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src/Sdk/DTObjectTemplating/ObjectTemplating/Tokens/ExpressionToken.cs

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+using System;
+using System.ComponentModel;
+using System.Linq;
+using System.Runtime.Serialization;
+using GitHub.DistributedTask.Expressions2;
+using GitHub.DistributedTask.Expressions2.Sdk;
+using GitHub.Services.WebApi.Internal;
+
+namespace GitHub.DistributedTask.ObjectTemplating.Tokens
+{
+    /// <summary>
+    /// Base class for all template expression tokens
+    /// </summary>
+    [DataContract]
+    [ClientIgnore]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public abstract class ExpressionToken : ScalarToken
+    {
+        internal ExpressionToken(
+            Int32 templateType,
+            Int32? fileId,
+            Int32? line,
+            Int32? column,
+            String directive)
+            : base(templateType, fileId, line, column)
+        {
+            Directive = directive;
+        }
+
+        [DataMember(Name = "directive", EmitDefaultValue = false)]
+        internal String Directive { get; }
+
+        internal static Boolean IsValidExpression(
+            String expression,
+            String[] allowedContext,
+            out Exception ex)
+        {
+            // Create dummy allowed contexts
+            INamedValueInfo[] namedValues = null;
+            if (allowedContext?.Length > 0)
+            {
+                namedValues = allowedContext.Select(x => new NamedValueInfo<ContextValueNode>(x)).ToArray();
+            }
+
+            // Parse
+            Boolean result;
+            ExpressionNode root = null;
+            try
+            {
+                root = new ExpressionParser().CreateTree(expression, null, namedValues, null) as ExpressionNode;
+
+                result = true;
+                ex = null;
+            }
+            catch (Exception exception)
+            {
+                result = false;
+                ex = exception;
+            }
+
+            return result;
+        }
+    }
+}