if (context.AllowUnknownKeywords)
+                    {
+                        node = new NoOperation();
+                        node.Name = function;
+                    }
+                    else
+                    {
+                        throw new ParseException(ParseExceptionKind.UnrecognizedFunction, context.Token, context.Expression);
+                    }
+                    break;
+
+                // Named-value
+                case TokenKind.NamedValue:
+                    var name = context.Token.RawValue;
+                    if (context.ExtensionNamedValues.TryGetValue(name, out var namedValueInfo))
+                    {
+                        node = namedValueInfo.CreateNode();
+                        node.Name = name;
+
+                    }
+                    else if (context.AllowUnknownKeywords)
+                    {
+                        node = new NoOperationNamedValue();
+                        node.Name = name;
+                    }
+                    else
+                    {
+                        throw new ParseException(ParseExceptionKind.UnrecognizedNamedValue, context.Token, context.Expression);
+                    }
+                    break;
+
+                // Otherwise simple
+                default:
+                    node = context.Token.ToNode();
+                    break;
+            }
+
+            // Push the operand
+            context.Operands.Push(node);
+        }
+
+        private static void PushOperator(ParseContext context)
+        {
+            // Flush higher or equal precedence
+            if (context.Token.Associativity == Associativity.LeftToRight)
+            {
+                var precedence = context.Token.Precedence;
+                while (context.Operators.Count > 0)
+                {
+                    var topOperator = context.Operators.Peek();
+                    if (precedence <= topOperator.Precedence &&
+                        topOperator.Kind != TokenKind.StartGroup &&     // Unless top is "(" logical grouping
+                        topOperator.Kind != TokenKind.StartIndex &&     // or unless top is "["
+                        topOperator.Kind != TokenKind.StartParameters &&// or unless top is "(" function call
+                        topOperator.Kind != TokenKind.Separator)        // or unless top is ","
+                    {
+                        FlushTopOperator(context);
+                        continue;
+                    }
+
+                    break;
+                }
+            }
+
+            // Push the operator
+            context.Operators.Push(context.Token);
+
+            // Process closing operators now, since context.LastToken is required
+            // to accurately process TokenKind.EndParameters
+            switch (context.Token.Kind)
+            {
+                case TokenKind.EndGroup:        // ")" logical grouping
+                case TokenKind.EndIndex:        // "]"
+                case TokenKind.EndParameters:   // ")" function call
+                    FlushTopOperator(context);
+                    break;
+            }
+        }
+
+        private static void FlushTopOperator(ParseContext context)
+        {
+            // Special handling for closing operators
+            switch (context.Operators.Peek().Kind)
+            {
+                case TokenKind.EndIndex:        // "]"
+                    FlushTopEndIndex(context);
+                    return;
+
+                case TokenKind.EndGroup:        // ")" logical grouping
+                    FlushTopEndGroup(context);
+                    return;
+
+                case TokenKind.EndParameters:   // ")" function call
+                    FlushTopEndParameters(context);
+                    return;
+            }
+
+            // Pop the operator
+            var @operator = context.Operators.Pop();
+
+            // Create the node
+            var node = (Container)@operator.ToNode();
+
+            // Pop the operands, add to the node
+            var operands = PopOperands(context, @operator.OperandCount);
+            foreach (var operand in operands)
+            {
+                // Flatten nested And
+                if (node is And)
+                {
+                    if (operand is And nestedAnd)
+                    {
+                        foreach (var nestedParameter in nestedAnd.Parameters)
+                        {
+                            node.AddParameter(nestedParameter);
+                        }
+
+                        continue;
+                    }
+                }
+                // Flatten nested Or
+                else if (node is Or)
+                {
+                    if (operand is Or nestedOr)
+                    {
+                        foreach (var nestedParameter in nestedOr.Parameters)
+                        {
+                            node.AddParameter(nestedParameter);
+                        }
+
+                        continue;
+                    }
+                }
+
+                node.AddParameter(operand);
+            }
+
+            // Push the node to the operand stack
+            context.Operands.Push(node);
+        }
+
+        /// <summary>
+        /// Flushes the ")" logical grouping operator
+        /// </summary>
+        private static void FlushTopEndGroup(ParseContext context)
+        {
+            // Pop the operators
+            PopOperator(context, TokenKind.EndGroup);   // ")" logical grouping
+            PopOperator(context, TokenKind.StartGroup); // "(" logical grouping
+        }
+
+        /// <summary>
+        /// Flushes the "]" operator
+        /// </summary>
+        private static void FlushTopEndIndex(ParseContext context)
+        {
+            // Pop the operators
+            PopOperator(context, TokenKind.EndIndex);                   // "]"
+            var @operator = PopOperator(context, TokenKind.StartIndex);  // "["
+
+            // Create the node
+            var node = (Container)@operator.ToNode();
+
+            // Pop the operands, add to the node
+            var operands = PopOperands(context, @operator.OperandCount);
+            foreach (var operand in operands)
+            {
+                node.AddParameter(operand);
+            }
+
+            // Push the node to the operand stack
+            context.Operands.Push(node);
+        }
+
+        // ")" function call
+        private static void FlushTopEndParameters(ParseContext context)
+        {
+            // Pop the operator
+            var @operator = PopOperator(context, TokenKind.EndParameters);   // ")" function call
+
+            // Sanity check top operator is the current token
+            if (!Object.ReferenceEquals(@operator, context.Token))
+            {
+                throw new InvalidOperationException("Expected the operator to be the current token");
+            }
+
+            var function = default(Function);
+
+            // No parameters
+            if (context.LastToken.Kind == TokenKind.StartParameters)
+            {
+                // Node already exists on the operand stack
+                function = (Function)context.Operands.Peek();
+            }
+            // Has parameters
+            else
+            {
+                // Pop the operands
+                var parameterCount = 1;
+                while (context.Operators.Peek().Kind == TokenKind.Separator)
+                {
+                    parameterCount++;
+                    context.Operators.Pop();
+                }
+                var functionOperands = PopOperands(context, parameterCount);
+                
+                // Node already exists on the operand stack
+                function = (Function)context.Operands.Peek();
+
+                // Add the operands to the node
+                foreach (var operand in functionOperands)
+                {
+                    function.AddParameter(operand);
+                }
+            }
+
+            // Pop the "(" operator too
+            @operator = PopOperator(context, TokenKind.StartParameters);
+
+            // Check min/max parameter count
+            TryGetFunctionInfo(context, function.Name, out var functionInfo);
+            if (functionInfo == null && context.AllowUnknownKeywords)
+            {
+                // Don't check min/max
+            }
+            else if (function.Parameters.Count < functionInfo.MinParameters)
+            {
+                throw new ParseException(ParseExceptionKind.TooFewParameters, token: @operator, expression: context.Expression);
+            }
+            else if (function.Parameters.Count > functionInfo.MaxParameters)
+            {
+                throw new ParseException(ParseExceptionKind.TooManyParameters, token: @operator, expression: context.Expression);
+            }
+        }
+
+        /// <summary>
+        /// Pops N operands from the operand stack. The operands are returned
+        /// in their natural listed order, i.e. not last-in-first-out.
+        /// </summary>
+        private static List<ExpressionNode> PopOperands(
+            ParseContext context,
+            Int32 count)
+        {
+            var result = new List<ExpressionNode>();
+            while (count-- > 0)
+            {
+                result.Add(context.Operands.Pop());
+            }
+
+            result.Reverse();
+            return result;
+        }
+
+        /// <summary>
+        /// Pops an operator and asserts it is the expected kind.
+        /// </summary>
+        private static Token PopOperator(
+            ParseContext context,
+            TokenKind expected)
+        {
+            var token = context.Operators.Pop();
+            if (token.Kind != expected)
+            {
+                throw new NotSupportedException($"Expected operator '{expected}' to be popped. Actual '{token.Kind}'.");
+            }
+            return token;
+        }
+
+        /// <summary>
+        /// Checks the max depth of the expression tree
+        /// </summary>
+        private static void CheckMaxDepth(
+            ParseContext context,
+            ExpressionNode node,
+            Int32 depth = 1)
+        {
+            if (depth > ExpressionConstants.MaxDepth)
+            {
+                throw new ParseException(ParseExceptionKind.ExceededMaxDepth, token: null, expression: context.Expression);
+            }
+
+            if (node is Container container)
+            {
+                foreach (var parameter in container.Parameters)
+                {
+                    CheckMaxDepth(context, parameter, depth + 1);
+                }
+            }
+        }
+
+        private static Boolean TryGetFunctionInfo(
+            ParseContext context,
+            String name,
+            out IFunctionInfo functionInfo)
+        {
+            return ExpressionConstants.WellKnownFunctions.TryGetValue(name, out functionInfo) ||
+                context.ExtensionFunctions.TryGetValue(name, out functionInfo);
+        }
+
+        private sealed class ParseContext
+        {
+            public Boolean AllowUnknownKeywords;
+            public readonly String Expression;
+            public readonly Dictionary<String, IFunctionInfo> ExtensionFunctions = new Dictionary<String, IFunctionInfo>(StringComparer.OrdinalIgnoreCase);
+            public readonly Dictionary<String, INamedValueInfo> ExtensionNamedValues = new Dictionary<String, INamedValueInfo>(StringComparer.OrdinalIgnoreCase);
+            public readonly LexicalAnalyzer LexicalAnalyzer;
+            public readonly Stack<ExpressionNode> Operands = new Stack<ExpressionNode>();
+            public readonly Stack<Token> Operators = new Stack<Token>();
+            public readonly ITraceWriter Trace;
+            public Token Token;
+            public Token LastToken;
+
+            public ParseContext(
+                String expression,
+                ITraceWriter trace,
+                IEnumerable<INamedValueInfo> namedValues,
+                IEnumerable<IFunctionInfo> functions,
+                Boolean allowUnknownKeywords = false)
+            {
+                Expression = expression ?? String.Empty;
+                if (Expression.Length > ExpressionConstants.MaxLength)
+                {
+                    throw new ParseException(ParseExceptionKind.ExceededMaxLength, token: null, expression: Expression);
+                }
+
+                Trace = trace ?? new NoOperationTraceWriter();
+                foreach (var namedValueInfo in (namedValues ?? new INamedValueInfo[0]))
+                {
+                    ExtensionNamedValues.Add(namedValueInfo.Name, namedValueInfo);
+                }
+
+                foreach (var functionInfo in (functions ?? new IFunctionInfo[0]))
+                {
+                    ExtensionFunctions.Add(functionInfo.Name, functionInfo);
+                }
+
+                LexicalAnalyzer = new LexicalAnalyzer(Expression);
+                AllowUnknownKeywords = allowUnknownKeywords;
+            }
+
+            private class NoOperationTraceWriter : ITraceWriter
+            {
+                public void Info(String message)
+                {
+                }
+
+                public void Verbose(String message)
+                {
+                }
+            }
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/FunctionInfo.cs

@@ -0,0 +1,29 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Expressions2.Sdk;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class FunctionInfo<T> : IFunctionInfo
+        where T : Function, new()
+    {
+        public FunctionInfo(String name, Int32 minParameters, Int32 maxParameters)
+        {
+            Name = name;
+            MinParameters = minParameters;
+            MaxParameters = maxParameters;
+        }
+
+        public String Name { get; }
+
+        public Int32 MinParameters { get; }
+
+        public Int32 MaxParameters { get; }
+
+        public Function CreateNode()
+        {
+            return new T();
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/IExpressionNode.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using GitHub.DistributedTask.Logging;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IExpressionNode
+    {
+        /// <summary>
+        /// Evaluates the expression and returns the result, wrapped in a helper
+        /// for converting, comparing, and traversing objects.
+        /// </summary>
+        /// <param name="trace">Optional trace writer</param>
+        /// <param name="secretMasker">Optional secret masker</param>
+        /// <param name="state">State object for custom evaluation function nodes and custom named-value nodes</param>
+        /// <param name="options">Evaluation options</param>
+        EvaluationResult Evaluate(
+            ITraceWriter trace,
+            ISecretMasker secretMasker,
+            Object state,
+            EvaluationOptions options);
+    }
+}

