Question

I’ve implemented a windows deskband (using the windows SDK sample) and need a way to communicate (one call to start IPC with another application, IPC is already working) with it.

My COM experience is very limited but extrapolating from what I’ve seen, I think it should be possible to create a new COM interface, implement it in the deskband object (which I have access to via IBandSite), call QueryInterface() for my own interface on it and then use it to call directly into the deskband.

I’ve tried this but ran into problems very quickly (main reason being: I’ve no idea what I’m actually doing most of the time …)

So, my questions are: Is this a viable approach and can someone give me an outline on how to proceed if it is (or point to some resource that could be helpful – short of reading a COM book, which would be my last approach). If it is not, do alternatives come to mind ?

Thank you for your time and best wishes,

Rene.

Answer 1

Here’s you path: you add a new interface into .idl file and also if you have a co-class in the .idl file that corresponds to you COM object you list that new interface in the co-class definition. Then you compile the .idl and this gets you a .h file and a .c file with identifiers – the C++ IID and C++ interface definition.

Then you inherit your COM object C++ class from the C++ interface and implement all methods of it. If for whatever reason you can’t or don’t want to implement a method you have to return E_NOTIMPL from that method implementation.

One very important final thing: you have to change QueryInterface() behavior in you COM object class. If you use ATL you have to add an entry into the COM map. If you don’t use ATL change you QueryInterface() – see this question for how to implement QueryInterface() in case of implementing several COM interfaces.