>I was thinking in terms of VFP pier-to-pier (apartment threaded?) COM. If two clients instanciate the same server, could the server be used to pass (push) a message from one of the clients to the other? I've had some experience with
>languages that offer an HWND port.
>
>I guess I am trying to understand if a server can be "aware" of the clients
>holding it's object reference, and can the server direct mesaages (providing a push mechanism is in place) to a specific client.

As I can understand, your question about Multi Use instancing of COM servers and not related to threading issues. IMO you can create a VFP COM out-of-process server with Multi Use instancing and declare a global array of created object references and counter of created objects. In object's constructor (Init event) you will increment a counter of objects and assign an object reference to array's element. Destructor of object (Destroy event) will decrement counter and clear object reference. Thus you will have all references to created objects.