Why is this C# COM class usable from VBScript but not JScript?

Question

Consider the automation-compatible COM library in C#, given below. It follows a common COM pattern of having a visible factory coclass FooFactory implementing ICreateFoos which creates an object of type IFoo. FooFactory is the only coclass in the type library. (The factory pattern is particularly useful with COM, as it does not allow for parameterized constructors).

In the code below, I'm finding that I cannot access the returned IFoo interface from jscript unless I make the FooImpl class ComVisible (by uncommenting commented lines; this causes it to appear as a coclass in the type library). There is no such problem accessing this from VBscript.

That is, I can run this VBScript:

set ff = CreateObject("jstest.FooFactory")
set foo = ff.CreateFoo(0)
foo.Foo

But this functionally identical JScript fails, with the error "C:\temp\jstest\jstest.js(4, 1) Microsoft JScript runtime error: 'foo' is null or not an object":

var ff = new ActiveXObject("jstest.FooFactory");
var foo = ff.CreateFoo(0)
//WScript.Stdout.WriteLine(null==foo)
foo.Foo();

If I uncomment the line, I can see that null==foo is false.

Why does this happen? Is it a bug? Note that I think this is a problem is a combination of JScript and the C#/.net-specific implementation (possibly of IDispatch), because I have other similar COM servers - implemented in C++ - that do not exhibit this problem from JScript.

The problem goes away if I uncomment the commented lines in the code below, making FooImpl visible as a coclass - but I specifically do not want to do this as I don't want to expose implementation details. A workaround seems to be to make FooImpl ComVisible, but mark its constructor internal, which prevents clients from being able to CoCreate it, but that's hardly elegant.

I'm running on WinXP SP3 with Visual Studio 2005, .net 2, and have been able to reproduce the issue on a completely fresh install of TinyXP on a VirtualBox (both with Windows Script Host 5.7), and also on Windows 7 Ultimate using .net SDKs 2.0, 3.0, 3.5 and 4.0 (WSH 5.8). All OSes were 32-bit.

The library code:

using System;
using System.Runtime.InteropServices;

[assembly: ComVisible(false)]

namespace jstest
{
    [ComVisible(true)]
    public interface ICreateFoos
    {
        IFoo CreateFoo(int importantNumber);
    }

    [ComVisible(true)]
    public interface IFoo
    {
        void Foo();
    }

    [ComVisible(true)]
    public class FooFactory : ICreateFoos
    {
        public IFoo CreateFoo(int importantNumber)
        {   // in *this* version, we don't use importantNumber yet
            return new FooImpl();
        }
    }

    //[ComVisible(true)]
    public class FooImpl : IFoo
    {
        public void Foo()
        {
            Console.WriteLine("Foo");
        }
    }
}

You can compile and register (you may have to run as admin to regasm) this with

csc /target:library jstest.cs
regasm /codebase jstest.dll

Answer 1

When QueryInterface is called against the IFoo object returned from CreateFoo for the IDispatch GUID it returns E_NOINTERFACE unless ComVisible is set for the actual implementing class.

When jscript prepares to call the Foo method it calls QueryInterface several times, including with this specific GUID, and since an error is returns it doesn't try to use Invoke.

When vbscript prepares to call the Foo method it does not check the interface supports IDispatch. QueryInterface is called, once, with the GUID for IDispatchEx but it seems to simply assume IDispatch will be supported.