Support for translating constant lambda expressions

Author: SteveWilkes

ReadableExpressions.UnitTests/WhenTranslatingConstants.cs

@@ -4,6 +4,7 @@
     using System.Collections.Generic;
     using System.Data.Common;
     using System.IO;
+    using System.Linq;
     using System.Linq.Expressions;
     using Microsoft.VisualStudio.TestTools.UnitTesting;
 
@@ -387,6 +388,21 @@ public void ShouldTranslateAnObjectConstant()
 
             Assert.AreEqual("123", translated);
         }
+
+        [TestMethod]
+        public void ShouldTranslateALambdaConstant()
+        {
+            Expression<Func<int, int>> lambda =
+                num => Enumerable.Range(num, 10).Select(i => new { Index = i }).Sum(d => d.Index);
+
+            var lambdaConstant = Expression.Constant(lambda, lambda.GetType());
+
+            var translated = lambdaConstant.ToReadableString();
+
+            const string EXPECTED = @"num => Enumerable.Range(num, 10).Select(i => new { Index = i }).Sum(d => d.Index)";
+
+            Assert.AreEqual(EXPECTED.TrimStart(), translated);
+        }
     }
 
     internal class GenericOne<T> { }

ReadableExpressions/Translators/ConstantExpressionTranslator.cs

@@ -31,15 +31,18 @@ public override string Translate(Expression expression, TranslationContext conte
                 return constant.Type.GetFriendlyName() + "." + constant.Value;
             }
 
-            if (TryTranslateByTypeCode(constant, out var translation))
+            if (TryTranslateByTypeCode(constant, context, out var translation))
             {
                 return translation;
             }
 
             return constant.Value.ToString();
         }
 
-        private static bool TryTranslateByTypeCode(ConstantExpression constant, out string translation)
+        private static bool TryTranslateByTypeCode(
+            ConstantExpression constant,
+            TranslationContext context,
+            out string translation)
         {
             switch ((Nullable.GetUnderlyingType(constant.Type) ?? constant.Type).GetTypeCode())
             {
@@ -78,6 +81,7 @@ private static bool TryTranslateByTypeCode(ConstantExpression constant, out stri
 
                 case NetStandardTypeCode.Object:
                     if (IsType(constant, out translation) ||
+                        IsLambda(constant, context, out translation) ||
                         IsFunc(constant, out translation) ||
                         IsDefault<Guid>(constant, out translation) ||
                         IsTimeSpan(constant, out translation))
@@ -236,6 +240,21 @@ private static bool IsType(ConstantExpression constant, out string translation)
             return false;
         }
 
+        private static bool IsLambda(
+            ConstantExpression constant,
+            TranslationContext context,
+            out string translation)
+        {
+            if (constant.Value is LambdaExpression lambda)
+            {
+                translation = context.TranslateAsCodeBlock(lambda);
+                return true;
+            }
+
+            translation = null;
+            return false;
+        }
+
         private static readonly Regex _funcMatcher = new Regex(@"^System\.(?:Func|Action)`\d+\[.+\]$");
 
         private static bool IsFunc(ConstantExpression constant, out string translation)