c# marshaling IDispatch pointer

Question

In an Unmanaged IDispatch interface my code calls, one of the functions is

    [id(1), helpstring("method")]
    HRESULT Test(IDispatch* pDispatch, [out, retval] long* pRC);

To call this I'm trying:

        Object myObj =  Activator.CreateInstance(
                        Type.GetTypeFromProgID("MyObject.ProgID"));
        Object [] args = new Object[1];
        args[0] = myObj
        Object result = So.GetType().InvokeMember("Test", BindingFlags.InvokeMethod, null, So,args);

Clearly this doesn't work. Does anyone have any advice on how to marshal my object as an IDispatch pointer?

Thanks