src/Sdk/DTExpressions2/Expressions2/IExpressionNodeExtensions.cs

@@ -0,0 +1,237 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GitHub.DistributedTask.Logging;
+using GitHub.DistributedTask.Expressions2.Sdk;
+using GitHub.DistributedTask.Expressions2.Sdk.Operators;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)]
+    public static class IExpressionNodeExtensions
+    {
+        /// <summary>
+        /// Returns the node and all descendant nodes
+        /// </summary>
+        public static IEnumerable<IExpressionNode> Traverse(this IExpressionNode node)
+        {
+            yield return node;
+
+            if (node is Container container && container.Parameters.Count > 0)
+            {
+                foreach (var parameter in container.Parameters)
+                {
+                    foreach (var descendant in parameter.Traverse())
+                    {
+                        yield return descendant;
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Checks whether specific contexts or sub-properties of contexts are referenced.
+        /// If a conclusive determination cannot be made, then the pattern is considered matched.
+        /// For example, the expression "toJson(github)" matches the pattern "github.event" because
+        /// the value is passed to a function. Not enough information is known to determine whether
+        /// the function requires the sub-property. Therefore, it is assumed that it may.
+        ///
+        /// Wildcards are supported in the pattern, and are treated as matching any literal.
+        /// For example, the expression "needs.my-job.outputs.my-output" matches the pattern "needs.*.outputs".
+        /// </summary>
+        public static Boolean[] CheckReferencesContext(
+            this IExpressionNode tree,
+            params String[] patterns)
+        {
+            var result = new Boolean[patterns.Length];
+
+            var segmentedPatterns = default(Stack<IExpressionNode>[]);
+
+            // Walk the tree
+            var stack = new Stack<IExpressionNode>();
+            stack.Push(tree);
+            while (stack.Count > 0)
+            {
+                var node = stack.Pop();
+
+                // Attempt to match a named-value or index operator.
+                // Note, do not push children of the index operator.
+                if (node is NamedValue || node is Index)
+                {
+                    // Lazy initialize the pattern segments
+                    if (segmentedPatterns is null)
+                    {
+                        segmentedPatterns = new Stack<IExpressionNode>[patterns.Length];
+                        var parser = new ExpressionParser();
+                        for (var i = 0; i < patterns.Length; i++)
+                        {
+                            var pattern = patterns[i];
+                            var patternTree = parser.ValidateSyntax(pattern, null);
+                            var patternSegments = GetMatchSegments(patternTree);
+                            if (patternSegments.Count == 0)
+                            {
+                                throw new InvalidOperationException($"Invalid context-match-pattern '{pattern}'");
+                            }
+                            segmentedPatterns[i] = patternSegments;
+                        }
+                    }
+
+                    // Match
+                    Match(node, segmentedPatterns, result);
+                }
+                // Push children of any other container node.
+                else if (node is Container container && container.Parameters.Count > 0)
+                {
+                    foreach (var child in container.Parameters)
+                    {
+                        stack.Push(child);
+                    }
+                }
+            }
+
+            return result;
+        }
+
+        private static void Match(
+            IExpressionNode node,
+            Stack<IExpressionNode>[] patterns,
+            Boolean[] result)
+        {
+            var nodeSegments = GetMatchSegments(node);
+
+            if (nodeSegments.Count == 0)
+            {
+                return;
+            }
+
+            var nodeNamedValue = nodeSegments.Peek() as NamedValue;
+            var originalNodeSegments = nodeSegments;
+
+            for (var i = 0; i < patterns.Length; i++)
+            {
+                var patternSegments = patterns[i];
+                var patternNamedValue = patternSegments.Peek() as NamedValue;
+
+                // Compare the named-value
+                if (String.Equals(nodeNamedValue.Name, patternNamedValue.Name, StringComparison.OrdinalIgnoreCase))
+                {
+                    // Clone the stacks before mutating
+                    nodeSegments = new Stack<IExpressionNode>(originalNodeSegments.Reverse());
+                    nodeSegments.Pop();
+                    patternSegments = new Stack<IExpressionNode>(patternSegments.Reverse());
+                    patternSegments.Pop();
+
+                    // Walk the stacks
+                    while (true)
+                    {
+                        // Every pattern segment was matched
+                        if (patternSegments.Count == 0)
+                        {
+                            result[i] = true;
+                            break;
+                        }
+                        // Every node segment was matched. Treat the pattern as matched. There is not
+                        // enough information to determine whether the property is required; assume it is.
+                        // For example, consider the pattern "github.event" and the expression "toJson(github)".
+                        // In this example the function requires the full structure of the named-value.
+                        else if (nodeSegments.Count == 0)
+                        {
+                            result[i] = true;
+                            break;
+                        }
+
+                        var nodeSegment = nodeSegments.Pop();
+                        var patternSegment = patternSegments.Pop();
+
+                        // The behavior of a wildcard varies depending on whether the left operand
+                        // is an array or an object. For simplicity, treat the pattern as matched.
+                        if (nodeSegment is Wildcard)
+                        {
+                            result[i] = true;
+                            break;
+                        }
+                        // Treat a wildcard pattern segment as matching any literal segment
+                        else if (patternSegment is Wildcard)
+                        {
+                            continue;
+                        }
+
+                        // Convert literals to string and compare
+                        var nodeLiteral = nodeSegment as Literal;
+                        var nodeEvaluationResult = EvaluationResult.CreateIntermediateResult(null, nodeLiteral.Value);
+                        var nodeString = nodeEvaluationResult.ConvertToString();
+                        var patternLiteral = patternSegment as Literal;
+                        var patternEvaluationResult = EvaluationResult.CreateIntermediateResult(null, patternLiteral.Value);
+                        var patternString = patternEvaluationResult.ConvertToString();
+                        if (String.Equals(nodeString, patternString, StringComparison.OrdinalIgnoreCase))
+                        {
+                            continue;
+                        }
+
+                        // Convert to number and compare
+                        var nodeNumber = nodeEvaluationResult.ConvertToNumber();
+                        if (!Double.IsNaN(nodeNumber) && nodeNumber >= 0d && nodeNumber <= (Double)Int32.MaxValue)
+                        {
+                            var patternNumber = patternEvaluationResult.ConvertToNumber();
+                            if (!Double.IsNaN(patternNumber) && patternNumber >= 0 && patternNumber <= (Double)Int32.MaxValue)
+                            {
+                                nodeNumber = Math.Floor(nodeNumber);
+                                patternNumber = Math.Floor(patternNumber);
+                                if (nodeNumber == patternNumber)
+                                {
+                                    continue;
+                                }
+                            }
+                        }
+
+                        // Not matched
+                        break;
+                    }
+                }
+            }
+        }
+
+        private static Stack<IExpressionNode> GetMatchSegments(IExpressionNode node)
+        {
+            var result = new Stack<IExpressionNode>();
+
+            // Node is a named-value
+            if (node is NamedValue)
+            {
+                result.Push(node);
+            }
+            // Node is an index
+            else if (node is Index index)
+            {
+                while (true)
+                {
+                    // Push parameter 1. Treat anything other than literal as a wildcard.
+                    var parameter1 = index.Parameters[1];
+                    result.Push(parameter1 is Literal ? parameter1 : new Wildcard());
+
+                    var parameter0 = index.Parameters[0];
+
+                    // Parameter 0 is a named-value
+                    if (parameter0 is NamedValue)
+                    {
+                        result.Push(parameter0);
+                        break;
+                    }
+                    // Parameter 0 is an index
+                    else if (parameter0 is Index index2)
+                    {
+                        index = index2;
+                    }
+                    // Otherwise clear
+                    else
+                    {
+                        result.Clear();
+                        break;
+                    }
+                }
+            }
+
+            return result;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/IFunctionInfo.cs

@@ -0,0 +1,15 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Expressions2.Sdk;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IFunctionInfo
+    {
+        String Name { get; }
+        Int32 MinParameters { get; }
+        Int32 MaxParameters { get; }
+        Function CreateNode();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/INamedValueInfo.cs

@@ -0,0 +1,13 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Expressions2.Sdk;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface INamedValueInfo
+    {
+        String Name { get; }
+        NamedValue CreateNode();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ITraceWriter.cs

@@ -0,0 +1,12 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface ITraceWriter
+    {
+        void Info(String message);
+        void Verbose(String message);
+    }
+}

src/Sdk/DTExpressions2/Expressions2/NamedValueInfo.cs

@@ -0,0 +1,23 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Expressions2.Sdk;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class NamedValueInfo<T> : INamedValueInfo
+        where T : NamedValue, new()
+    {
+        public NamedValueInfo(String name)
+        {
+            Name = name;
+        }
+
+        public String Name { get; }
+
+        public NamedValue CreateNode()
+        {
+            return new T();
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ParseException.cs

@@ -0,0 +1,68 @@
+using System;
+using System.ComponentModel;
+using GitHub.DistributedTask.Expressions2.Tokens;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class ParseException : ExpressionException
+    {
+        internal ParseException(ParseExceptionKind kind, Token token, String expression)
+            : base(secretMasker: null, message: String.Empty)
+        {
+            Expression = expression;
+            Kind = kind;
+            RawToken = token?.RawValue;
+            TokenIndex = token?.Index ?? 0;
+            String description;
+            switch (kind)
+            {
+                case ParseExceptionKind.ExceededMaxDepth:
+                    description = $"Exceeded max expression depth {ExpressionConstants.MaxDepth}";
+                    break;
+                case ParseExceptionKind.ExceededMaxLength:
+                    description = $"Exceeded max expression length {ExpressionConstants.MaxLength}";
+                    break;
+                case ParseExceptionKind.TooFewParameters:
+                    description = "Too few parameters supplied";
+                    break;
+                case ParseExceptionKind.TooManyParameters:
+                    description = "Too many parameters supplied";
+                    break;
+                case ParseExceptionKind.UnexpectedEndOfExpression:
+                    description = "Unexpected end of expression";
+                    break;
+                case ParseExceptionKind.UnexpectedSymbol:
+                    description = "Unexpected symbol";
+                    break;
+                case ParseExceptionKind.UnrecognizedFunction:
+                    description = "Unrecognized function";
+                    break;
+                case ParseExceptionKind.UnrecognizedNamedValue:
+                    description = "Unrecognized named-value";
+                    break;
+                default: // Should never reach here.
+                    throw new Exception($"Unexpected parse exception kind '{kind}'.");
+            }
+
+            if (token == null)
+            {
+                Message = description;
+            }
+            else
+            {
+                Message = $"{description}: '{RawToken}'. Located at position {TokenIndex + 1} within expression: {Expression}";
+            }
+        }
+
+        internal String Expression { get; }
+
+        internal ParseExceptionKind Kind { get; }
+
+        internal String RawToken { get; }
+
+        internal Int32 TokenIndex { get; }
+
+        public sealed override String Message { get; }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ParseExceptionKind.cs

@@ -0,0 +1,14 @@
+namespace GitHub.DistributedTask.Expressions2
+{
+    internal enum ParseExceptionKind
+    {
+        ExceededMaxDepth,
+        ExceededMaxLength,
+        TooFewParameters,
+        TooManyParameters,
+        UnexpectedEndOfExpression,
+        UnexpectedSymbol,
+        UnrecognizedFunction,
+        UnrecognizedNamedValue,
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Container.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public abstract class Container : ExpressionNode
+    {
+        public IReadOnlyList<ExpressionNode> Parameters => m_parameters.AsReadOnly();
+
+        public void AddParameter(ExpressionNode node)
+        {
+            m_parameters.Add(node);
+            node.Container = this;
+        }
+
+        private readonly List<ExpressionNode> m_parameters = new List<ExpressionNode>();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/EvaluationContext.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using GitHub.DistributedTask.Logging;
+using GitHub.Services.Common;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class EvaluationContext
+    {
+        internal EvaluationContext(
+            ITraceWriter trace,
+            ISecretMasker secretMasker,
+            Object state,
+            EvaluationOptions options,
+            ExpressionNode node)
+        {
+            ArgumentUtility.CheckForNull(trace, nameof(trace));
+            ArgumentUtility.CheckForNull(secretMasker, nameof(secretMasker));
+            Trace = trace;
+            SecretMasker = secretMasker;
+            State = state;
+
+            // Copy the options
+            options = new EvaluationOptions(copy: options);
+            if (options.MaxMemory == 0)
+            {
+                // Set a reasonable default max memory
+                options.MaxMemory = 1048576; // 1 mb
+            }
+            Options = options;
+            Memory = new EvaluationMemory(options.MaxMemory, node);
+
+            m_traceResults = new Dictionary<ExpressionNode, String>();
+            m_traceMemory = new MemoryCounter(null, options.MaxMemory);
+        }
+
+        public ITraceWriter Trace { get; }
+
+        public ISecretMasker SecretMasker { get; }
+
+        public Object State { get; }
+
+        internal EvaluationMemory Memory { get; }
+
+        internal EvaluationOptions Options { get; }
+
+        internal void SetTraceResult(
+            ExpressionNode node,
+            EvaluationResult result)
+        {
+            // Remove if previously added. This typically should not happen. This could happen
+            // due to a badly authored function. So we'll handle it and track memory correctly.
+            if (m_traceResults.TryGetValue(node, out String oldValue))
+            {
+                m_traceMemory.Remove(oldValue);
+                m_traceResults.Remove(node);
+            }
+
+            // Check max memory
+            String value = ExpressionUtility.FormatValue(SecretMasker, result);
+            if (m_traceMemory.TryAdd(value))
+            {
+                // Store the result
+                m_traceResults[node] = value;
+            }
+        }
+
+        internal Boolean TryGetTraceResult(ExpressionNode node, out String value)
+        {
+            return m_traceResults.TryGetValue(node, out value);
+        }
+
+        private readonly Dictionary<ExpressionNode, String> m_traceResults = new Dictionary<ExpressionNode, String>();
+        private readonly MemoryCounter m_traceMemory;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/EvaluationMemory.cs

