Calling CoCreateInstance CLSCTX_INPROC_SERVER from a 32 bits app, but the COM server .dll is x64

Question

Hi!
I have a question... I call CoCreateInstance with CLSCTX_INPROC_SERVER from an x86 app but the registered .dll is a 64 bits library.
Is there a way to instantiate it by doing a trick? I mean... I see for example there is an undocumented SHCreateLocalServerRundll API that may be run using rundll32.exe but it's not documented...

Thanks

Answer 1

You can instantiate a 64-bit com server from a 32-bit client if the server is an out-of-process server. A COM server hosted in a 64-bit DLL can be instantiated as an out-of-process server by using the system COM surrogate.

https://learn.microsoft.com/en-us/windows/win32/com/dllsurrogate
https://learn.microsoft.com/en-us/windows/win32/com/registering-the-dll-server-for-surrogate-activation

Example usage update -
The screenshot below shows a COM server hosted in a 64-bit DLL being instantiated by a 64-bit client directly and as an out-of-process server using the system's COM Surrogate.

It also shows a 32-bit client failing when attempting to instantiate it as an in-process server but succeeding when using the COM surrogate to instantiate the 64-bit server as out-of-process.