Question

Problem statement

I have a worker thread that basically scans a folder, going into the files within it, and then sleeps for a while. The scanning operation might take 2-3 seconds but not much more. I’m looking for a way to stop this thread elegantly.

Clarification: I want to stop the thread while it’s sleeping, and not while it’s scanning. However, the problem is that I do not know what is the current state of the thread. If it’s sleeping I want it to exit immediately. If it’s scanning, I want it to exit the moment it tries to block.

Attempts at a solution

At first I was using Sleep and Interrupt. Then I found out that Interrupt doesn’t really interrupt the Sleep – it only works when the threads TRIES to go into sleeping.

So I switched to Monitor Wait&Pulse. Then I found out that the Pulse only works when I’m actually in the Wait. So now I have a thread which looks like that:

while (m_shouldRun)
{
    try
    {
        DoSomethingThatTakesSeveralSeconds();
        lock (this)
        {
            Monitor.Wait(this, 5000);
        }
    }
    catch (ThreadInterruptedException)
    {
        m_shouldRun = false;
    }
}

And now I need to craft my Stop function. So I started with:

public void Stop()
{
    m_shouldRun = false;
    lock (this)
    {
        Monitor.Pulse(this);
    }
    thread.Join();
}

But this doesn’t work because I may be pulsing while the thread works (while it’s not waiting). So I added Interrupt:

public void Stop()
{
    m_shouldRun = false;
    thread.Interrupt();
    lock (this)
    {
        Monitor.Pulse(this);
    }
    thread.Join();
}

Another option is to use:

public void Stop()
{
    m_shouldRun = false;
    while (!thread.Join(1000))
    {
        lock (this)
        {
            Monitor.Pulse(this);
        }
    }
}

The question

What is the preferred method? Is there a third method which is preferable?

Answer 1

Another alternative is to use events:

private ManualResetEvent _event = new ManualResetEvent(false);


public void Run() 
{
 while (true)
 {
    DoSomethingThatTakesSeveralSeconds();
    if (_event.WaitOne(timeout))
      break;
 }
}

public void Stop() 
{
   _event.Set();
   thread.Join();
}