Error: Unable to cast COM object of type ‘mshtml.HTMLDocumentClass’ to interface type ‘ICustomDoc’

This operation failed because the QueryInterface call on the COM component for the interface with IID ‘{3050F3F0-98B5-11CF-BB82-00AA00BDCE0B}’ failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

My first reaction was:”What the hell? HTMLDocumentClass is the managed wrapper of MSHTML, and MSHTML is supposed to support the ICustomDoc interface!”

Now I started wondering why the interfaces don’t work I created a sandbox project and tried to cast interface there, but it works smoothly. I played with strong name and found no luck. Finally, I found out that it is the frame document that does not support this interface.


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