Is there a way to hook or intercept CoGetClassObject and/or CoCreateInstance calls?

Question

I'd like to intercept the COM CoCreateInstanceEx and/or CoGetClassObject functions to replace a class with a testing shim in a unit test. This will only be for a single CLSID; all others can go through unchanged. Is there a way to do this without horrible, evil hacks?

Answer 1

It depends on what exactly you want.

If you want to detect who is loading that class or to find whether it is loaded at all you can use Process Monitor. A call to CoGetClassObject() (or CoCreateInstanceEx()) will lead to a HKCR\CLSID\{Class of interest id} key being read and Process Monitor will show you what process and when does this and how successful it is.

If you want to replace an existing class with yours - compile a library with your version of class with the same class id and change the path to the COM server inside HKCR\CLSID\{Class of interest id} so that your library is used to serve a class with that id. You can do that manually or with regsvr32 - first register the original library, then yours to override the class of interest registartion. COM does class id -> library mapping through that key when a consumer calls CoGetClassObject() or CoCreateInstanceEx().

Answer 2

There's always the CoTreatAsClass function: http://msdn.microsoft.com/en-us/library/ms693452%28VS.85%29.aspx

But, as you noted, it will be a system-wide replacement of the class, not a local change.

Alternatively, you could look into hooking CoCreateInstance as suggested in the post referenced by Shay Erlichmen's comment.