Re: CoCreateInstance Cannot Fetch IUnkown?

"Morgan Cheng" <>
8 Jun 2006 01:59:41 -0700
I did some further investigation.

After CoGetClassObject from some troublesome component, I got pointer
to IClassFactory. The I invoke IClassFactory::CreateInstance to get
IUnknown interface. It fails. Now, I know the problem is not due to
CoCreateInstance, but due to implementation of some component, because
IClassFactory::CreateInstance doens't respond correctly if IID is

Morgan Cheng =E5=86=99=E9=81=93=EF=BC=9A

I am working on a Disk-Cleanup program. Disk-Cleanup utility in works
works with COM. In Windows Registry
there are info about all COM components that is for different kinds of
temp files.

I used _com_ptr_t and found it cannot create instance for some
component. So I checked code in _com_ptr_t::CreateInstance(). It is as

    // Loads an interface for the provided CLSID.
    // Returns an HRESULT. Any previous interface is unconditionally
    HRESULT CreateInstance(const CLSID& rclsid, IUnknown* pOuter =
NULL, DWORD dwClsContext = CLSCTX_ALL) throw()
        HRESULT hr;


        if (dwClsContext & (CLSCTX_LOCAL_SERVER |
            IUnknown* pIUnknown;
            hr = CoCreateInstance(rclsid, pOuter, dwClsContext,
__uuidof(IUnknown), reinterpret_cast<void**>(&pIUnknown));

            if (SUCCEEDED(hr)) {
                hr = OleRun(pIUnknown);

                if (SUCCEEDED(hr)) {
                    hr = pIUnknown->QueryInterface(GetIID(),

        else {
            hr = CoCreateInstance(rclsid, pOuter, dwClsContext,
GetIID(), reinterpret_cast<void**>(&m_pInterface));

        if (FAILED(hr)) {
            // just in case refcount = 0 and dtor gets called
            m_pInterface = NULL;

        return hr;

I set some point and found that the CreateInstance breaks at
"CoCreateInstance(rclsid, pOuter, dwClsContext, __uuidof(IUnknown),
reinterpret_cast<void**>(&pIUnknown))". For Some some components it
fails, for others it is OK.

This is weird. Any component should support IUnkown interface. I write
some code the check the component by CoGetClassObject and get IUnkown
successfully. It confuses me. I believe that CoCreateInstance must do
something strange so that it cannot load some (not all) components.

Is it possible that component disable client to get its IUnkown through

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