Com Interop Order of Declarations

Question

I have an interface declaration like this:

[ComImport]
[Guid("79EAC9E4-BAF9-11CE-8C82-00AA004BA90B")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IInternetProtocol {
    //IInternetProtcolRoot
    void Start(
        [ MarshalAs(UnmanagedType.LPWStr) ] string szURL, 
        IInternetProtocolSink Sink, 
        IInternetBindInfo pOIBindInfo, 
        UInt32 grfPI, 
        UInt32 dwReserved);
    void Continue(ref _tagPROTOCOLDATA pProtocolData);
    void Abort(Int32 hrReason, UInt32 dwOptions);
    void Terminate(UInt32 dwOptions);
    void Suspend();
    void Resume();
    //IInternetProtocol
    [PreserveSig()] UInt32 Read(IntPtr pv, UInt32 cb, out UInt32 pcbRead);
    void Seek(_LARGE_INTEGER dlibMove, UInt32 dwOrigin, out _ULARGE_INTEGER plibNewPosition);
    void LockRequest(UInt32 dwOptions);
    void UnlockRequest();
}

The object that implements this interface is supposed to have it's Start method called. However, that is not happening. But curiously, I find that the Terminate method is being called instead, and if I set a break point on the Terminate method, and then look at the dwOptions parameter, it actually contains an IntPtr which can be cast into a string - which happens to contain what would be first parameter to Start.

I assume this has something to do with the order of declaration, even though the above declaration is the canonical one I've seen everywhere.

Also, I find that if I add arbitrary IntPtr arguments to the definition of Terminate so it looks like this:

Terminate(IntPtr a1, IntPtr a2, IntPtr a3, IntPtr a4, IntPtr a5, IntPtr a6)

the method is still successfully being called with the string as a pointer in a1 and other fields being populated with either 13 0 or what looks like another memory address.

Any idea what's going on here? The Start method only takes 5 parameters. Yet here, I've declared Terminate with 6 and its still being called in the place where Start is expected to be called.

Answer 1

Make sure that the dispatch ids (dispid) are set correctly by comparing the generated callable wrapper with the IDL file.

Answer 2

You forgot the 3 IUnknown methods. IInternetProtocolRoot::Start() is the 4th method in the vtable.