Javascript unable to do dynamic type cast

Question

It looks like javascript in WinRT is unable to do dynamic type cast when calling into WINRT DLL component. Here is a simple example:

// In js/default.js, I added the following lines:

+   var testObj = new WinRTComponentDll.Test();
+   var derivedClass = new WinRTComponentDll.DerivedClass();
+   testObj.testTypeDynamicCast(derivedClass);

Here we pass a DerivedClass object pointer although the method is defined with an argument of BaseClass type. When running the app, javascript fails to dynamically cast DerivedClass type to BaseClass type, throws a "Type mismatch" exception:

"Unhandled exception at line 33, column 5 in ms-appx://e10bc6bb-8304-4b2a-a9a4-7311b5807830-4sdaedqydyjdm/js/default.js
0x800a000d - JavaScript runtime error: Type mismatch
File: default.js, line: 33 column: 5"

The definition of the BaseClass, DerivedClass and Test classes is as follows:
////////////////////////////////////////////////////////////////

// WinRTComponent.h

#pragma once

namespace WinRTComponentDll
{
    public ref class BaseClass
    {
    public:
        BaseClass();
    };

 public ref class DerivedClass sealed : public BaseClass
    {
    public:
        DerivedClass();
    };

 public ref class Test sealed
 {
 public:
  void TestTypeDynamicCast (BaseClass^ base) {}
 };
};

// WinRTComponent.cpp
#include "pch.h"
#include "WinRTComponent.h"

using namespace WinRTComponentDll;
using namespace Platform;

BaseClass::BaseClass()
{}

DerivedClass::DerivedClass()
{}

Answer 1

That does not surprise me.  Remember you need to ensure only classes that are sealed can be passed through WinRT.  You cannot derive from sealed classes so I would expect this to fail.

---

Jeff Sanders (MSFT)

Answer 2

That does not surprise me.  Remember you need to ensure only classes that are sealed can be passed through WinRT.  You cannot derive from sealed classes so I would expect this to fail.

---

Jeff Sanders (MSFT)

This same sample works in C#. Is the CLI for javascript missing this?

Answer 3

In the WIN8 developer preview this problem does not happen for javascript.

Answer 4

That is great info, thanks.  I have not had time to look yet but did you check the Migration document to see if this is listed?

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Jeff Sanders (MSFT)

Answer 5

hmm, good info John.  I will check this out.

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Jeff Sanders (MSFT)

Answer 6

I did check the Migration document, but this is not listed.

Answer 7

I got some more background on this.  WinJS will not support the class unless it is marked Sealed.  JS can only consume sealed classes.  This is by design.

---

Jeff Sanders (MSFT)

Answer 8

I got some more background on this.  WinJS will not support the class unless it is marked Sealed.  JS can only consume sealed classes.  This is by design.

---

Jeff Sanders (MSFT)

Jeff,

Since Developer Preview (before the type system was complete), we observed that both JS and C# CLI bindings had no problems with accessing types that at least had their parent sealed. C# in Consumer Preview still supports this. Should JS be less functional than C# or is C# broken in that it should never have supported this?