[C#] Stopping timer in its callback method

Question

I have a System.Threading.Timer that calls its appropriate event handler (callback) every 10 ms. The method itself is not reentrant and can sometimes take way longer than 10 ms. Thus, I want to stop the timer during method execution.

Code:

private Timer _creatorTimer;

// BackgroundWorker's work
private void CreatorWork(object sender, DoWorkEventArgs e) {
      _creatorTimer = new Timer(CreatorLoop, null, 0, 10);

      // some other code that worker is doing while the timer is active
      // ...
      // ...
}

private void CreatorLoop(object state) {
      // Stop timer (prevent reentering)
      _creatorTimer.Change(Timeout.Infinite, 0);

      /*
          ... Work here
      */

      // Reenable timer
      _creatorTimer.Change(10, 0);
}

MSDN states that the callback method is called (every time the timer fires) in separate thread from the thread pool. That means that if I stop the timer the first thing in method it still doesn't neccessarily prevent the timer to fire and run another instance of the method before the first one had a chance to stop the timer.

Should maybe the timer (or even the non-reentrant method itself) be locked? What is the right way to prevent timer from firing during execution of its callback (and non-reentrant) method?

Answer 1

I've had similar situation with a System.Timers.Timer, where the elapsed event is executed from a threadpool and needs to be reentrant.

I used this method to get around the issue:

private void tmr_Elapsed(object sender, EventArgs e)
{
    tmr.Enabled = false;
    // Do Stuff
    tmr.Enabled = true;
}

Depending on what you're doing you may want to consider a System.Timers.Timer, here's a nice summary from MSDN

                                         System.Windows.Forms    System.Timers         System.Threading  
Timer event runs on what thread?         UI thread               UI or worker thread   Worker thread
Instances are thread safe?               No                      Yes                   No
Familiar/intuitive object model?         Yes                     Yes                   No
Requires Windows Forms?                  Yes                     No                    No
Metronome-quality beat?                  No                      Yes*                  Yes*
Timer event supports state object?       No                      No                    Yes
Initial timer event can be scheduled?    No                      No                    Yes
Class supports inheritance?              Yes                     Yes                   No

* Depending on the availability of system resources (for example, worker threads)

Answer 2

You could let the timer continue firing the callback method but wrap your non-reentrant code in a Monitor.TryEnter/Exit. No need to stop/restart the timer in that case; overlapping calls will not acquire the lock and return immediately.

 private void CreatorLoop(object state) 
 {
   if (Monitor.TryEnter(lockObject)
   {
     try
     {
       // Work here
     }
     finally
     {
       Monitor.Exit(lockObject);
     }
   }
 }

Answer 3

A couple possible solutions:

• have the real work done in yet another thread delegate that's waiting on an event. The timer callback merely signals the event. The worker thread cannot be reentered, as it's a single thread that does its work only when the event is signaled. The timer is reentrant, since all it does is signal the event (seems a little roundabout and wasteful, but it'll work)
• have the timer created with only a start timeout and no periodic timeout so it'll fire only once. The timer callback will dispose of that timer object and create a new one when it has completed its work that will also only fire once.

You may be able to manage option #2 without disposing/creating a new object by using the Change() method of the original timer object, but I'm not sure what the behavior is exactly of calling Change() with a new start timeout after the first timeout has expired. That would be worth a test or two.

Edit:

---

I did the test - manipulating the timer as a restartable one-shot seems to work perfectly, and it's much simpler than the other methods. Here's some sample code based on yours as a starting point (a few details may have changed to get it to compile on my machine):

private Timer _creatorTimer;

// BackgroundWorker's work
private void CreatorWork(object sender, EventArgs e) {
    // note: there's only a start timeout, and no repeat timeout
    //   so this will fire only once
    _creatorTimer = new Timer(CreatorLoop, null, 1000, Timeout.Infinite);

    // some other code that worker is doing while the timer is active
    // ...
    // ...
}

private void CreatorLoop(object state) {
    Console.WriteLine( "In CreatorLoop...");
    /*
        ... Work here
    */
    Thread.Sleep( 3000);

    // Reenable timer
    Console.WriteLine( "Exiting...");

    // now we reset the timer's start time, so it'll fire again
    //   there's no chance of reentrancy, except for actually
    //   exiting the method (and there's no danger even if that
    //   happens because it's safe at this point).
    _creatorTimer.Change(1000, Timeout.Infinite);
}