Set EnableConstantParameterization as false will make enum filter fail.

 #202

Author: xuzhg

OData/src/System.Web.OData/OData/Query/Expressions/FilterBinder.cs

@@ -1416,12 +1416,6 @@ private static Expression ConvertToEnumUnderlyingType(Expression expression, Typ
                             parameterizedConstantValue, enumUnderlyingType, CultureInfo.InvariantCulture));
                 }
             }
-            else if (expression.NodeType == ExpressionType.Constant)
-            {
-                // only null constants are not parameterized.
-                Contract.Assert((expression as ConstantExpression).Value == null);
-                return expression;
-            }
             else if (expression.Type == enumType)
             {
                 return Expression.Convert(expression, enumUnderlyingType);
@@ -1430,6 +1424,10 @@ private static Expression ConvertToEnumUnderlyingType(Expression expression, Typ
             {
                 return Expression.Convert(expression, typeof(Nullable<>).MakeGenericType(enumUnderlyingType));
             }
+            else if (expression.NodeType == ExpressionType.Constant && ((ConstantExpression)expression).Value == null)
+            {
+                return expression;
+            }
             else
             {
                 throw Error.NotSupported(SRResources.ConvertToEnumFailed, enumType, expression.Type);

OData/test/System.Web.OData.Test/OData/Query/FilterQueryOptionTest.cs

@@ -661,6 +661,31 @@ public void ApplyToEnums_ReturnsCorrectQueryable(string filter, int[] enumModelI
                 actualCustomers.Select(enumModel => enumModel.Id));
         }
 
+        [Theory]
+        [PropertyData("EnumModelTestFilters")]
+        public void ApplyToEnums_ReturnsCorrectQueryabl_IfEnableConstantParameterizationSetFalse(string filter, int[] enumModelIds)
+        {
+            // Arrange
+            var model = GetEnumModel();
+            var context = new ODataQueryContext(model, typeof(EnumModel));
+            var filterOption = new FilterQueryOption(filter, context);
+            IEnumerable<EnumModel> enumModels = EnumModelTestData;
+
+            // Act
+            IQueryable queryable = filterOption.ApplyTo(enumModels.AsQueryable(),
+                new ODataQuerySettings
+                {
+                    HandleNullPropagation = HandleNullPropagationOption.True,
+                    EnableConstantParameterization = false
+                });
+
+            // Assert
+            Assert.NotNull(queryable);
+            IEnumerable<EnumModel> actualCustomers = Assert.IsAssignableFrom<IEnumerable<EnumModel>>(queryable);
+            Assert.Equal(
+                enumModelIds,
+                actualCustomers.Select(enumModel => enumModel.Id));
+        }
         [Theory]
         [InlineData("Simple has null", typeof(ODataException))]
         [InlineData("null has Microsoft.TestCommon.Types.SimpleEnum'First'", typeof(ODataException))]