@@ -0,0 +1,112 @@
+using System;
+using System.Collections.Generic;
+using ExpressionResources = GitHub.DistributedTask.Expressions.ExpressionResources;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    /// <summary>
+    /// This is an internal class only.
+    ///
+    /// This class is used to track current memory consumption
+    /// across the entire expression evaluation.
+    /// </summary>
+    internal sealed class EvaluationMemory
+    {
+        internal EvaluationMemory(
+            Int32 maxBytes,
+            ExpressionNode node)
+        {
+            m_maxAmount = maxBytes;
+            m_node = node;
+        }
+
+        internal void AddAmount(
+            Int32 depth,
+            Int32 bytes,
+            Boolean trimDepth = false)
+        {
+            // Trim deeper depths
+            if (trimDepth)
+            {
+                while (m_maxActiveDepth > depth)
+                {
+                    var amount = m_depths[m_maxActiveDepth];
+
+                    if (amount > 0)
+                    {
+                        // Sanity check
+                        if (amount > m_totalAmount)
+                        {
+                            throw new InvalidOperationException("Bytes to subtract exceeds total bytes");
+                        }
+
+                        // Subtract from the total
+                        checked
+                        {
+                            m_totalAmount -= amount;
+                        }
+
+                        // Reset the amount
+                        m_depths[m_maxActiveDepth] = 0;
+                    }
+
+                    m_maxActiveDepth--;
+                }
+            }
+
+            // Grow the depths
+            if (depth > m_maxActiveDepth)
+            {
+                // Grow the list
+                while (m_depths.Count <= depth)
+                {
+                    m_depths.Add(0);
+                }
+
+                // Adjust the max active depth
+                m_maxActiveDepth = depth;
+            }
+
+            checked
+            {
+                // Add to the depth
+                m_depths[depth] += bytes;
+
+                // Add to the total
+                m_totalAmount += bytes;
+            }
+
+            // Check max
+            if (m_totalAmount > m_maxAmount)
+            {
+                throw new InvalidOperationException(ExpressionResources.ExceededAllowedMemory(m_node?.ConvertToExpression()));
+            }
+        }
+
+        internal static Int32 CalculateBytes(Object obj)
+        {
+            if (obj is String str)
+            {
+                // This measurement doesn't have to be perfect
+                // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+
+                checked
+                {
+                    return c_stringBaseOverhead + ((str?.Length ?? 0) * sizeof(Char));
+                }
+            }
+            else
+            {
+                return c_minObjectSize;
+            }
+        }
+
+        private const Int32 c_minObjectSize = 24;
+        private const Int32 c_stringBaseOverhead = 26;
+        private readonly List<Int32> m_depths = new List<Int32>();
+        private readonly Int32 m_maxAmount;
+        private readonly ExpressionNode m_node;
+        private Int32 m_maxActiveDepth = -1;
+        private Int32 m_totalAmount;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/EvaluationTraceWriter.cs

@@ -0,0 +1,37 @@
+using System;
+using GitHub.DistributedTask.Logging;
+using GitHub.Services.Common;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    internal sealed class EvaluationTraceWriter : ITraceWriter
+    {
+        public EvaluationTraceWriter(ITraceWriter trace, ISecretMasker secretMasker)
+        {
+            ArgumentUtility.CheckForNull(secretMasker, nameof(secretMasker));
+            m_trace = trace;
+            m_secretMasker = secretMasker;
+        }
+
+        public void Info(String message)
+        {
+            if (m_trace != null)
+            {
+                message = m_secretMasker.MaskSecrets(message);
+                m_trace.Info(message);
+            }
+        }
+
+        public void Verbose(String message)
+        {
+            if (m_trace != null)
+            {
+                message = m_secretMasker.MaskSecrets(message);
+                m_trace.Verbose(message);
+            }
+        }
+
+        private readonly ISecretMasker m_secretMasker;
+        private readonly ITraceWriter m_trace;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/ExpressionNode.cs

@@ -0,0 +1,192 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using GitHub.DistributedTask.Logging;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public abstract class ExpressionNode : IExpressionNode
+    {
+        internal Container Container { get; set; }
+
+        internal Int32 Level { get; private set; }
+
+        /// <summary>
+        /// The name is used for tracing. Normally the parser will set the name. However if a node
+        /// is added manually, then the name may not be set and will fallback to the type name.
+        /// </summary>
+        protected internal String Name
+        {
+            get
+            {
+                return !String.IsNullOrEmpty(m_name) ? m_name : this.GetType().Name;
+            }
+
+            set
+            {
+                m_name = value;
+            }
+        }
+
+        /// <summary>
+        /// Indicates whether the evalation result should be stored on the context and used
+        /// when the realized result is traced.
+        /// </summary>
+        protected abstract Boolean TraceFullyRealized { get; }
+
+        /// <summary>
+        /// IExpressionNode entry point.
+        /// </summary>
+        EvaluationResult IExpressionNode.Evaluate(
+            ITraceWriter trace,
+            ISecretMasker secretMasker,
+            Object state,
+            EvaluationOptions options)
+        {
+            if (Container != null)
+            {
+                // Do not localize. This is an SDK consumer error.
+                throw new NotSupportedException($"Expected {nameof(IExpressionNode)}.{nameof(Evaluate)} to be called on root node only.");
+            }
+
+
+            var originalSecretMasker = secretMasker;
+            try
+            {
+                // Evaluate
+                secretMasker = secretMasker?.Clone() ?? new SecretMasker();
+                trace = new EvaluationTraceWriter(trace, secretMasker);
+                var context = new EvaluationContext(trace, secretMasker, state, options, this);
+                trace.Info($"Evaluating: {ConvertToExpression()}");
+                var result = Evaluate(context);
+
+                // Trace the result
+                TraceTreeResult(context, result.Value, result.Kind);
+
+                return result;
+            }
+            finally
+            {
+                if (secretMasker != null && secretMasker != originalSecretMasker)
+                {
+                    (secretMasker as IDisposable)?.Dispose();
+                    secretMasker = null;
+                }
+            }
+        }
+
+        /// <summary>
+        /// This function is intended only for ExpressionNode authors to call. The EvaluationContext
+        /// caches result-state specific to the evaluation instance.
+        /// </summary>
+        public EvaluationResult Evaluate(EvaluationContext context)
+        {
+            // Evaluate
+            Level = Container == null ? 0 : Container.Level + 1;
+            TraceVerbose(context, Level, $"Evaluating {Name}:");
+            var coreResult = EvaluateCore(context, out ResultMemory coreMemory);
+
+            if (coreMemory == null)
+            {
+                coreMemory = new ResultMemory();
+            }
+
+            // Convert to canonical value
+            var val = ExpressionUtility.ConvertToCanonicalValue(coreResult, out ValueKind kind, out Object raw);
+
+            // The depth can be safely trimmed when the total size of the core result is known,
+            // or when the total size of the core result can easily be determined.
+            var trimDepth = coreMemory.IsTotal || (Object.ReferenceEquals(raw, null) && ExpressionUtility.IsPrimitive(kind));
+
+            // Account for the memory overhead of the core result
+            var coreBytes = coreMemory.Bytes ?? EvaluationMemory.CalculateBytes(raw ?? val);
+            context.Memory.AddAmount(Level, coreBytes, trimDepth);
+
+            // Account for the memory overhead of the conversion result
+            if (!Object.ReferenceEquals(raw, null))
+            {
+                var conversionBytes = EvaluationMemory.CalculateBytes(val);
+                context.Memory.AddAmount(Level, conversionBytes);
+            }
+
+            var result = new EvaluationResult(context, Level, val, kind, raw);
+
+            // Store the trace result
+            if (this.TraceFullyRealized)
+            {
+                context.SetTraceResult(this, result);
+            }
+
+            return result;
+        }
+
+        internal abstract String ConvertToExpression();
+
+        internal abstract String ConvertToRealizedExpression(EvaluationContext context);
+
+        /// <summary>
+        /// Evaluates the node
+        /// </summary>
+        /// <param name="context">The current expression context</param>
+        /// <param name="resultMemory">
+        /// Helps determine how much memory is being consumed across the evaluation of the expression.
+        /// </param>
+        protected abstract Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory);
+
+        protected MemoryCounter CreateMemoryCounter(EvaluationContext context)
+        {
+            return new MemoryCounter(this, context.Options.MaxMemory);
+        }
+
+        private void TraceTreeResult(
+            EvaluationContext context,
+            Object result,
+            ValueKind kind)
+        {
+            // Get the realized expression
+            String realizedExpression = ConvertToRealizedExpression(context);
+
+            // Format the result
+            String traceValue = ExpressionUtility.FormatValue(context.SecretMasker, result, kind);
+
+            // Only trace the realized expression if it is meaningfully different
+            if (!String.Equals(realizedExpression, traceValue, StringComparison.Ordinal))
+            {
+                if (kind == ValueKind.Number &&
+                    String.Equals(realizedExpression, $"'{traceValue}'", StringComparison.Ordinal))
+                {
+                    // Don't bother tracing the realized expression when the result is a number and the
+                    // realized expresion is a precisely matching string.
+                }
+                else
+                {
+                    context.Trace.Info($"Expanded: {realizedExpression}");
+                }
+            }
+
+            // Always trace the result
+            context.Trace.Info($"Result: {traceValue}");
+        }
+
+        private static void TraceVerbose(
+            EvaluationContext context,
+            Int32 level,
+            String message)
+        {
+            context.Trace.Verbose(String.Empty.PadLeft(level * 2, '.') + (message ?? String.Empty));
+        }
+
+        private static readonly ValueKind[] s_simpleKinds = new[]
+        {
+            ValueKind.Boolean,
+            ValueKind.Null,
+            ValueKind.Number,
+            ValueKind.String,
+        };
+
+        private String m_name;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/ExpressionUtility.cs

@@ -0,0 +1,265 @@
+using System;
+using System.ComponentModel;
+using System.Globalization;
+using System.Linq;
+using System.Reflection;
+using GitHub.DistributedTask.Logging;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    internal static class ExpressionUtility
+    {
+        internal static Object ConvertToCanonicalValue(
+            Object val,
+            out ValueKind kind,
+            out Object raw)
+        {
+            raw = null;
+
+            if (Object.ReferenceEquals(val, null))
+            {
+                kind = ValueKind.Null;
+                return null;
+            }
+            else if (val is Boolean)
+            {
+                kind = ValueKind.Boolean;
+                return val;
+            }
+            else if (val is Double)
+            {
+                kind = ValueKind.Number;
+                return val;
+            }
+            else if (val is String)
+            {
+                kind = ValueKind.String;
+                return val;
+            }
+            else if (val is INull n)
+            {
+                kind = ValueKind.Null;
+                raw = val;
+                return null;
+            }
+            else if (val is IBoolean boolean)
+            {
+                kind = ValueKind.Boolean;
+                raw = val;
+                return boolean.GetBoolean();
+            }
+            else if (val is INumber number)
+            {
+                kind = ValueKind.Number;
+                raw = val;
+                return number.GetNumber();
+            }
+            else if (val is IString str)
+            {
+                kind = ValueKind.String;
+                raw = val;
+                return str.GetString();
+            }
+            else if (val is IReadOnlyObject)
+            {
+                kind = ValueKind.Object;
+                return val;
+            }
+            else if (val is IReadOnlyArray)
+            {
+                kind = ValueKind.Array;
+                return val;
+            }
+            else if (!val.GetType().GetTypeInfo().IsClass)
+            {
+                if (val is Decimal || val is Byte || val is SByte || val is Int16 || val is UInt16 || val is Int32 || val is UInt32 || val is Int64 || val is UInt64 || val is Single)
+                {
+                    kind = ValueKind.Number;
+                    return Convert.ToDouble(val);
+                }
+                else if (val is Enum)
+                {
+                    var strVal = String.Format(CultureInfo.InvariantCulture, "{0:G}", val);
+                    if (Double.TryParse(strVal, NumberStyles.AllowLeadingSign, CultureInfo.InvariantCulture, out Double doubleValue))
+                    {
+                        kind = ValueKind.Number;
+                        return doubleValue;
+                    }
+
+                    kind = ValueKind.String;
+                    return strVal;
+                }
+            }
+
+            kind = ValueKind.Object;
+            return val;
+        }
+
+        internal static String FormatValue(
+            ISecretMasker secretMasker,
+            EvaluationResult evaluationResult)
+        {
+            return FormatValue(secretMasker, evaluationResult.Value, evaluationResult.Kind);
+        }
+
+        internal static String FormatValue(
+            ISecretMasker secretMasker,
+            Object value,
+            ValueKind kind)
+        {
+            switch (kind)
+            {
+                case ValueKind.Null:
+                    return ExpressionConstants.Null;
+
+                case ValueKind.Boolean:
+                    return ((Boolean)value) ? ExpressionConstants.True : ExpressionConstants.False;
+
+                case ValueKind.Number:
+                    var strNumber = ((Double)value).ToString(ExpressionConstants.NumberFormat, CultureInfo.InvariantCulture);
+                    return secretMasker != null ? secretMasker.MaskSecrets(strNumber) : strNumber;
+
+                case ValueKind.String:
+                    // Mask secrets before string-escaping.
+                    var strValue = secretMasker != null ? secretMasker.MaskSecrets(value as String) : value as String;
+                    return $"'{StringEscape(strValue)}'";
+
+                case ValueKind.Array:
+                case ValueKind.Object:
+                    return kind.ToString();
+
+                default: // Should never reach here.
+                    throw new NotSupportedException($"Unable to convert to realized expression. Unexpected value kind: {kind}");
+            }
+        }
+
+        internal static bool IsLegalKeyword(String str)
+        {
+            if (String.IsNullOrEmpty(str))
+            {
+                return false;
+            }
+
+            var first = str[0];
+            if ((first >= 'a' && first <= 'z') ||
+                (first >= 'A' && first <= 'Z') ||
+                first == '_')
+            {
+                for (var i = 1; i < str.Length; i++)
+                {
+                    var c = str[i];
+                    if ((c >= 'a' && c <= 'z') ||
+                        (c >= 'A' && c <= 'Z') ||
+                        (c >= '0' && c <= '9') ||
+                        c == '_' ||
+                        c == '-')
+                    {
+                        // OK
+                    }
+                    else
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+            else
+            {
+                return false;
+            }
+
+        }
+
+        internal static Boolean IsPrimitive(ValueKind kind)
+        {
+            switch (kind)
+            {
+                case ValueKind.Null:
+                case ValueKind.Boolean:
+                case ValueKind.Number:
+                case ValueKind.String:
+                    return true;
+                default:
+                    return false;
+            }
+        }
+
+        /// <summary>
+        /// The rules here attempt to follow Javascript rules for coercing a string into a number
+        /// for comparison. That is, the Number() function in Javascript.
+        /// </summary>
+        internal static Double ParseNumber(String str)
+        {
+            // Trim
+            str = str?.Trim() ?? String.Empty;
+
+            // Empty
+            if (String.IsNullOrEmpty(str))
+            {
+                return 0d;
+            }
+            // Try parse
+            else if (Double.TryParse(str, NumberStyles.AllowLeadingSign | NumberStyles.AllowDecimalPoint | NumberStyles.AllowExponent, CultureInfo.InvariantCulture, out var value))
+            {
+                return value;
+            }
+            // Check for 0x[0-9a-fA-F]+
+            else if (str[0] == '0' &&
+                str.Length > 2 &&
+                str[1] == 'x' &&
+                str.Skip(2).All(x => (x >= '0' && x <= '9') || (x >= 'a' && x <= 'f') || (x >= 'A' && x <= 'F')))
+            {
+                // Try parse
+                if (Int32.TryParse(str.Substring(2), NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture, out var integer))
+                {
+                    return (Double)integer;
+                }
+
+                // Otherwise exceeds range
+            }
+            // Check for 0o[0-9]+
+            else if (str[0] == '0' &&
+                str.Length > 2 &&
+                str[1] == 'o' &&
+                str.Skip(2).All(x => x >= '0' && x <= '7'))
+            {
+                // Try parse
+                var integer = default(Int32?);
+                try
+                {
+                    integer = Convert.ToInt32(str.Substring(2), 8);
+                }
+                // Otherwise exceeds range
+                catch (Exception)
+                {
+                }
+
+                // Success
+                if (integer != null)
+                {
+                    return (Double)integer.Value;
+                }
+            }
+            // Infinity
+            else if (String.Equals(str, ExpressionConstants.Infinity, StringComparison.Ordinal))
+            {
+                return Double.PositiveInfinity;
+            }
+            // -Infinity
+            else if (String.Equals(str, ExpressionConstants.NegativeInfinity, StringComparison.Ordinal))
+            {
+                return Double.NegativeInfinity;
+            }
+
+            // Otherwise NaN
+            return Double.NaN;
+        }
+
+        internal static String StringEscape(String value)
+        {
+            return String.IsNullOrEmpty(value) ? String.Empty : value.Replace("'", "''");
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Function.cs

