VSTO Background Threading with STA and COM

Question

I'm building a VSTO add-in: I need to run a background thread, but this thread needs to access COM components (I've implemented IMessageFilter to make this messaging as safe as possible). All threads which access OLE or COM have to be STA I believe; thus, BackgroundWorker is off the table.

I am happy to use the normal thread and set the ApartmentState to STA, but I also need to know when this thread as finished so I can update some UI stuff. I do not want to Join() because it will lock the UI (which is the whole reason I'm using threads!).

Is the best practice to trigger some kind of event at the end of my threaded method, and have the event do the onfinished stuff I need?

OR

Is it better to poll

   while(!_threadFinishedStarting)
   {
        Application.DoEvents();
        Thread.Sleep(100);
   }

As mentioned here: C#.net - How to alert program that the thread is finished (event driven)?

Thanks so much

Answer 1

You can call Thread.SetApartmentState() to select an STA before you start the thread. You must also pump a message loop, a requirement for STA threads. That tends to be difficult and is often skipped, you know you need one if your code deadlocks or your IMessageFilter complains.

Beware of the other requirement, an STA is only efficient when you also create the object you want to use on that same thread. That tends to be a problem in a VSTO add-in, you normally want to use an object provided by the Office object model. You don't get to choose where that object was created.

If that's accurate at all then you are not ahead with creating your own STA thread since the calls get marshaled anyway. In which case a BGW is just fine since you can't get ahead anyway, the call gets marshaled in both cases. There is no free lunch for thread-safety.