Holy cow, I wrote a book!
In the absence of an explicit threading model for your COM object,
you get the "main" threading model.
The "main" threading model is little-known, and that's a good thing.
It's a relic from the days before multi-threading.
The first thread in a process to initialize COM becomes declared
the "main" thread.
(It might be the first thread to initialize COM in apartment model;
When a "main" threaded object is created, COM marshals the creation
call to the main thread, creates the object, then marshals the result
back to the creator's thread.
Similarly, when you invoke any method on the object, the call is
marshalled to the main thread, invoked, then the result is marshalled back.
In other words, a "main" threaded object is like an apartment threaded
object, with the additional constraint that the only apartment that
can use it is the one that the "main" thread belongs to.
As you can imagine, this is a horrific performance penalty
in any multithreaded application, since there is so much marshalling
going on. Even worse, it completely bottlenecks the main thread
because there are now all these objects that must
be serviced on that thread and no other thread.
Even worse than worse, all this marshalling creates
new opportunities for re-entrancy.
While waiting for the main thread to do its thing, the calling
thread will likely process messages, which means that you can
receive a window message at a time when you didn't expect it.
So why does this awful threading model exist at all?
For backwards compatibility with COM objects written before
multithreaded support was added to COM.
Back in those days, there was only one thread, so COM objects
could be extremely lazy with their synchronization.
In fact, they didn't need any! If you have only one thread,
then you certainly don't need to coordinate your actions with
other threads because there are none.
That's also why "main" threading model is the default.
Threading models were invented when multithreading support
was added to COM. Before then, there were no threads, so no
threading models. All old objects therefore didn't specify
a threading model in their registration.
The only reason you should even be aware of this ancient threading model
in the first place is that
if you forget to specify a threading model in your object
registration, you will get the dreaded "main" threading model by
And then you will wonder why your application's performance is horrible,
and why you have all these strange re-entrancy problems.