@@ -0,0 +1,45 @@
+using System;
+using System.ComponentModel;
+using System.Globalization;
+using System.Linq;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public abstract class Function : Container
+    {
+        /// <summary>
+        /// Generally this should not be overridden. True indicates the result of the node is traced as part of the "expanded"
+        /// (i.e. "realized") trace information. Otherwise the node expression is printed, and parameters to the node may or
+        /// may not be fully realized - depending on each respective parameter's trace-fully-realized setting.
+        /// 
+        /// The purpose is so the end user can understand how their expression expanded at run time. For example, consider
+        /// the expression: eq(variables.publish, 'true'). The runtime-expanded expression may be: eq('true', 'true')
+        /// </summary>
+        protected override Boolean TraceFullyRealized => true;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "{0}({1})",
+                Name,
+                String.Join(", ", Parameters.Select(x => x.ConvertToExpression())));
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "{0}({1})",
+                Name,
+                String.Join(", ", Parameters.Select(x => x.ConvertToRealizedExpression(context))));
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/Contains.cs

@@ -0,0 +1,44 @@
+using System;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class Contains : Function
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            if (left.IsPrimitive)
+            {
+                var leftString = left.ConvertToString();
+
+                var right = Parameters[1].Evaluate(context);
+                if (right.IsPrimitive)
+                {
+                    var rightString = right.ConvertToString();
+                    return leftString.IndexOf(rightString, StringComparison.OrdinalIgnoreCase) >= 0;
+                }
+            }
+            else if (left.TryGetCollectionInterface(out var collection) &&
+                collection is IReadOnlyArray array &&
+                array.Count > 0)
+            {
+                var right = Parameters[1].Evaluate(context);
+                foreach (var item in array)
+                {
+                    var itemResult = EvaluationResult.CreateIntermediateResult(context, item);
+                    if (right.AbstractEqual(itemResult))
+                    {
+                        return true;
+                    }
+                }
+            }
+
+            return false;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/EndsWith.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class EndsWith : Function
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            if (left.IsPrimitive)
+            {
+                var leftString = left.ConvertToString();
+
+                var right = Parameters[1].Evaluate(context);
+                if (right.IsPrimitive)
+                {
+                    var rightString = right.ConvertToString();
+                    return leftString.EndsWith(rightString, StringComparison.OrdinalIgnoreCase);
+                }
+            }
+
+            return false;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/Format.cs

@@ -0,0 +1,298 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using ExpressionResources = GitHub.DistributedTask.Expressions.ExpressionResources;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class Format : Function
+    {
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var format = Parameters[0].Evaluate(context).ConvertToString();
+            var index = 0;
+            var result = new FormatResultBuilder(this, context, CreateMemoryCounter(context));
+            while (index < format.Length)
+            {
+                var lbrace = format.IndexOf('{', index);
+                var rbrace = format.IndexOf('}', index);
+
+                // Left brace
+                if (lbrace >= 0 && (rbrace < 0 || rbrace > lbrace))
+                {
+                    // Escaped left brace
+                    if (SafeCharAt(format, lbrace + 1) == '{')
+                    {
+                        result.Append(format.Substring(index, lbrace - index + 1));
+                        index = lbrace + 2;
+                    }
+                    // Left brace, number, optional format specifiers, right brace
+                    else if (rbrace > lbrace + 1 &&
+                        ReadArgIndex(format, lbrace + 1, out Byte argIndex, out Int32 endArgIndex) &&
+                        ReadFormatSpecifiers(format, endArgIndex + 1, out String formatSpecifiers, out rbrace))
+                    {
+                        // Check parameter count
+                        if (argIndex > Parameters.Count - 2)
+                        {
+                            throw new FormatException(ExpressionResources.InvalidFormatArgIndex(format));
+                        }
+
+                        // Append the portion before the left brace
+                        if (lbrace > index)
+                        {
+                            result.Append(format.Substring(index, lbrace - index));
+                        }
+
+                        // Append the arg
+                        result.Append(argIndex, formatSpecifiers);
+                        index = rbrace + 1;
+                    }
+                    else
+                    {
+                        throw new FormatException(ExpressionResources.InvalidFormatString(format));
+                    }
+                }
+                // Right brace
+                else if (rbrace >= 0)
+                {
+                    // Escaped right brace
+                    if (SafeCharAt(format, rbrace + 1) == '}')
+                    {
+                        result.Append(format.Substring(index, rbrace - index + 1));
+                        index = rbrace + 2;
+                    }
+                    else
+                    {
+                        throw new FormatException(ExpressionResources.InvalidFormatString(format));
+                    }
+                }
+                // Last segment
+                else
+                {
+                    result.Append(format.Substring(index));
+                    break;
+                }
+            }
+
+            return result.ToString();
+        }
+
+        private Boolean ReadArgIndex(
+            String str,
+            Int32 startIndex,
+            out Byte result,
+            out Int32 endIndex)
+        {
+            // Count the number of digits
+            var length = 0;
+            while (Char.IsDigit(SafeCharAt(str, startIndex + length)))
+            {
+                length++;
+            }
+
+            // Validate at least one digit
+            if (length < 1)
+            {
+                result = default;
+                endIndex = default;
+                return false;
+            }
+
+            // Parse the number
+            endIndex = startIndex + length - 1;
+            return Byte.TryParse(str.Substring(startIndex, length), NumberStyles.None, CultureInfo.InvariantCulture, out result);
+        }
+
+        private Boolean ReadFormatSpecifiers(
+            String str,
+            Int32 startIndex,
+            out String result,
+            out Int32 rbrace)
+        {
+            // No format specifiers
+            var c = SafeCharAt(str, startIndex);
+            if (c == '}')
+            {
+                result = String.Empty;
+                rbrace = startIndex;
+                return true;
+            }
+
+            // Validate starts with ":"
+            if (c != ':')
+            {
+                result = default;
+                rbrace = default;
+                return false;
+            }
+
+            // Read the specifiers
+            var specifiers = new StringBuilder();
+            var index = startIndex + 1;
+            while (true)
+            {
+                // Validate not the end of the string
+                if (index >= str.Length)
+                {
+                    result = default;
+                    rbrace = default;
+                    return false;
+                }
+
+                c = str[index];
+
+                // Not right-brace
+                if (c != '}')
+                {
+                    specifiers.Append(c);
+                    index++;
+                }
+                // Escaped right-brace
+                else if (SafeCharAt(str, index + 1) == '}')
+                {
+                    specifiers.Append('}');
+                    index += 2;
+                }
+                // Closing right-brace
+                else
+                {
+                    result = specifiers.ToString();
+                    rbrace = index;
+                    return true;
+                }
+            }
+        }
+
+        private Char SafeCharAt(
+            String str,
+            Int32 index)
+        {
+            if (str.Length > index)
+            {
+                return str[index];
+            }
+
+            return '\0';
+        }
+
+        private sealed class FormatResultBuilder
+        {
+            internal FormatResultBuilder(
+                Format node,
+                EvaluationContext context,
+                MemoryCounter counter)
+            {
+                m_node = node;
+                m_context = context;
+                m_counter = counter;
+                m_cache = new ArgValue[node.Parameters.Count - 1];
+            }
+
+            // Build the final string. This is when lazy segments are evaluated.
+            public override String ToString()
+            {
+                return String.Join(
+                    String.Empty,
+                    m_segments.Select(obj =>
+                    {
+                        if (obj is Lazy<String> lazy)
+                        {
+                            return lazy.Value;
+                        }
+                        else
+                        {
+                            return obj as String;
+                        }
+                    }));
+            }
+
+            // Append a static value
+            internal void Append(String value)
+            {
+                if (value?.Length > 0)
+                {
+                    // Track memory
+                    m_counter.Add(value);
+
+                    // Append the segment
+                    m_segments.Add(value);
+                }
+            }
+
+            // Append an argument
+            internal void Append(
+                Int32 argIndex,
+                String formatSpecifiers)
+            {
+                // Delay execution until the final ToString
+                m_segments.Add(new Lazy<String>(() =>
+                {
+                    String result;
+
+                    // Get the arg from the cache
+                    var argValue = m_cache[argIndex];
+
+                    // Evaluate the arg and cache the result
+                    if (argValue == null)
+                    {
+                        // The evaluation result is required when format specifiers are used. Otherwise the string
+                        // result is required. Go ahead and store both values. Since ConvertToString produces tracing,
+                        // we need to run that now so the tracing appears in order in the log.
+                        var evaluationResult = m_node.Parameters[argIndex + 1].Evaluate(m_context);
+                        var stringResult = evaluationResult.ConvertToString();
+                        argValue = new ArgValue(evaluationResult, stringResult);
+                        m_cache[argIndex] = argValue;
+                    }
+
+                    // No format specifiers
+                    if (String.IsNullOrEmpty(formatSpecifiers))
+                    {
+                        result = argValue.StringResult;
+                    }
+                    // Invalid
+                    else
+                    {
+                        throw new FormatException(ExpressionResources.InvalidFormatSpecifiers(formatSpecifiers, argValue.EvaluationResult.Kind));
+                    }
+
+                    // Track memory
+                    if (!String.IsNullOrEmpty(result))
+                    {
+                        m_counter.Add(result);
+                    }
+
+                    return result;
+                }));
+            }
+
+            private readonly ArgValue[] m_cache;
+            private readonly EvaluationContext m_context;
+            private readonly MemoryCounter m_counter;
+            private readonly Format m_node;
+            private readonly List<Object> m_segments = new List<Object>();
+        }
+
+        /// <summary>
+        /// Stores an EvaluateResult and the value converted to a String.
+        /// </summary>
+        private sealed class ArgValue
+        {
+            public ArgValue(
+                EvaluationResult evaluationResult,
+                String stringResult)
+            {
+                EvaluationResult = evaluationResult;
+                StringResult = stringResult;
+            }
+
+            public EvaluationResult EvaluationResult { get; }
+
+            public String StringResult { get; }
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/HashFiles.cs

