Re: Is there a way to implement pure IDispatch interfaces in C++?

"raj" <>
28 Sep 2006 11:33:18 -0700

Thanks for the response. I will try it out. But, I am still confused
why VB didnt complain????

Igor Tandetnik wrote:

"raj" <> wrote in message

Thanks for the quick response. I guess I failed to get my point across
correctly. When I meant the VB app calls release method, I think they
have an actual "Release()" method on this particular interface( not
the one of IUnknown::Release()). This is what I am confused.
To make it clear: Let's say we have a client VB app VBCli and a COM
object in VB - call it VBSrvr.

Let me see if I understand. You have a COM server implemented in VB.
This server implements a dispinterface that happens to have a method
called Release (quite apart from IUnknown::Release). A, shall we say,
questionable choice for a method name, but you are stuck with it. You
now want to implement the same interface in C++, but the name Release
conflicts with IUnknown::Release.

Here's one way to handle the situation. Suppose your dispinterface looks
like this in IDL:

[object, uuid("1234...")]
dispinterface IA {
    [id(1)] void Release();
    [id(2)] void AnotherMethod();

Define a new dual interface having the same methods with the same
signatures and, importantly, same DISPIDs, but rename Release to
something else (e.g. Release2, Dispose - doesn't matter):

[object, dual, uuid("4567...")]
interface IB : IDispatch {
    [id(1)] HRESULT Release2();
    [id(2)] HRESULT AnotherMethod();

Implement this dual interface as you normally would, with a slight twist
to interface map:

class CMyClass :
    public IDispatchImpl<IB, IID_IB, ...>
        // This is the punch line:

    STDMETHODIMP Release2();
    STDMETHODIMP AnotherMethod();

    // Override GetIdsOfNames so that "Release" is treated as an alias
for "Release2"
cNames, ...) {
        LPOLESTR* names = rgszNames;
        if (lstrcmpiW(rgszNames[0], OLESTR("Release")) == 0) {
            names = new LPOLESTR[cNames];
            names[0] = OLESTR("Release2");
            for (int i = 1; i < cNames; ++i) {
                names[i] = rgszNames[i];

        HRESULT hr = IDispatchImpl<...>::GetIdsOfNames(riid, names,
cNames, ...);

        if (names != rgszNames) delete[] names;
        return hr;

With best wishes,
    Igor Tandetnik

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