How does COM/Automation do IPC under the hood?

Question

In its simplest form, COM allows you to instantiate C++-like classes from DLL in your application. Basically it's a glorified wrapper around LoadLibrary and some conventions regarding the interface. This is called using an in-process component.

But COM also supports out-of-process components. If you instantiate a class from such a component, COM starts a new process. Your objects live in said process, and are marshalled transparently over to you, so you don't care too much about where they live. They might even be on a different computer (DCOM). You can also fetch objects from already running applications. A well-known example is controlling MS Office via a script. This is called Automation (formerly OLE Automation, and there is a bit of confusion around what exactly this term encompasses).

There are a couple of nice articles explaining how (in-process) COM works low-level (e.g. COM from scratch. I'd like to know how it works when your component is out-of-process. Especially, what IPC does COM use beneath the hood to communicate between the processes? Window messages, shared memory, sockets, or something else? MSDN lists COM as an IPC method by itself, but I'm guessing it has to use something else underneath. Are different IPC methods used in different cases (instantiating an OOP component from C++, accessing an Excel document from VBScript, embedding a document in another via OLE)? It seems like it is all the same underlying technology. And lastly, how does marshalling fit in the picture? I believe it is neccessary to serialize method parameters for transmitting between processes, correct?

Answer 1

According to this MSDN article, it's RPC.

When you instantiate an OOP component, the COM subsystem generates an in-process proxy. This proxy is responsible for packing parameters and unpacking return values. It also generates a stub in the server process, which, expectably, unpacks parameters and packs return values.

Interestingly enough, the whole marshaling process can be customized, by implementing IMarshal.