ToCSharpCode: add comments to invalid block expressions

Author: exyi

src/FastExpressionCompiler/FastExpressionCompiler.cs

@@ -6978,7 +6978,6 @@ internal static StringBuilder ToCSharpString(this Expression e, StringBuilder sb
                         var x = (ConditionalExpression)e;
                         if (e.Type == typeof(void)) // otherwise output as ternary expression
                         {
-                            sb.NewLine(lineIdent, identSpaces);
                             sb.Append("if (");
                             x.Test.ToCSharpString(sb, EnclosedIn.IfTest, lineIdent, stripNamespace, printType, identSpaces, notRecognizedToCode);
                             sb.Append(')');
@@ -7016,9 +7015,15 @@ internal static StringBuilder ToCSharpString(this Expression e, StringBuilder sb
                             return BlockToCSharpString((BlockExpression)e, sb, lineIdent, stripNamespace, printType, identSpaces, notRecognizedToCode: notRecognizedToCode);
                         else
                         {
-                            sb.Append('{');
+                            var isExpressionBlock = e.Type != typeof(void) && sb.Length > 0;
+                            sb.Append("{");
+                            if (isExpressionBlock)
+                                sb.Append(" /* BlockExpression cannot be written in C#. Please rewrite the code inside these braces as a C# expression, or reorganize the parent expression as a block. */");
                             BlockToCSharpString((BlockExpression)e, sb, lineIdent + identSpaces, stripNamespace, printType, identSpaces, notRecognizedToCode: notRecognizedToCode);
-                            return sb.AddSemicolonIfFits().NewLineIdent(lineIdent).Append('}');
+
+                        if (isExpressionBlock)
+                                sb.Append("/* <- block result */");
+                            return sb.NewLineIdent(lineIdent).Append('}');
                         }
                     }
                 case ExpressionType.Loop:
@@ -7170,12 +7175,11 @@ void PrintPart(Expression part)
                     }
                 case ExpressionType.Default:
                     {
-                        return e.Type == typeof(void) ? sb : // `default(void)` does not make sense in the C#
-                            !e.Type.IsValueType && !e.Type.IsGenericParameter ? sb.Append("null") :
-                            sb.Append("default(").Append(e.Type.ToCode(stripNamespace, printType)).Append(')');
+                        return e.Type == typeof(void) ? sb // `default(void)` does not make sense in the C#
+                            : sb.Append("default(").Append(e.Type.ToCode(stripNamespace, printType)).Append(')');
                     }
                 case ExpressionType.TypeIs:
-                case ExpressionType.TypeEqual: // TODO: type equal
+                case ExpressionType.TypeEqual:
                     {
                         var x = (TypeBinaryExpression)e;
                         sb.Append('(');
@@ -8127,4 +8131,3 @@ public RequiresUnreferencedCodeAttribute(string message)
     }
 }
 #endif
-}

test/FastExpressionCompiler.UnitTests/ToCSharpStringTests.cs

@@ -32,15 +32,62 @@ public void Outputs_closed_generic_type_constant_correctly()
         }
 
         [Test]
-        public void Outputs_default_reference_type_is_just_null()
+        public void Lambda_with_block_body()
         {
-            var cs = Default(typeof(string)).ToCSharpString();
-            Assert.AreEqual("null;", cs);
+            var variable = Parameter(typeof(int), "variable");
+            var e = Lambda<Func<int>>(Block(new [] { variable }, Assign(variable,  Constant(1)), Add(variable, Constant(2))));
+
+            var cs = e.ToCSharpString();
 
-            cs = Default(typeof(System.Collections.Generic.List<string>)).ToCSharpString();
-            Assert.AreEqual("null;", cs);
+            Assert.AreEqual("""
+                (Func<int>)(() =>
+                {
+                    int variable = default;
+                    variable = 1;
+                    return (variable + 2);
+                });
+                """, cs);
         }
 
+        [Test]
+        public void Nested_blocks()
+        {
+            var v1 = Parameter(typeof(int), "v1");
+            var v2 = Parameter(typeof(int), "v2");
+            var cs = Block(new [] { v1 },
+                Assign(v1, Constant(2)),
+                Block(new [] { v2 },
+                    Assign(v2, Constant(3)),
+                    AddAssign(v1, v2),
+                    IfThen(
+                        Equal(v1, Constant(5)),
+                        Block(
+                            Assign(v2, Constant(7)),
+                            AddAssign(v1, v2)
+                        )
+                    )
+                ),
+                v1
+            ).ToCSharpString();
+            Console.WriteLine(cs);
+            Assert.AreEqual("""
+                    {
+                        int v1 = default;
+                        v1 = 2;
+                        int v2 = default;
+                        v2 = 3;
+                        v1 += v2;
+                        if (v1 == 5)
+                        {
+                            v2 = 7;
+                            v1 += v2;
+                        }
+                        v1;
+                    };
+                    """, cs);
+        }
+
+
         [Test]
         public void Somehow_handles_block_in_expression()
         {
@@ -51,10 +98,10 @@ public void Somehow_handles_block_in_expression()
                 variable
             )).ToCSharpString();
             Assert.AreEqual("""
-                    (1 + {
-                        int variable;
+                    (1 + { /* BlockExpression cannot be written in C#. Please rewrite the code inside these braces as a C# expression, or reorganize the parent expression as a block. */
+                        int variable = default;
                         variable = 2;
-                        variable;
+                        variable;/* <- block result */
                     });
                     """, cs);
         }