CComPtr CoCreateInstance() fails

Question

I have a COM .dll registered successfully with regsvr32 but somehow CoCreateInstance() fails to create one of its interfaces. Is there a freeware tool which can determine the reason for the failure?

Answer 1

First of all, check the return value of the CoCreateInstance() call. Second, you can use a tool like Regmon or Process Monitor to see what registry lookup fails. This way, you can quickly determine what exactly wasn't registered the way you'd expect it to be.

Answer 2

If your com dll is implemented in C++ & has debug info, you could also try debugging with MSVC to step into CoCreateInstance.

My guess is that you missed associating your class with one of the interfaces properly. I've done that a number of times by mistake. If you're using ATL you need to make sure your implementing class derives from the interface & also you have added COM_INTERFACE_ENTRY(I____) for your interface in the COM_MAP:

BEGIN_COM_MAP(CFileHelper)
    COM_INTERFACE_ENTRY(IFileHelper)
    COM_INTERFACE_ENTRY(IDispatch)
    COM_INTERFACE_ENTRY(IStream)
    COM_INTERFACE_ENTRY(ISupportErrorInfo)
END_COM_MAP()

Or maybe the GUID differs between the IDL file and what's in the implementing C++ file. MSVC6 has an annoying bug where if the wizard fails to create a new class because of file permissions (e.g. some of the files it wants to change are read-only) but it has already added a GUID to the IDL file, and you fix the file permissions and go to do it again, there will be an inconsistency in the GUIDs and it's a pain to catch this. In an ideal world, the GUID definitions would reside ONLY in one file and then you wouldn't have to worry about this.

If not that, sometimes there are weird errors regarding marshaling & apartments but that only pops up if you've got multiple threads and are sending interface pointers across thread or interface boundaries.