@@ -0,0 +1,93 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Minimatch;
+using System.IO;
+using System.Security.Cryptography;
+using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.Pipelines.ObjectTemplating;
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class HashFiles : Function
+    {
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+
+            // hashFiles() only works on the runner and only works with files under GITHUB_WORKSPACE
+            // Since GITHUB_WORKSPACE is set by runner, I am using that as the fact of this code runs on server or runner.
+            if (context.State is ObjectTemplating.TemplateContext templateContext &&
+                templateContext.ExpressionValues.TryGetValue(PipelineTemplateConstants.GitHub, out var githubContextData) &&
+                githubContextData is DictionaryContextData githubContext &&
+                githubContext.TryGetValue(PipelineTemplateConstants.Workspace, out var workspace) == true &&
+                workspace is StringContextData workspaceData)
+            {
+                string searchRoot = workspaceData.Value;
+                string pattern = Parameters[0].Evaluate(context).ConvertToString();
+
+                context.Trace.Info($"Search root directory: '{searchRoot}'");
+                context.Trace.Info($"Search pattern: '{pattern}'");
+                var files = Directory.GetFiles(searchRoot, "*", SearchOption.AllDirectories).OrderBy(x => x).ToList();
+                if (files.Count == 0)
+                {
+                    throw new ArgumentException($"'hashFiles({pattern})' failed. Directory '{searchRoot}' is empty");
+                }
+                else
+                {
+                    context.Trace.Info($"Found {files.Count} files");
+                }
+
+                var matcher = new Minimatcher(pattern, s_minimatchOptions);
+                files = matcher.Filter(files).ToList();
+                if (files.Count == 0)
+                {
+                    throw new ArgumentException($"'hashFiles({pattern})' failed. Search pattern '{pattern}' doesn't match any file under '{searchRoot}'");
+                }
+                else
+                {
+                    context.Trace.Info($"{files.Count} matches to hash");
+                }
+
+                List<byte> filesSha256 = new List<byte>();
+                foreach (var file in files)
+                {
+                    context.Trace.Info($"Hash {file}");
+                    using (SHA256 sha256hash = SHA256.Create())
+                    {
+                        using (var fileStream = File.OpenRead(file))
+                        {
+                            filesSha256.AddRange(sha256hash.ComputeHash(fileStream));
+                        }
+                    }
+                }
+
+                using (SHA256 sha256hash = SHA256.Create())
+                {
+                    var hashBytes = sha256hash.ComputeHash(filesSha256.ToArray());
+                    StringBuilder hashString = new StringBuilder();
+                    for (int i = 0; i < hashBytes.Length; i++)
+                    {
+                        hashString.Append(hashBytes[i].ToString("x2"));
+                    }
+                    var result = hashString.ToString();
+                    context.Trace.Info($"Final hash result: '{result}'");
+                    return result;
+                }
+            }
+            else
+            {
+                throw new InvalidOperationException("'hashfiles' expression function is only supported under runner context.");
+            }
+        }
+
+        private static readonly Options s_minimatchOptions = new Options
+        {
+            Dot = true,
+            NoBrace = true,
+            NoCase = Environment.OSVersion.Platform != PlatformID.Unix && Environment.OSVersion.Platform != PlatformID.MacOSX
+        };
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/Join.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Text;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class Join : Function
+    {
+        protected sealed override Boolean TraceFullyRealized => true;
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var items = Parameters[0].Evaluate(context);
+
+            // Array
+            if (items.TryGetCollectionInterface(out var collection) &&
+                collection is IReadOnlyArray array &&
+                array.Count > 0)
+            {
+                var result = new StringBuilder();
+                var memory = new MemoryCounter(this, context.Options.MaxMemory);
+
+                // Append the first item
+                var item = array[0];
+                var itemResult = EvaluationResult.CreateIntermediateResult(context, item);
+                var itemString = itemResult.ConvertToString();
+                memory.Add(itemString);
+                result.Append(itemString);
+
+                // More items?
+                if (array.Count > 1)
+                {
+                    var separator = ",";
+                    if (Parameters.Count > 1)
+                    {
+                        var separatorResult = Parameters[1].Evaluate(context);
+                        if (separatorResult.IsPrimitive)
+                        {
+                            separator = separatorResult.ConvertToString();
+                        }
+                    }
+
+                    for (var i = 1; i < array.Count; i++)
+                    {
+                        // Append the separator
+                        memory.Add(separator);
+                        result.Append(separator);
+
+                        // Append the next item
+                        var nextItem = array[i];
+                        var nextItemResult = EvaluationResult.CreateIntermediateResult(context, nextItem);
+                        var nextItemString = nextItemResult.ConvertToString();
+                        memory.Add(nextItemString);
+                        result.Append(nextItemString);
+                    }
+                }
+
+                return result.ToString();
+            }
+            // Primitive
+            else if (items.IsPrimitive)
+            {
+                return items.ConvertToString();
+            }
+            // Otherwise return empty string
+            else
+            {
+                return String.Empty;
+            }
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/NoOperation.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    /// <summary>
+    /// Used for building expression parse trees.
+    /// </summary>
+    internal sealed class NoOperation : Function
+    {
+        protected override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            return null;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/StartsWith.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class StartsWith : Function
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            if (left.IsPrimitive)
+            {
+                var leftString = left.ConvertToString();
+
+                var right = Parameters[1].Evaluate(context);
+                if (right.IsPrimitive)
+                {
+                    var rightString = right.ConvertToString();
+                    return leftString.StartsWith(rightString, StringComparison.OrdinalIgnoreCase);
+                }
+            }
+
+            return false;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/ToJson.cs

@@ -0,0 +1,390 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Text;
+using GitHub.Services.WebApi;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions
+{
+    internal sealed class ToJson : Function
+    {
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var result = new StringBuilder();
+            var memory = new MemoryCounter(this, context.Options.MaxMemory);
+            var current = Parameters[0].Evaluate(context);
+            var ancestors = new Stack<ICollectionEnumerator>();
+
+            do
+            {
+                // Descend as much as possible
+                while (true)
+                {
+                    // Collection
+                    if (current.TryGetCollectionInterface(out Object collection))
+                    {
+                        // Array
+                        if (collection is IReadOnlyArray array)
+                        {
+                            if (array.Count > 0)
+                            {
+                                // Write array start
+                                WriteArrayStart(result, memory, ancestors);
+
+                                // Move to first item
+                                var enumerator = new ArrayEnumerator(context, current, array);
+                                enumerator.MoveNext();
+                                ancestors.Push(enumerator);
+                                current = enumerator.Current;
+                            }
+                            else
+                            {
+                                // Write empty array
+                                WriteEmptyArray(result, memory, ancestors);
+                                break;
+                            }
+                        }
+                        // Mapping
+                        else if (collection is IReadOnlyObject obj)
+                        {
+                            if (obj.Count > 0)
+                            {
+                                // Write mapping start
+                                WriteMappingStart(result, memory, ancestors);
+
+                                // Move to first pair
+                                var enumerator = new ObjectEnumerator(context, current, obj);
+                                enumerator.MoveNext();
+                                ancestors.Push(enumerator);
+
+                                // Write mapping key
+                                WriteMappingKey(context, result, memory, enumerator.Current.Key, ancestors);
+
+                                // Move to mapping value
+                                current = enumerator.Current.Value;
+                            }
+                            else
+                            {
+                                // Write empty mapping
+                                WriteEmptyMapping(result, memory, ancestors);
+                                break;
+                            }
+                        }
+                        else
+                        {
+                            throw new NotSupportedException($"Unexpected type '{collection?.GetType().FullName}'");
+                        }
+                    }
+                    // Not a collection
+                    else
+                    {
+                        // Write value
+                        WriteValue(context, result, memory, current, ancestors);
+                        break;
+                    }
+                }
+
+                // Next sibling or ancestor sibling
+                do
+                {
+                    if (ancestors.Count > 0)
+                    {
+                        var parent = ancestors.Peek();
+
+                        // Parent array
+                        if (parent is ArrayEnumerator arrayEnumerator)
+                        {
+                            // Move to next item
+                            if (arrayEnumerator.MoveNext())
+                            {
+                                current = arrayEnumerator.Current;
+
+                                break;
+                            }
+                            // Move to parent
+                            else
+                            {
+                                ancestors.Pop();
+                                current = arrayEnumerator.Array;
+
+                                // Write array end
+                                WriteArrayEnd(result, memory, ancestors);
+                            }
+                        }
+                        // Parent mapping
+                        else if (parent is ObjectEnumerator objectEnumerator)
+                        {
+                            // Move to next pair
+                            if (objectEnumerator.MoveNext())
+                            {
+                                // Write mapping key
+                                WriteMappingKey(context, result, memory, objectEnumerator.Current.Key, ancestors);
+
+                                // Move to mapping value
+                                current = objectEnumerator.Current.Value;
+
+                                break;
+                            }
+                            // Move to parent
+                            else
+                            {
+                                ancestors.Pop();
+                                current = objectEnumerator.Object;
+
+                                // Write mapping end
+                                WriteMappingEnd(result, memory, ancestors);
+                            }
+                        }
+                        else
+                        {
+                            throw new NotSupportedException($"Unexpected type '{parent?.GetType().FullName}'");
+                        }
+                    }
+                    else
+                    {
+                        current = null;
+                    }
+
+                } while (current != null);
+
+            } while (current != null);
+
+            return result.ToString();
+        }
+
+        private void WriteArrayStart(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = PrefixValue("[", ancestors);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteMappingStart(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = PrefixValue("{", ancestors);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteArrayEnd(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = $"\n{new String(' ', ancestors.Count * 2)}]";
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteMappingEnd(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = $"\n{new String(' ', ancestors.Count * 2)}}}";
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteEmptyArray(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = PrefixValue("[]", ancestors);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteEmptyMapping(
+            StringBuilder writer,
+            MemoryCounter memory,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = PrefixValue("{}", ancestors);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteMappingKey(
+            EvaluationContext context,
+            StringBuilder writer,
+            MemoryCounter memory,
+            EvaluationResult key,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            var str = PrefixValue(JsonUtility.ToString(key.ConvertToString()), ancestors, isMappingKey: true);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private void WriteValue(
+            EvaluationContext context,
+            StringBuilder writer,
+            MemoryCounter memory,
+            EvaluationResult value,
+            Stack<ICollectionEnumerator> ancestors)
+        {
+            String str;
+            switch (value.Kind)
+            {
+                case ValueKind.Null:
+                    str = "null";
+                    break;
+
+                case ValueKind.Boolean:
+                    str = (Boolean)value.Value ? "true" : "false";
+                    break;
+
+                case ValueKind.Number:
+                    str = value.ConvertToString();
+                    break;
+
+                case ValueKind.String:
+                    str = JsonUtility.ToString(value.Value);
+                    break;
+
+                default:
+                    str = "{}"; // The value is an object we don't know how to traverse
+                    break;
+            }
+
+            str = PrefixValue(str, ancestors);
+            memory.Add(str);
+            writer.Append(str);
+        }
+
+        private String PrefixValue(
+            String value,
+            Stack<ICollectionEnumerator> ancestors,
+            Boolean isMappingKey = false)
+        {
+            var level = ancestors.Count;
+            var parent = level > 0 ? ancestors.Peek() : null;
+
+            if (!isMappingKey && parent is ObjectEnumerator)
+            {
+                return $": {value}";
+            }
+            else if (level > 0)
+            {
+                return $"{(parent.IsFirst ? String.Empty : ",")}\n{new String(' ', level * 2)}{value}";
+            }
+            else
+            {
+                return value;
+            }
+        }
+
+        private interface ICollectionEnumerator : IEnumerator
+        {
+            Boolean IsFirst { get; }
+        }
+
+        private sealed class ArrayEnumerator : ICollectionEnumerator
+        {
+            public ArrayEnumerator(
+                EvaluationContext context,
+                EvaluationResult result,
+                IReadOnlyArray array)
+            {
+                m_context = context;
+                m_result = result;
+                m_enumerator = array.GetEnumerator();
+            }
+
+            public EvaluationResult Array => m_result;
+
+            public EvaluationResult Current => m_current;
+
+            Object IEnumerator.Current => m_current;
+
+            public Boolean IsFirst => m_index == 0;
+
+            public Boolean MoveNext()
+            {
+                if (m_enumerator.MoveNext())
+                {
+                    m_current = EvaluationResult.CreateIntermediateResult(m_context, m_enumerator.Current);
+                    m_index++;
+                    return true;
+                }
+                else
+                {
+                    m_current = null;
+                    return false;
+                }
+            }
+
+            public void Reset()
+            {
+                throw new NotSupportedException(nameof(Reset));
+            }
+
+            private readonly EvaluationContext m_context;
+            private readonly IEnumerator m_enumerator;
+            private readonly EvaluationResult m_result;
+            private EvaluationResult m_current;
+            private Int32 m_index = -1;
+        }
+
+        private sealed class ObjectEnumerator : ICollectionEnumerator
+        {
+            public ObjectEnumerator(
+                EvaluationContext context,
+                EvaluationResult result,
+                IReadOnlyObject obj)
+            {
+                m_context = context;
+                m_result = result;
+                m_enumerator = obj.GetEnumerator();
+            }
+
+            public KeyValuePair<EvaluationResult, EvaluationResult> Current => m_current;
+
+            Object IEnumerator.Current => m_current;
+
+            public Boolean IsFirst => m_index == 0;
+
+            public EvaluationResult Object => m_result;
+
+            public Boolean MoveNext()
+            {
+                if (m_enumerator.MoveNext())
+                {
+                    var current = (KeyValuePair<String, Object>)m_enumerator.Current;
+                    var key = EvaluationResult.CreateIntermediateResult(m_context, current.Key);
+                    var value = EvaluationResult.CreateIntermediateResult(m_context, current.Value);
+                    m_current = new KeyValuePair<EvaluationResult, EvaluationResult>(key, value);
+                    m_index++;
+                    return true;
+                }
+                else
+                {
+                    m_current = default(KeyValuePair<EvaluationResult, EvaluationResult>);
+                    return false;
+                }
+            }
+
+            public void Reset()
+            {
+                throw new NotSupportedException(nameof(Reset));
+            }
+
+            private readonly EvaluationContext m_context;
+            private readonly IEnumerator m_enumerator;
+            private readonly EvaluationResult m_result;
+            private KeyValuePair<EvaluationResult, EvaluationResult> m_current;
+            private Int32 m_index = -1;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/IBoolean.cs

@@ -0,0 +1,11 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IBoolean
+    {
+        Boolean GetBoolean();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/INull.cs

@@ -0,0 +1,9 @@
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface INull
+    {
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/INumber.cs

@@ -0,0 +1,11 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface INumber
+    {
+        Double GetNumber();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/IReadOnlyArray.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IReadOnlyArray
+    {
+        Int32 Count { get; }
+
+        Object this[Int32 index] { get; }
+
+        IEnumerator GetEnumerator();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/IReadOnlyObject.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IReadOnlyObject
+    {
+        Int32 Count { get; }
+
+        IEnumerable<String> Keys { get; }
+
+        IEnumerable<Object> Values { get; }
+
+        Object this[String key] { get; }
+
+        Boolean ContainsKey(String key);
+
+        IEnumerator GetEnumerator();
+
+        Boolean TryGetValue(
+            String key,
+            out Object value);
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/IString.cs

