Before I ask this question I would like to make it clear that I know there are libraries and techniques for Inter process communcation. This though, is a learning question about COM.
I also do know about out-of-proc servers but that's not what I am looking for.
What I want to know, because I don't know this, is it possible, and if yes how, to share an in-proc COM object (object defined in a DLL) living in one process (has been instantiated in the process) across another process? Ie, how do I obtain a pointer to the in-proc object from proces A in process B?
Thanks in advance.
Yes, it's possible. The underlying principle is the same regardless of whether you are sharing a single object instance between apartments in a single process, or between separate processes.
There's two approaches here: perhaps the simplest is to use the Running Object Table: this is essentially a workstation-wide table of named COM objects. You have one process add an object to the table with a well-known name, and have the other process look up that object.
The other approach is to use marshaling. Marshaling is the process of using a COM API to get a series of bytes that describe the location of an object. You can then copy that series of bytes to another process using any means you want to (shared memory, file, pipe, etc), and then use another COM API in the receiving process to unmarshal the object; COM then creates a suitable remoting proxy in that process that communicates back to the original one. Check out the APIs CoMarshalIntefcace and CoUnmarshalInterface for more details.
Note that both of these require that you have suitable remoting support in place for the object; the interfaces you are using need to be described in IDL and compiled and registered appropriately.
I don't have code handy for either of these cases unfortunately.
For the CoMarshalInterface approach, the process is something like:
On the far side, use:
Normal COM and Windows refcounting/resource rules apply across all of this; AddRef/Release as appropriate; use GlobalFree to free any HGLOBALs that you allocate, etc.