Thottam R. Sriram has an article in the January MSDN Magazine called CLR Inside Out: Introduction to COM Interop. As the introduction says:
COM is a wonderful technology. One aspect of the common language runtime (CLR) that makes it an extremely powerful platform is that it allows seamless interactions between Microsoft® .NET applications and unmanaged COM components. However, when I searched the Web, I found few working samples demonstrating the very basic concepts of COM interop. The purpose of this column is to illustrate those basic concepts in order to provide solid working examples that can jump-start users in this technology.
I'll start off with a simple Active Template Library (ATL) COM server and try accessing methods in this server using an unmanaged COM client, then do the same thing with a managed client. I'll walk through the various DLLs to illustrate the translation from unmanaged to managed, and I'll show how to access an exported method in an unmanaged DLL using P/Invoke. The toughest part of this is to figure out marshaling of complex structures, which I don't cover exhaustively in this column-it would be a complete column (or book) on its own. I'll show you how unmanaged code can call back into managed code using interfaces. (You could do this with delegates as well, but I won't cover that in this column.)
Finally, I'll discuss debugging your COM interop project using public symbols. This will give you very basic introduction to WinDbg.exe, unmanaged debugging, and managed debugging using SOS. I'll demonstrate the stack from managed to unmanaged as calls are made in either direction.
The managed client is in C# -- when you write your clients in C++/CLI you don't have to figure out how to declare and marshal complex structures, after all. And seeing how to call back from native code to managed code is helpful indeed. I'm not sure if Reverse P/Invoke is the official name for this, but it works for me. You set up an interface in your C# code and decorate it with attributes for COM. Then you implement that interface in your code and use tlbexp to make a COM Callable Wrapper around your .NET object. Then you hand-define the same interface as a COM interface in your native code and you're all set. The .NET code can P/Invoke some native function and pass in a reference to the .NET interface. By the time it's unmarshaled over in the native world, it's become a smart pointer to that COM interface and you can invoke the method from the native code. It's a little tricky, but it's not hard - once you've seen it done.
Since the article wraps up by touching on Windbg, there's something for everyone. Check it out!