Fix "in" for nullable Enums (#932)

Author: StefH

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

@@ -374,8 +374,8 @@ private Expression ParseIn()
                     // we need to parse unary expressions because otherwise 'in' clause will fail in use cases like 'in (-1, -1)' or 'in (!true)'
                     Expression right = ParseUnary();
 
-                    // if the identifier is an Enum, try to convert the right-side also to an Enum.
-                    if (left.Type.GetTypeInfo().IsEnum)
+                    // if the identifier is an Enum (or nullable Enum), try to convert the right-side also to an Enum.
+                    if (TypeHelper.GetNonNullableType(left.Type).GetTypeInfo().IsEnum)
                     {
                         if (right is ConstantExpression constantExprRight)
                         {

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

@@ -1269,10 +1269,6 @@ public void ExpressionTests_HexadecimalInteger()
         [Fact]
         public void ExpressionTests_In_Enum()
         {
-            var config = new ParsingConfig();
-#if NETSTANDARD
-            // config.CustomTypeProvider = new NetStandardCustomTypeProvider();
-#endif
             // Arrange
             var model1 = new ModelWithEnum { TestEnum = TestEnum.Var1 };
             var model2 = new ModelWithEnum { TestEnum = TestEnum.Var2 };
@@ -1281,8 +1277,28 @@ public void ExpressionTests_In_Enum()
 
             // Act
             var expected = qry.Where(x => new[] { TestEnum.Var1, TestEnum.Var2 }.Contains(x.TestEnum)).ToArray();
-            var result1 = qry.Where(config, "it.TestEnum in (\"Var1\", \"Var2\")").ToArray();
-            var result2 = qry.Where(config, "it.TestEnum in (0, 1)").ToArray();
+            var result1 = qry.Where("it.TestEnum in (\"Var1\", \"Var2\")").ToArray();
+            var result2 = qry.Where("it.TestEnum in (0, 1)").ToArray();
+
+            // Assert
+            Check.That(result1).ContainsExactly(expected);
+            Check.That(result2).ContainsExactly(expected);
+        }
+
+        [Fact]
+        public void ExpressionTests_In_EnumIsNullable()
+        {
+            // Arrange
+            var model1 = new ModelWithEnum { TestEnumNullable = TestEnum.Var1 };
+            var model2 = new ModelWithEnum { TestEnumNullable = TestEnum.Var2 };
+            var model3 = new ModelWithEnum { TestEnumNullable = TestEnum.Var3 };
+            var model4 = new ModelWithEnum { TestEnumNullable = null };
+            var qry = new[] { model1, model2, model3, model4 }.AsQueryable();
+
+            // Act
+            var expected = new[] { model1, model2 };
+            var result1 = qry.Where("it.TestEnumNullable in (\"Var1\", \"Var2\")").ToArray();
+            var result2 = qry.Where("it.TestEnumNullable in (0, 1)").ToArray();
 
             // Assert
             Check.That(result1).ContainsExactly(expected);

test/System.Linq.Dynamic.Core.Tests/Helpers/Models/ModelWithEnum.cs

@@ -1,10 +1,10 @@
-
-namespace System.Linq.Dynamic.Core.Tests.Helpers.Models
+namespace System.Linq.Dynamic.Core.Tests.Helpers.Models;
+
+public class ModelWithEnum
 {
-    public class ModelWithEnum
-    {
-        public string Name { get; set; }
+    public string Name { get; set; } = null!;
+
+    public TestEnum TestEnum { get; set; }
 
-        public TestEnum TestEnum { get; set; }
-    }
+    public TestEnum? TestEnumNullable { get; set; }
 }