Why code in finally block doesn't execute?

Question

It seems that finally block doesn't execute if it other than main thread execute code. Is it possible to force execute finally in that case?

Environment: VS 2010, .Net Framework 4.0.3

class Program
{
    static void Main(string[] args)
    {
        var h = new AutoResetEvent(false);

        ThreadPool.QueueUserWorkItem(
            obj => TestProc(h));

        h.WaitOne();
    }

    private static void TestProc(EventWaitHandle h)
    {
        try
        {
            Trace.WriteLine("Try");
            h.Set();
        }
        catch(Exception)
        {
            Trace.WriteLine("Catch");
        }
        finally
        {
            Thread.Sleep(2000);
            Trace.WriteLine("Finally");
        }
    }
}

---

Update:

I found mentions and explanation about that case in MSDN:

ThreadAbortException Class http://msdn.microsoft.com/en-us/library/system.threading.threadabortexception.aspx

When a call is made to the Abort method to destroy a thread, the common language runtime throws a ThreadAbortException. ThreadAbortException is a special exception that can be caught, but it will automatically be raised again at the end of the catch block. When this exception is raised, the runtime executes all the finally blocks before ending the thread. Because the thread can do an unbounded computation in the finally blocks or call Thread.ResetAbort to cancel the abort, there is no guarantee that the thread will ever end. If you want to wait until the aborted thread has ended, you can call the Thread.Join method. Join is a blocking call that does not return until the thread actually stops executing.

Note:

When the common language runtime (CLR) stops background threads after all foreground threads in a managed executable have ended, it does not use Thread.Abort. Therefore, you cannot use ThreadAbortException to detect when background threads are being terminated by the CLR.

---

Foreground and Background Threads http://msdn.microsoft.com/en-us/library/h339syd0.aspx

When the runtime stops a background thread because the process is shutting down, no exception is thrown in the thread. However, when threads are stopped because the AppDomain.Unload method unloads the application domain, a ThreadAbortException is thrown in both foreground and background threads.

---

So why at the end of application CLR doesn't use AppDomain.Unload method for unloading the application domain before end (kill) of main process? Because http://msdn.microsoft.com/en-us/library/system.appdomain.unload.aspx:

When a thread calls Unload, the target domain is marked for unloading. The dedicated thread attempts to unload the domain, and all threads in the domain are aborted. If a thread does not abort, for example because it is executing unmanaged code, or because it is executing a finally block, then after a period of time a CannotUnloadAppDomainException is thrown in the thread that originally called Unload. If the thread that could not be aborted eventually ends, the target domain is not unloaded. Thus, in the .NET Framework version 2.0 domain is not guaranteed to unload, because it might not be possible to terminate executing threads.

Conclusion: In some cases I need to consider if will my code be executing in background or foreground thread? Is it possible that my code will not finished before application main thread ends all work?

Answer 1

Your code is running in a background thread. When you set the AutoResetEvent, your single foreground thread terminates (as you reach the end of the Main method) and the process is torn down "immediately".

In fact, I think it likely that your finally block starts executing, but as the first thing you do is sleep for two seconds, the process quits before it gets to your WriteLine call.

If your Main method were still running, or any other foreground thread were keeping the process alive, you'd see your finally block complete as normal. This isn't really a matter of "finally on other threads" - it's a matter of "the process only stays alive while there are foreground threads".

Answer 2

You can prevent the main method from exiting until the finally has executed. There are many possible approaches.

• You can use synchronization to achieve this. For example using a ResetEvent, similar to what you are already doing, or creating a thread explicitly and joining with it.

• You could just a simple sleep or readline at the end of the Main method:

  h.WaitOne();
  Console.ReadLine();
  

Then the user can control when the program exits.

• You can use a non-background thread instead of a thread from the thread pool. Then the program won't exit until the thread has also terminated. This is probably the best and simpest option if you want your program not to terminate until the thread has completed.