Issue #2255,Create the correct expression when casting entity/complex type to derived type. PR #2299

Author: KenitoInc

src/Microsoft.AspNet.OData.Shared/Query/Expressions/FilterBinder.cs

@@ -337,8 +337,19 @@ private Expression BindSingleResourceCastFunctionCall(SingleResourceFunctionCall
             else if (arguments[0].Type.IsAssignableFrom(targetClrType))
             {
                 // To support to cast Entity/Complex type to the sub type now.
-                Expression source = BindCastSourceNode(node.Source);
-
+                Expression source;
+                if(node.Source != null)
+                {
+                    source = BindCastSourceNode(node.Source);
+                }
+                else
+                {
+                    // if the cast is on the root i.e $it (~/Products?$filter=NS.PopularProducts/.....),
+                    // node.Source would be null. Calling BindCastSourceNode will always return '$it'.
+                    // In scenarios where we are casting a navigation property to return an expression that queries against the parent property,
+                    // we need to have a memberAccess expression e.g '$it.Category'. We can get this from arguments[0].
+                    source = arguments[0];
+                }
                 return Expression.TypeAs(source, targetClrType);
             }
             else

test/UnitTest/Microsoft.AspNet.OData.Test.Shared/Query/Expressions/FilterBinderTests.cs

@@ -2316,7 +2316,7 @@ public void CastToUnquotedEntityType_ThrowsODataException(string filter, string
 
         [Theory]
         [InlineData("cast('Microsoft.AspNet.OData.Test.Query.Expressions.DerivedProduct')/DerivedProductName eq null", "$it => (($it As DerivedProduct).DerivedProductName == null)","$it => (IIF((($it As DerivedProduct) == null), null, ($it As DerivedProduct).DerivedProductName) == null)")]
-        [InlineData("cast(Category,'Microsoft.AspNet.OData.Test.Query.Expressions.DerivedCategory')/DerivedCategoryName eq null", "$it => (($it As DerivedCategory).DerivedCategoryName == null)", "$it => (IIF((($it As DerivedCategory) == null), null, ($it As DerivedCategory).DerivedCategoryName) == null)")]
+        [InlineData("cast(Category,'Microsoft.AspNet.OData.Test.Query.Expressions.DerivedCategory')/DerivedCategoryName eq null", "$it => (($it.Category As DerivedCategory).DerivedCategoryName == null)", "$it => (IIF((($it.Category As DerivedCategory) == null), null, ($it.Category As DerivedCategory).DerivedCategoryName) == null)")]
         public void CastToQuotedEntityOrComplexType_DerivedProductName(string filter, string expectedExpression, string expectedExpressionWithNullCheck)
         {
             // Arrange, Act & Assert