@@ -0,0 +1,11 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public interface IString
+    {
+        String GetString();
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Literal.cs

@@ -0,0 +1,43 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class Literal : ExpressionNode
+    {
+        public Literal(Object val)
+        {
+            Value = ExpressionUtility.ConvertToCanonicalValue(val, out var kind, out _);
+            Kind = kind;
+            Name = kind.ToString();
+        }
+
+        public ValueKind Kind { get; }
+
+        public Object Value { get; }
+
+        // Prevent the value from being stored on the evaluation context.
+        // This avoids unneccessarily duplicating the value in memory.
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return ExpressionUtility.FormatValue(null, Value, Kind);
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            return ExpressionUtility.FormatValue(null, Value, Kind);
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            return Value;
+        }
+    }
+
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/MemoryCounter.cs

@@ -0,0 +1,94 @@
+using System;
+using System.ComponentModel;
+using Newtonsoft.Json.Linq;
+using ExpressionResources = GitHub.DistributedTask.Expressions.ExpressionResources;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    /// <summary>
+    /// Helper class for ExpressionNode authors. This class helps calculate memory overhead for a result object.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class MemoryCounter
+    {
+        internal MemoryCounter(
+            ExpressionNode node,
+            Int32? maxBytes)
+        {
+            m_node = node;
+            m_maxBytes = (maxBytes ?? 0) > 0 ? maxBytes.Value : Int32.MaxValue;
+        }
+
+        public Int32 CurrentBytes => m_currentBytes;
+
+        public void Add(Int32 amount)
+        {
+            if (!TryAdd(amount))
+            {
+                throw new InvalidOperationException(ExpressionResources.ExceededAllowedMemory(m_node?.ConvertToExpression()));
+            }
+        }
+
+        public void Add(String value)
+        {
+            Add(CalculateSize(value));
+        }
+
+        public void AddMinObjectSize()
+        {
+            Add(MinObjectSize);
+        }
+
+        public void Remove(String value)
+        {
+            m_currentBytes -= CalculateSize(value);
+        }
+
+        public static Int32 CalculateSize(String value)
+        {
+            // This measurement doesn't have to be perfect.
+            // https://codeblog.jonskeet.uk/2011/04/05/of-memory-and-strings/
+
+            Int32 bytes;
+            checked
+            {
+                bytes = StringBaseOverhead + ((value?.Length ?? 0) * 2);
+            }
+            return bytes;
+        }
+
+        internal Boolean TryAdd(Int32 amount)
+        {
+            try
+            {
+                checked
+                {
+                    amount += m_currentBytes;
+                }
+
+                if (amount > m_maxBytes)
+                {
+                    return false;
+                }
+
+                m_currentBytes = amount;
+                return true;
+            }
+            catch (OverflowException)
+            {
+                return false;
+            }
+        }
+
+        internal Boolean TryAdd(String value)
+        {
+            return TryAdd(CalculateSize(value));
+        }
+
+        internal const Int32 MinObjectSize = 24;
+        internal const Int32 StringBaseOverhead = 26;
+        private readonly Int32 m_maxBytes;
+        private readonly ExpressionNode m_node;
+        private Int32 m_currentBytes;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/NamedValue.cs

@@ -0,0 +1,24 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public abstract class NamedValue : ExpressionNode
+    {
+        internal sealed override string ConvertToExpression() => Name;
+
+        protected sealed override Boolean TraceFullyRealized => true;
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return Name;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/NoOperationNamedValue.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    internal sealed class NoOperationNamedValue : NamedValue
+    {
+        protected override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            return null;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/And.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Globalization;
+using System.Linq;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class And : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0})",
+                String.Join(" && ", Parameters.Select(x => x.ConvertToExpression())));
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0})",
+                String.Join(" && ", Parameters.Select(x => x.ConvertToRealizedExpression(context))));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var result = default(EvaluationResult);
+            foreach (var parameter in Parameters)
+            {
+                result = parameter.Evaluate(context);
+                if (result.IsFalsy)
+                {
+                    return result.Value;
+                }
+            }
+
+            return result?.Value;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/Equal.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class Equal : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} == {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} == {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractEqual(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/GreaterThan.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class GreaterThan : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} > {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} > {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractGreaterThan(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/GreaterThanOrEqual.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class GreaterThanOrEqual : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} >= {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} >= {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractGreaterThanOrEqual(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/Index.cs

@@ -0,0 +1,286 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class Index : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => true;
+
+        internal sealed override String ConvertToExpression()
+        {
+            // Verify if we can simplify the expression, we would rather return 
+            // github.sha then github['sha'] so we check if this is a simple case.
+            if (Parameters[1] is Literal literal &&
+                literal.Value is String literalString &&
+                ExpressionUtility.IsLegalKeyword(literalString))
+            {
+                return String.Format(
+                    CultureInfo.InvariantCulture,
+                    "{0}.{1}",
+                    Parameters[0].ConvertToExpression(),
+                    literalString);
+            }
+            else
+            {
+                return String.Format(
+                    CultureInfo.InvariantCulture,
+                    "{0}[{1}]",
+                    Parameters[0].ConvertToExpression(),
+                    Parameters[1].ConvertToExpression());
+            }
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "{0}[{1}]",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            var left = Parameters[0].Evaluate(context);
+
+            // Not a collection
+            if (!left.TryGetCollectionInterface(out Object collection))
+            {
+                resultMemory = null;
+                return Parameters[1] is Wildcard ? new FilteredArray() : null;
+            }
+            // Filtered array
+            else if (collection is FilteredArray filteredArray)
+            {
+                return HandleFilteredArray(context, filteredArray, out resultMemory);
+            }
+            // Object
+            else if (collection is IReadOnlyObject obj)
+            {
+                return HandleObject(context, obj, out resultMemory);
+            }
+            // Array
+            else if (collection is IReadOnlyArray array)
+            {
+                return HandleArray(context, array, out resultMemory);
+            }
+
+            resultMemory = null;
+            return null;
+        }
+
+        private Object HandleFilteredArray(
+            EvaluationContext context,
+            FilteredArray filteredArray,
+            out ResultMemory resultMemory)
+        {
+            var result = new FilteredArray();
+            var counter = new MemoryCounter(this, context.Options.MaxMemory);
+
+            var index = new IndexHelper(context, Parameters[1]);
+
+            foreach (var item in filteredArray)
+            {
+                // Leverage the expression SDK to traverse the object
+                var itemResult = EvaluationResult.CreateIntermediateResult(context, item);
+                if (itemResult.TryGetCollectionInterface(out var nestedCollection))
+                {
+                    // Apply the index to each child object
+                    if (nestedCollection is IReadOnlyObject nestedObject)
+                    {
+                        // Wildcard
+                        if (index.IsWildcard)
+                        {
+                            foreach (var val in nestedObject.Values)
+                            {
+                                result.Add(val);
+                                counter.Add(IntPtr.Size);
+                            }
+                        }
+                        // String
+                        else if (index.HasStringIndex)
+                        {
+                            if (nestedObject.TryGetValue(index.StringIndex, out var nestedObjectValue))
+                            {
+                                result.Add(nestedObjectValue);
+                                counter.Add(IntPtr.Size);
+                            }
+                        }
+                    }
+                    // Apply the index to each child array
+                    else if (nestedCollection is IReadOnlyArray nestedArray)
+                    {
+                        // Wildcard
+                        if (index.IsWildcard)
+                        {
+                            foreach (var val in nestedArray)
+                            {
+                                result.Add(val);
+                                counter.Add(IntPtr.Size);
+                            }
+                        }
+                        // String
+                        else if (index.HasIntegerIndex &&
+                            index.IntegerIndex < nestedArray.Count)
+                        {
+                            result.Add(nestedArray[index.IntegerIndex]);
+                            counter.Add(IntPtr.Size);
+                        }
+                    }
+                }
+            }
+
+            resultMemory = new ResultMemory { Bytes = counter.CurrentBytes };
+            return result;
+        }
+
+        private Object HandleObject(
+            EvaluationContext context,
+            IReadOnlyObject obj,
+            out ResultMemory resultMemory)
+        {
+            var index = new IndexHelper(context, Parameters[1]);
+
+            // Wildcard
+            if (index.IsWildcard)
+            {
+                var filteredArray = new FilteredArray();
+                var counter = new MemoryCounter(this, context.Options.MaxMemory);
+                counter.AddMinObjectSize();
+
+                foreach (var val in obj.Values)
+                {
+                    filteredArray.Add(val);
+                    counter.Add(IntPtr.Size);
+                }
+
+                resultMemory = new ResultMemory { Bytes = counter.CurrentBytes };
+                return filteredArray;
+            }
+            // String
+            else if (index.HasStringIndex &&
+                obj.TryGetValue(index.StringIndex, out var result))
+            {
+                resultMemory = null;
+                return result;
+            }
+
+            resultMemory = null;
+            return null;
+        }
+
+        private Object HandleArray(
+            EvaluationContext context,
+            IReadOnlyArray array,
+            out ResultMemory resultMemory)
+        {
+            var index = new IndexHelper(context, Parameters[1]);
+
+            // Wildcard
+            if (index.IsWildcard)
+            {
+                var filtered = new FilteredArray();
+                var counter = new MemoryCounter(this, context.Options.MaxMemory);
+                counter.AddMinObjectSize();
+
+                foreach (var item in array)
+                {
+                    filtered.Add(item);
+                    counter.Add(IntPtr.Size);
+                }
+
+                resultMemory = new ResultMemory { Bytes = counter.CurrentBytes };
+                return filtered;
+            }
+            // Integer
+            else if (index.HasIntegerIndex && index.IntegerIndex < array.Count)
+            {
+                resultMemory = null;
+                return array[index.IntegerIndex];
+            }
+
+            resultMemory = null;
+            return null;
+        }
+        
+        private class FilteredArray : IReadOnlyArray
+        {
+            public FilteredArray()
+            {
+                m_list = new List<Object>();
+            }
+
+            public void Add(Object o)
+            {
+                m_list.Add(o);
+            }
+
+            public Int32 Count => m_list.Count;
+
+            public Object this[Int32 index] => m_list[index];
+
+            public IEnumerator GetEnumerator() => m_list.GetEnumerator();
+
+            private readonly IList<Object> m_list;
+        }
+
+        private class IndexHelper
+        {
+            public IndexHelper(
+                EvaluationContext context,
+                ExpressionNode parameter)
+            {
+                m_parameter = parameter;
+                m_result = parameter.Evaluate(context);
+
+                m_integerIndex = new Lazy<Int32?>(() =>
+                {
+                    var doubleIndex = m_result.ConvertToNumber();
+                    if (Double.IsNaN(doubleIndex) || doubleIndex < 0d)
+                    {
+                        return null;
+                    }
+
+                    doubleIndex = Math.Floor(doubleIndex);
+                    if (doubleIndex > (Double)Int32.MaxValue)
+                    {
+                        return null;
+                    }
+
+                    return (Int32)doubleIndex;
+                });
+
+                m_stringIndex = new Lazy<String>(() =>
+                {
+                    return m_result.IsPrimitive ? m_result.ConvertToString() : null;
+                });
+            }
+
+            public Boolean HasIntegerIndex => m_integerIndex.Value != null;
+
+            public Boolean HasStringIndex => m_stringIndex.Value != null;
+
+            public Boolean IsWildcard => m_parameter is Wildcard;
+
+            public Int32 IntegerIndex => m_integerIndex.Value ?? default(Int32);
+
+            public String StringIndex => m_stringIndex.Value;
+
+            private readonly ExpressionNode m_parameter;
+            private readonly EvaluationResult m_result;
+            private readonly Lazy<Int32?> m_integerIndex;
+            private readonly Lazy<String> m_stringIndex;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/LessThan.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class LessThan : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} < {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} < {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractLessThan(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/LessThanOrEqual.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class LessThanOrEqual : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} <= {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} <= {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractLessThanOrEqual(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/Not.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class Not : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "!{0}",
+                Parameters[0].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "!{0}",
+                Parameters[0].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var result = Parameters[0].Evaluate(context);
+            return result.IsFalsy;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/NotEqual.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class NotEqual : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} != {1})",
+                Parameters[0].ConvertToExpression(),
+                Parameters[1].ConvertToExpression());
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0} != {1})",
+                Parameters[0].ConvertToRealizedExpression(context),
+                Parameters[1].ConvertToRealizedExpression(context));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var left = Parameters[0].Evaluate(context);
+            var right = Parameters[1].Evaluate(context);
+            return left.AbstractNotEqual(right);
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Operators/Or.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Globalization;
+using System.Linq;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Operators
+{
+    internal sealed class Or : Container
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0})",
+                String.Join(" || ", Parameters.Select(x => x.ConvertToExpression())));
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            // Check if the result was stored
+            if (context.TryGetTraceResult(this, out String result))
+            {
+                return result;
+            }
+
+            return String.Format(
+                CultureInfo.InvariantCulture,
+                "({0})",
+                String.Join(" || ", Parameters.Select(x => x.ConvertToRealizedExpression(context))));
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            var result = default(EvaluationResult);
+            foreach (var parameter in Parameters)
+            {
+                result = parameter.Evaluate(context);
+                if (result.IsTruthy)
+                {
+                    break;
+                }
+            }
+
+            return result?.Value;
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/ResultMemory.cs

