Fix certain cases where implicit conversions aren't correctly detected when parsing comparison operators (#285)

* Add failing test for one-way implicit type conversions

* Search both types for implicit conversion methods, not just base type

* Respect implicit conversions for non-value types

* Make comparisons in HasImplicitConversion more explicit

Author: alexweav

src/System.Linq.Dynamic.Core/Parser/ExpressionParser.cs

@@ -436,11 +436,11 @@ Expression ParseComparisonOperator()
                     // If left or right is NullLiteral, just continue. Else check if the types differ.
                     if (!(Constants.IsNull(left) || Constants.IsNull(right)) && left.Type != right.Type)
                     {
-                        if (left.Type.IsAssignableFrom(right.Type))
+                        if (left.Type.IsAssignableFrom(right.Type) || HasImplicitConversion(right.Type, left.Type))
                         {
                             right = Expression.Convert(right, left.Type);
                         }
-                        else if (right.Type.IsAssignableFrom(left.Type))
+                        else if (right.Type.IsAssignableFrom(left.Type) || HasImplicitConversion(left.Type, right.Type))
                         {
                             left = Expression.Convert(left, right.Type);
                         }
@@ -549,7 +549,16 @@ Expression ParseComparisonOperator()
 
         private bool HasImplicitConversion(Type baseType, Type targetType)
         {
-            return baseType.GetMethods(BindingFlags.Public | BindingFlags.Static)
+            var baseTypeHasConversion = baseType.GetMethods(BindingFlags.Public | BindingFlags.Static)
+                .Where(mi => mi.Name == "op_Implicit" && mi.ReturnType == targetType)
+                .Any(mi => mi.GetParameters().FirstOrDefault()?.ParameterType == baseType);
+
+            if (baseTypeHasConversion)
+            {
+                return true;
+            }
+
+            return targetType.GetMethods(BindingFlags.Public | BindingFlags.Static)
                 .Where(mi => mi.Name == "op_Implicit" && mi.ReturnType == targetType)
                 .Any(mi => mi.GetParameters().FirstOrDefault()?.ParameterType == baseType);
         }

test/System.Linq.Dynamic.Core.Tests/DynamicExpressionParserTests.cs

@@ -1,10 +1,10 @@
-using NFluent;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Linq.Dynamic.Core.CustomTypeProviders;
 using System.Linq.Dynamic.Core.Exceptions;
 using System.Linq.Dynamic.Core.Tests.Helpers.Models;
 using System.Linq.Expressions;
 using System.Reflection;
+using NFluent;
 using Xunit;
 using User = System.Linq.Dynamic.Core.Tests.Helpers.Models.User;
 
@@ -64,6 +64,36 @@ public override string ToString()
             }
         }
 
+        public class CustomClassWithOneWayImplicitConversion
+        {
+            public CustomClassWithOneWayImplicitConversion(string origin)
+            {
+                Origin = origin;
+            }
+
+            public string Origin { get; }
+
+            public static implicit operator CustomClassWithOneWayImplicitConversion(string origin)
+            {
+                return new CustomClassWithOneWayImplicitConversion(origin);
+            }
+
+            public override string ToString()
+            {
+                return Origin;
+            }
+        }
+
+        public class TestImplicitConversionContainer
+        {
+            public TestImplicitConversionContainer(CustomClassWithOneWayImplicitConversion oneWay)
+            {
+                OneWay = oneWay;
+            }
+
+            public CustomClassWithOneWayImplicitConversion OneWay { get; }
+        }
+
         public class TextHolder
         {
             public TextHolder(string name, CustomTextClass note)
@@ -757,6 +787,21 @@ public void DynamicExpressionParser_ParseLambda_With_Concat_CustomType_String()
             Assert.Equal("note1 (name1)", result);
         }
 
+        [Fact]
+        public void DynamicExpressionParser_ParseLambda_With_One_Way_Implicit_Conversions()
+        {
+            // Arrange
+            var testValue = "test";
+            var container = new TestImplicitConversionContainer(testValue);
+            var expressionText = $"OneWay == \"{testValue}\"";
+
+            // Act
+            var lambda = DynamicExpressionParser.ParseLambda<TestImplicitConversionContainer, bool>(ParsingConfig.Default, false, expressionText);
+
+            // Assert
+            Assert.NotNull(lambda);
+        }
+
         [Fact]
         public void DynamicExpressionParser_ParseLambda_Operator_Less_Greater_With_Guids()
         {