in a thread, is there a difference if I use
Coinitialize(nil)
instead of
CoInitializeEx(0, COINIT_MULTITHREADED);
I use Delphi 7 but I presume the question can remain for other programming languages Thanks for your help.
The former initializes COM in a way that puts the calling thread into its own single-threaded apartment (STA). The latter initializes COM in a way that puts the calling thread into a shared multi-threaded apartment (MTA). The two apartments have very different semantics, especially in how COM objects are accessed across thread boundaries. Threads in different apartments must use proxies to share COM objects, but COM provides synchronization for you (via per-thread messages queues). Threads in the same apartment can share COM objects without using proxies, but must synchronize manually, such as with critical sections or mutexes.
So yes, there is a difference and it can be very significant. Please read the documentation on MSDN, it is very detailed.