@@ -0,0 +1,58 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public class ResultMemory
+    {
+        /// <summary>
+        /// Only set a non-null value when both of the following conditions are met:
+        /// 1) The result is a complex object. In other words, the result is
+        /// not a simple type: string, boolean, number, or null.
+        /// 2) The result is a newly created object.
+        ///
+        /// <para>
+        /// For example, consider a function jsonParse() which takes a string parameter,
+        /// and returns a JToken object. The JToken object is newly created and a rough
+        /// measurement should be returned for the number of bytes it consumes in memory.
+        /// </para>
+        ///
+        /// <para>
+        /// For another example, consider a function which returns a sub-object from a
+        /// complex parameter value. From the perspective of an individual function,
+        /// the size of the complex parameter value is unknown. In this situation, set the
+        /// value to IntPtr.Size.
+        /// </para>
+        ///
+        /// <para>
+        /// When you are unsure, set the value to null. Null indicates the overhead of a
+        /// new pointer should be accounted for.
+        /// </para>
+        /// </summary>
+        public Int32? Bytes { get; set; }
+
+        /// <summary>
+        /// Indicates whether <c ref="Bytes" /> represents the total size of the result.
+        /// True indicates the accounting-overhead of downstream parameters can be discarded.
+        ///
+        /// For <c ref="EvaluationOptions.Converters" />, this value is currently ignored.
+        ///
+        /// <para>
+        /// For example, consider a funciton jsonParse() which takes a string paramter,
+        /// and returns a JToken object. The JToken object is newly created and a rough
+        /// measurement should be returned for the amount of bytes it consumes in memory.
+        /// Set the <c ref="IsTotal" /> to true, since new object contains no references
+        /// to previously allocated memory.
+        /// </para>
+        ///
+        /// <para>
+        /// For another example, consider a function which wraps a complex parameter result.
+        /// <c ref="Bytes" /> should be set to the amount of newly allocated memory.
+        /// However since the object references previously allocated memory, set <c ref="IsTotal" />
+        /// to false.
+        /// </para>
+        /// </summary>
+        public Boolean IsTotal { get; set; }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Sdk/Wildcard.cs

@@ -0,0 +1,32 @@
+using System;
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class Wildcard : ExpressionNode
+    {
+        // Prevent the value from being stored on the evaluation context.
+        // This avoids unneccessarily duplicating the value in memory.
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        internal sealed override String ConvertToExpression()
+        {
+            return ExpressionConstants.Wildcard.ToString();
+        }
+
+        internal sealed override String ConvertToRealizedExpression(EvaluationContext context)
+        {
+            return ExpressionConstants.Wildcard.ToString();
+        }
+
+        protected sealed override Object EvaluateCore(
+            EvaluationContext context,
+            out ResultMemory resultMemory)
+        {
+            resultMemory = null;
+            return ExpressionConstants.Wildcard.ToString();
+        }
+    }
+
+}

src/Sdk/DTExpressions2/Expressions2/Tokens/Associativity.cs

@@ -0,0 +1,9 @@
+namespace GitHub.DistributedTask.Expressions2.Tokens
+{
+    internal enum Associativity
+    {
+        None,
+        LeftToRight,
+        RightToLeft,
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Tokens/LexicalAnalyzer.cs

@@ -0,0 +1,491 @@
+using GitHub.DistributedTask.Expressions2.Sdk;
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+
+namespace GitHub.DistributedTask.Expressions2.Tokens
+{
+    internal sealed class LexicalAnalyzer
+    {
+        public LexicalAnalyzer(String expression)
+        {
+            m_expression = expression;
+        }
+
+        public IEnumerable<Token> UnclosedTokens => m_unclosedTokens;
+
+        public Boolean TryGetNextToken(ref Token token)
+        {
+            // Skip whitespace
+            while (m_index < m_expression.Length && Char.IsWhiteSpace(m_expression[m_index]))
+            {
+                m_index++;
+            }
+
+            // Test end of string
+            if (m_index >= m_expression.Length)
+            {
+                token = null;
+                return false;
+            }
+
+            // Read the first character to determine the type of token.
+            var c = m_expression[m_index];
+            switch (c)
+            {
+                case ExpressionConstants.StartGroup:    // "("
+                    // Function call
+                    if (m_lastToken?.Kind == TokenKind.Function)
+                    {
+                        token = CreateToken(TokenKind.StartParameters, c, m_index++);
+                    }
+                    // Logical grouping
+                    else
+                    {
+                        token = CreateToken(TokenKind.StartGroup, c, m_index++);
+                    }
+                    break;
+                case ExpressionConstants.StartIndex:    // "["
+                    token = CreateToken(TokenKind.StartIndex, c, m_index++);
+                    break;
+                case ExpressionConstants.EndGroup:      // ")"
+                    // Function call
+                    if (m_unclosedTokens.FirstOrDefault()?.Kind == TokenKind.StartParameters) // "(" function call
+                    {
+                        token = CreateToken(TokenKind.EndParameters, c, m_index++);
+                    }
+                    // Logical grouping
+                    else
+                    {
+                        token = CreateToken(TokenKind.EndGroup, c, m_index++);
+                    }
+                    break;
+                case ExpressionConstants.EndIndex:      // "]"
+                    token = CreateToken(TokenKind.EndIndex, c, m_index++);
+                    break;
+                case ExpressionConstants.Separator:     // ","
+                    token = CreateToken(TokenKind.Separator, c, m_index++);
+                    break;
+                case ExpressionConstants.Wildcard:      // "*"
+                    token = CreateToken(TokenKind.Wildcard, c, m_index++);
+                    break;
+                case '\'':
+                    token = ReadStringToken();
+                    break;
+                case '!':   // "!" and "!="
+                case '>':   // ">" and ">="
+                case '<':   // "<" and "<="
+                case '=':   // "=="
+                case '&':   // "&&"
+                case '|':   // "||"
+                    token = ReadOperator();
+                    break;
+                default:
+                    if (c == '.')
+                    {
+                        // Number
+                        if (m_lastToken == null ||
+                            m_lastToken.Kind == TokenKind.Separator ||          // ","
+                            m_lastToken.Kind == TokenKind.StartGroup ||         // "(" logical grouping
+                            m_lastToken.Kind == TokenKind.StartIndex ||         // "["
+                            m_lastToken.Kind == TokenKind.StartParameters ||    // "(" function call
+                            m_lastToken.Kind == TokenKind.LogicalOperator)      // "!", "==", etc
+                        {
+                            token = ReadNumberToken();
+                        }
+                        // "."
+                        else
+                        {
+                            token = CreateToken(TokenKind.Dereference, c, m_index++);
+                        }
+                    }
+                    else if (c == '-' || c == '+' || (c >= '0' && c <= '9'))
+                    {
+                        token = ReadNumberToken();
+                    }
+                    else
+                    {
+                        token = ReadKeywordToken();
+                    }
+
+                    break;
+            }
+
+            m_lastToken = token;
+            return true;
+        }
+
+        private Token ReadNumberToken()
+        {
+            var startIndex = m_index;
+            do
+            {
+                m_index++;
+            }
+            while (m_index < m_expression.Length && (!TestTokenBoundary(m_expression[m_index]) || m_expression[m_index] == '.'));
+
+            var length = m_index - startIndex;
+            var str = m_expression.Substring(startIndex, length);
+            var d = ExpressionUtility.ParseNumber(str);
+
+            if (Double.IsNaN(d))
+            {
+                return CreateToken(TokenKind.Unexpected, str, startIndex);
+            }
+
+            return CreateToken(TokenKind.Number, str, startIndex, d);
+        }
+
+        private Token ReadKeywordToken()
+        {
+            // Read to the end of the keyword.
+            var startIndex = m_index;
+            m_index++; // Skip the first char. It is already known to be the start of the keyword.
+            while (m_index < m_expression.Length && !TestTokenBoundary(m_expression[m_index]))
+            {
+                m_index++;
+            }
+
+            // Test if valid keyword character sequence.
+            var length = m_index - startIndex;
+            var str = m_expression.Substring(startIndex, length);
+            if (ExpressionUtility.IsLegalKeyword(str))
+            {
+                // Test if follows property dereference operator.
+                if (m_lastToken != null && m_lastToken.Kind == TokenKind.Dereference)
+                {
+                    return CreateToken(TokenKind.PropertyName, str, startIndex);
+                }
+
+                // Null
+                if (str.Equals(ExpressionConstants.Null, StringComparison.Ordinal))
+                {
+                    return CreateToken(TokenKind.Null, str, startIndex);
+                }
+                // Boolean
+                else if (str.Equals(ExpressionConstants.True, StringComparison.Ordinal))
+                {
+                    return CreateToken(TokenKind.Boolean, str, startIndex, true);
+                }
+                else if (str.Equals(ExpressionConstants.False, StringComparison.Ordinal))
+                {
+                    return CreateToken(TokenKind.Boolean, str, startIndex, false);
+                }
+                // NaN
+                else if (str.Equals(ExpressionConstants.NaN, StringComparison.Ordinal))
+                {
+                    return CreateToken(TokenKind.Number, str, startIndex, Double.NaN);
+                }
+                // Infinity
+                else if (str.Equals(ExpressionConstants.Infinity, StringComparison.Ordinal))
+                {
+                    return CreateToken(TokenKind.Number, str, startIndex, Double.PositiveInfinity);
+                }
+
+                // Lookahead
+                var tempIndex = m_index;
+                while (tempIndex < m_expression.Length && Char.IsWhiteSpace(m_expression[tempIndex]))
+                {
+                    tempIndex++;
+                }
+
+                // Function
+                if (tempIndex < m_expression.Length && m_expression[tempIndex] == ExpressionConstants.StartGroup)   // "("
+                {
+                    return CreateToken(TokenKind.Function, str, startIndex);
+                }
+                // Named-value
+                else
+                {
+                    return CreateToken(TokenKind.NamedValue, str, startIndex);
+                }
+            }
+            else
+            {
+                // Invalid keyword
+                return CreateToken(TokenKind.Unexpected, str, startIndex);
+            }
+        }
+
+        private Token ReadStringToken()
+        {
+            var startIndex = m_index;
+            var c = default(Char);
+            var closed = false;
+            var str = new StringBuilder();
+            m_index++; // Skip the leading single-quote.
+            while (m_index < m_expression.Length)
+            {
+                c = m_expression[m_index++];
+                if (c == '\'')
+                {
+                    // End of string.
+                    if (m_index >= m_expression.Length || m_expression[m_index] != '\'')
+                    {
+                        closed = true;
+                        break;
+                    }
+
+                    // Escaped single quote.
+                    m_index++;
+                }
+
+                str.Append(c);
+            }
+
+            var length = m_index - startIndex;
+            var rawValue = m_expression.Substring(startIndex, length);
+            if (closed)
+            {
+                return CreateToken(TokenKind.String, rawValue, startIndex, str.ToString());
+            }
+
+            return CreateToken(TokenKind.Unexpected, rawValue, startIndex);
+        }
+
+        private Token ReadOperator()
+        {
+            var startIndex = m_index;
+            var raw = default(String);
+            m_index++;
+
+            // Check for a two-character operator
+            if (m_index < m_expression.Length)
+            {
+                m_index++;
+                raw = m_expression.Substring(startIndex, 2);
+                switch (raw)
+                {
+                    case ExpressionConstants.NotEqual:
+                    case ExpressionConstants.GreaterThanOrEqual:
+                    case ExpressionConstants.LessThanOrEqual:
+                    case ExpressionConstants.Equal:
+                    case ExpressionConstants.And:
+                    case ExpressionConstants.Or:
+                        return CreateToken(TokenKind.LogicalOperator, raw, startIndex);
+                }
+
+                // Backup
+                m_index--;
+            }
+
+            // Check for one-character operator
+            raw = m_expression.Substring(startIndex, 1);
+            switch (raw)
+            {
+                case ExpressionConstants.Not:
+                case ExpressionConstants.GreaterThan:
+                case ExpressionConstants.LessThan:
+                    return CreateToken(TokenKind.LogicalOperator, raw, startIndex);
+            }
+
+            // Unexpected
+            while (m_index < m_expression.Length && !TestTokenBoundary(m_expression[m_index]))
+            {
+                m_index++;
+            }
+
+            var length = m_index - startIndex;
+            raw = m_expression.Substring(startIndex, length);
+            return CreateToken(TokenKind.Unexpected, raw, startIndex);
+        }
+
+        private static Boolean TestTokenBoundary(Char c)
+        {
+            switch (c)
+            {
+                case ExpressionConstants.StartGroup:    // "("
+                case ExpressionConstants.StartIndex:    // "["
+                case ExpressionConstants.EndGroup:      // ")"
+                case ExpressionConstants.EndIndex:      // "]"
+                case ExpressionConstants.Separator:     // ","
+                case ExpressionConstants.Dereference:   // "."
+                case '!': // "!" and "!="
+                case '>': // ">" and ">="
+                case '<': // "<" and "<="
+                case '=': // "=="
+                case '&': // "&&"
+                case '|': // "||"
+                    return true;
+                default:
+                    return char.IsWhiteSpace(c);
+            }
+        }
+
+        private Token CreateToken(
+            TokenKind kind,
+            Char rawValue,
+            Int32 index,
+            Object parsedValue = null)
+        {
+            return CreateToken(kind, rawValue.ToString(), index, parsedValue);
+        }
+
+        private Token CreateToken(
+            TokenKind kind,
+            String rawValue,
+            Int32 index,
+            Object parsedValue = null)
+        {
+            // Check whether the current token is legal based on the last token
+            var legal = false;
+            switch (kind)
+            {
+                case TokenKind.StartGroup:      // "(" logical grouping
+                    // Is first or follows "," or "(" or "[" or a logical operator
+                    legal = CheckLastToken(null, TokenKind.Separator, TokenKind.StartGroup, TokenKind.StartParameters, TokenKind.StartIndex, TokenKind.LogicalOperator);
+                    break;
+                case TokenKind.StartIndex:      // "["
+                    // Follows ")", "]", "*", a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.StartParameters: // "(" function call
+                    // Follows a function
+                    legal = CheckLastToken(TokenKind.Function);
+                    break;
+                case TokenKind.EndGroup:        // ")" logical grouping
+                    // Follows ")", "]", "*", a literal, a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.Null, TokenKind.Boolean, TokenKind.Number, TokenKind.String, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.EndIndex:        // "]"
+                    // Follows ")", "]", "*", a literal, a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.Null, TokenKind.Boolean, TokenKind.Number, TokenKind.String, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.EndParameters:   // ")" function call
+                    // Follows "(" function call, ")", "]", "*", a literal, a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.StartParameters, TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.Null, TokenKind.Boolean, TokenKind.Number, TokenKind.String, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.Separator:       // ","
+                    // Follows ")", "]", "*", a literal, a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.Null, TokenKind.Boolean, TokenKind.Number, TokenKind.String, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.Dereference:     // "."
+                    // Follows ")", "]", "*", a property name, or a named-value
+                    legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.PropertyName, TokenKind.NamedValue);
+                    break;
+                case TokenKind.Wildcard:        // "*"
+                    // Follows "[" or "."
+                    legal = CheckLastToken(TokenKind.StartIndex, TokenKind.Dereference);
+                    break;
+                case TokenKind.LogicalOperator: // "!", "==", etc
+                    switch (rawValue)
+                    {
+                        case ExpressionConstants.Not:
+                            // Is first or follows "," or "(" or "[" or a logical operator
+                            legal = CheckLastToken(null, TokenKind.Separator, TokenKind.StartGroup, TokenKind.StartParameters, TokenKind.StartIndex, TokenKind.LogicalOperator);
+                            break;
+                        default:
+                            // Follows ")", "]", "*", a literal, a property name, or a named-value
+                            legal = CheckLastToken(TokenKind.EndGroup, TokenKind.EndParameters, TokenKind.EndIndex, TokenKind.Wildcard, TokenKind.Null, TokenKind.Boolean, TokenKind.Number, TokenKind.String, TokenKind.PropertyName, TokenKind.NamedValue);
+                            break;
+                    }
+                    break;
+                case TokenKind.Null:
+                case TokenKind.Boolean:
+                case TokenKind.Number:
+                case TokenKind.String:
+                    // Is first or follows "," or "[" or "(" or a logical operator (e.g. "!" or "==" etc)
+                    legal = CheckLastToken(null, TokenKind.Separator, TokenKind.StartIndex, TokenKind.StartGroup, TokenKind.StartParameters, TokenKind.LogicalOperator);
+                    break;
+                case TokenKind.PropertyName:
+                    // Follows "."
+                    legal = CheckLastToken(TokenKind.Dereference);
+                    break;
+                case TokenKind.Function:
+                    // Is first or follows "," or "[" or "(" or a logical operator (e.g. "!" or "==" etc)
+                    legal = CheckLastToken(null, TokenKind.Separator, TokenKind.StartIndex, TokenKind.StartGroup, TokenKind.StartParameters, TokenKind.LogicalOperator);
+                    break;
+                case TokenKind.NamedValue:
+                    // Is first or follows "," or "[" or "(" or a logical operator (e.g. "!" or "==" etc)
+                    legal = CheckLastToken(null, TokenKind.Separator, TokenKind.StartIndex, TokenKind.StartGroup, TokenKind.StartParameters, TokenKind.LogicalOperator);
+                    break;
+            }
+
+            // Illegal
+            if (!legal)
+            {
+                return new Token(TokenKind.Unexpected, rawValue, index);
+            }
+
+            // Legal so far
+            var token = new Token(kind, rawValue, index, parsedValue);
+
+            switch (kind)
+            {
+                case TokenKind.StartGroup:      // "(" logical grouping
+                case TokenKind.StartIndex:      // "["
+                case TokenKind.StartParameters: // "(" function call
+                    // Track start token
+                    m_unclosedTokens.Push(token);
+                    break;
+
+                case TokenKind.EndGroup:        // ")" logical grouping
+                    // Check inside logical grouping
+                    if (m_unclosedTokens.FirstOrDefault()?.Kind != TokenKind.StartGroup)
+                    {
+                        return new Token(TokenKind.Unexpected, rawValue, index);
+                    }
+
+                    // Pop start token
+                    m_unclosedTokens.Pop();
+                    break;
+
+                case TokenKind.EndIndex:        // "]"
+                    // Check inside indexer
+                    if (m_unclosedTokens.FirstOrDefault()?.Kind != TokenKind.StartIndex)
+                    {
+                        return new Token(TokenKind.Unexpected, rawValue, index);
+                    }
+
+                    // Pop start token
+                    m_unclosedTokens.Pop();
+                    break;
+
+                case TokenKind.EndParameters:   // ")" function call
+                    // Check inside function call
+                    if (m_unclosedTokens.FirstOrDefault()?.Kind != TokenKind.StartParameters)
+                    {
+                        return new Token(TokenKind.Unexpected, rawValue, index);
+                    }
+
+                    // Pop start token
+                    m_unclosedTokens.Pop();
+                    break;
+
+                case TokenKind.Separator:       // ","
+                    // Check inside function call
+                    if (m_unclosedTokens.FirstOrDefault()?.Kind != TokenKind.StartParameters)
+                    {
+                        return new Token(TokenKind.Unexpected, rawValue, index);
+                    }
+                    break;
+            }
+
+            return token;
+        }
+
+        /// <summary>
+        /// Checks whether the last token kind is in the array of allowed kinds.
+        /// </summary>
+        private Boolean CheckLastToken(params TokenKind?[] allowed)
+        {
+            var lastKind = m_lastToken?.Kind;
+            foreach (var kind in allowed)
+            {
+                if (kind == lastKind)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private readonly String m_expression; // Raw expression string
+        private readonly Stack<Token> m_unclosedTokens = new Stack<Token>(); // Unclosed start tokens
+        private Int32 m_index; // Index of raw expression string
+        private Token m_lastToken;
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Tokens/Token.cs

