Application doesn't exit with 0 threads

Question

We have a WinForms desktop application, which is heavily multithreaded. 3 threads run with Application.Run and a bunch of other background worker threads. Getting all the threads to shut down properly was kind of tricky, but I thought I finally got it right.

But when we actually deployed the application, users started experiencing the application not exiting. There's a System.Threading.Mutex to prevent them from running the app multiple times, so they have to go into task manager and kill the old one before they can run it again.

Every thread gets a Thread.Join before the main thread exits, and I added logging to each thread I spawn. According to the log, every single thread that starts also exits, and the main thread also exits. Even stranger, running SysInternals ProcessExplorer show all the threads disappear when the application exits. As in, there are 0 threads (managed or unmanaged), but the process is still running.

I can't reproduce this on any developers computers or our test environment, and so far I've only seen it happen on Windows XP (not Vista or Windows 7 or any Windows Server). How can a process keep running with 0 threads?

Edit:

Here's a bit more detail. One of event loops is hosting an Win32 interop DLL which uses a COM object to talk to a device driver. I put it in its own thread because the device driver is time sensitive, and whenever the UI thread would block for a significant amount of time (such as waiting for a database call to finish), it would interfere with the device driver.

So I changed the code so the main thread would do a Thread.Join with the device driver thread. That actually caused the application to lock up... it logs a few more calls on the UI thread after the Join completed and then everything stops. If the device is powered off, the driver never starts, and the problem goes away. So it looks like the driver must be responsible for keeping the application alive, even after it's supposedly been shut down.

Answer 1

Is it possible that the children of your Application.Run calls aren't terminating? Also, what's actually causing the application to exit - does it automatically close when all the threads have terminated (automatically meaning you wrote some code to do that), or is it user-imitated?

I had an issue once where there was a race condition in my "thread complete" event code that would sometimes result in what you're seeing. The last two threads would finish at the same time, fire the event simultaneously, and each event would decide it wasn't the last thread, so the application would continue running, even though the thread count was zero. To resolve this, I was able to find and eliminate the race condition, but you could also use a timer that checks every second or two, gets a count of the threads, and if none are still open, it kills the application.

Answer 2

When you create your threads, set IsBackground=true on them. When your main ui thread/app is closed, all created threads will automatically be shut down.

http://msdn.microsoft.com/en-us/library/system.threading.thread.isbackground.aspx

Answer 3

We never did figure out the root programmatic cause, but it was a specific driver version that caused the issue, and upgrading to a new driver fixed the problem.

Unfortunately, this is all the answer I can give, if someone else someday runs into a similar problem...