What's the advantage of using COM over a plain DLL?

Question

Assume that you work only in the C++ world (cross-language interop is not required). What advantages/inconvenients do you see in using COM instead of a plain basic DLL? Do you think using COM is worth the trouble if you are not going to use the interface from different languages?

Answer 1

Everybody is mentioning things that are in COM's plus column. I'll mention a couple of detractions.

• When you implement your system using COM, you need to register the COM 'servers' (be they in-proc or out-of-proc) at setup and unregister them at uninstall. This could increase the complexity of your setup system slightly and tends to require a reboot unless the user carefully tears down running processes first.

• COM is slow compared to other standard ways of doing the same thing. This comment will probably generate a lot of hate and maybe some downvotes, but the fact of the matter is that at some point you will need to marshall data, and that is expensive.

• According to the Rules of COM, once an interface has been published it can never be changed. That in itself is not a negative, and you might even argue that it forces you to do thorough design before shipping the interface. But the truth is there's no such thing as never, and in production code interfaces change. You will undoubtedly need to either add methods or change the signatures of existing methods. In order to accomplish this you have to either break the rules of COM -- which has bad effects -- or follow the rules of COM which are more complicated than just adding a parameter to a function like you would with a astraight DLL.

Answer 2

COM can be useful in plain old C++ for:

• Interprocess communication
• Plugin architectures
• Late binding scenarios
• "Much, much, more..." (tm)

That said, if you don't need it, don't use it.