@@ -0,0 +1,209 @@
+using System;
+using GitHub.DistributedTask.Expressions2.Sdk;
+using GitHub.DistributedTask.Expressions2.Sdk.Operators;
+
+namespace GitHub.DistributedTask.Expressions2.Tokens
+{
+    internal sealed class Token
+    {
+        public Token(
+            TokenKind kind,
+            String rawValue,
+            Int32 index,
+            Object parsedValue = null)
+        {
+            Kind = kind;
+            RawValue = rawValue;
+            Index = index;
+            ParsedValue = parsedValue;
+        }
+
+        public TokenKind Kind { get; }
+
+        public String RawValue { get; }
+
+        public Int32 Index { get; }
+
+        public Object ParsedValue { get; }
+
+        public Associativity Associativity
+        {
+            get
+            {
+                switch (Kind)
+                {
+                    case TokenKind.StartGroup:
+                        return Associativity.None;
+                    case TokenKind.LogicalOperator:
+                        switch (RawValue)
+                        {
+                            case ExpressionConstants.Not:   // "!"
+                                return Associativity.RightToLeft;
+                        }
+                        break;
+                }
+
+                return IsOperator ? Associativity.LeftToRight : Associativity.None;
+            }
+        }
+
+        public Boolean IsOperator
+        {
+            get
+            {
+                switch (Kind)
+                {
+                    case TokenKind.StartGroup:      // "(" logical grouping
+                    case TokenKind.StartIndex:      // "["
+                    case TokenKind.StartParameters: // "(" function call
+                    case TokenKind.EndGroup:        // ")" logical grouping
+                    case TokenKind.EndIndex:        // "]"
+                    case TokenKind.EndParameters:   // ")" function call
+                    case TokenKind.Separator:       // ","
+                    case TokenKind.Dereference:     // "."
+                    case TokenKind.LogicalOperator: // "!", "==", etc
+                        return true;
+                    default:
+                        return false;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Operator precedence. The value is only meaningful for operator tokens.
+        /// </summary>
+        public Int32 Precedence
+        {
+            get
+            {
+                switch (Kind)
+                {
+                    case TokenKind.StartGroup:      // "(" logical grouping
+                        return 20;
+                    case TokenKind.StartIndex:      // "["
+                    case TokenKind.StartParameters: // "(" function call
+                    case TokenKind.Dereference:     // "."
+                        return 19;
+                    case TokenKind.LogicalOperator:
+                        switch (RawValue)
+                        {
+                            case ExpressionConstants.Not:               // "!"
+                                return 16;
+                            case ExpressionConstants.GreaterThan:       // ">"
+                            case ExpressionConstants.GreaterThanOrEqual:// ">="
+                            case ExpressionConstants.LessThan:          // "<"
+                            case ExpressionConstants.LessThanOrEqual:   // "<="
+                                return 11;
+                            case ExpressionConstants.Equal:             // "=="
+                            case ExpressionConstants.NotEqual:          // "!="
+                                return 10;
+                            case ExpressionConstants.And:               // "&&"
+                                return 6;
+                            case ExpressionConstants.Or:                // "||"
+                                return 5;
+                        }
+                        break;
+                    case TokenKind.EndGroup:        // ")" logical grouping
+                    case TokenKind.EndIndex:        // "]"
+                    case TokenKind.EndParameters:   // ")" function call
+                    case TokenKind.Separator:       // ","
+                        return 1;
+                }
+
+                return 0;
+            }
+        }
+
+        /// <summary>
+        /// Expected number of operands. The value is only meaningful for standalone unary operators and binary operators.
+        /// </summary>
+        public Int32 OperandCount
+        {
+            get
+            {
+                switch (Kind)
+                {
+                    case TokenKind.StartIndex:      // "["
+                    case TokenKind.Dereference:     // "."
+                        return 2;
+                    case TokenKind.LogicalOperator:
+                        switch (RawValue)
+                        {
+                            case ExpressionConstants.Not:               // "!"
+                                return 1;
+                            case ExpressionConstants.GreaterThan:       // ">"
+                            case ExpressionConstants.GreaterThanOrEqual:// ">="
+                            case ExpressionConstants.LessThan:          // "<"
+                            case ExpressionConstants.LessThanOrEqual:   // "<="
+                            case ExpressionConstants.Equal:             // "=="
+                            case ExpressionConstants.NotEqual:          // "!="
+                            case ExpressionConstants.And:               // "&&"
+                            case ExpressionConstants.Or:                // "|"
+                                return 2;
+                        }
+                        break;
+                }
+
+                return 0;
+            }
+        }
+
+        public ExpressionNode ToNode()
+        {
+            switch (Kind)
+            {
+                case TokenKind.StartIndex:          // "["
+                case TokenKind.Dereference:         // "."
+                    return new Index();
+
+                case TokenKind.LogicalOperator:
+                    switch (RawValue)
+                    {
+                        case ExpressionConstants.Not:               // "!"
+                            return new Not();
+
+                        case ExpressionConstants.NotEqual:          // "!="
+                            return new NotEqual();
+
+                        case ExpressionConstants.GreaterThan:       // ">"
+                            return new GreaterThan();
+
+                        case ExpressionConstants.GreaterThanOrEqual:// ">="
+                            return new GreaterThanOrEqual();
+
+                        case ExpressionConstants.LessThan:          // "<"
+                            return new LessThan();
+
+                        case ExpressionConstants.LessThanOrEqual:   // "<="
+                            return new LessThanOrEqual();
+
+                        case ExpressionConstants.Equal:             // "=="
+                            return new Equal();
+
+                        case ExpressionConstants.And:               // "&&"
+                            return new And();
+
+                        case ExpressionConstants.Or:                // "||"
+                            return new Or();
+
+                        default:
+                            throw new NotSupportedException($"Unexpected logical operator '{RawValue}' when creating node");
+                    }
+
+                case TokenKind.Null:
+                case TokenKind.Boolean:
+                case TokenKind.Number:
+                case TokenKind.String:
+                    return new Literal(ParsedValue);
+
+                case TokenKind.PropertyName:
+                    return new Literal(RawValue);
+
+                case TokenKind.Wildcard:            // "*"
+                    return new Wildcard();
+            }
+
+            throw new NotSupportedException($"Unexpected kind '{Kind}' when creating node");
+        }
+    }
+}

src/Sdk/DTExpressions2/Expressions2/Tokens/TokenKind.cs

@@ -0,0 +1,28 @@
+namespace GitHub.DistributedTask.Expressions2.Tokens
+{
+    internal enum TokenKind
+    {
+        // Punctuation
+        StartGroup,         // "(" logical grouping
+        StartIndex,         // "["
+        StartParameters,    // "(" function call
+        EndGroup,           // ")" logical grouping
+        EndIndex,           // "]"
+        EndParameters,      // ")" function call
+        Separator,          // ","
+        Dereference,        // "."
+        Wildcard,           // "*"
+        LogicalOperator,    // "!", "==", etc
+
+        // Values
+        Null,
+        Boolean,
+        Number,
+        String,
+        PropertyName,
+        Function,
+        NamedValue,
+
+        Unexpected,
+    }
+}

src/Sdk/DTExpressions2/Expressions2/ValueKind.cs

@@ -0,0 +1,15 @@
+using System.ComponentModel;
+
+namespace GitHub.DistributedTask.Expressions2
+{
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public enum ValueKind
+    {
+        Array,
+        Boolean,
+        Null,
+        Number,
+        Object,
+        String,
+    }
+}