Question

I have a function where I want to execute in a separate thread avoiding two threads to access the same resources. Also I want to make sure that if the thread is currently executing then stop that thread and start executing the new thread. This is what I have:

volatile int threadCount = 0; // use it to know the number of threads being executed
private void DoWork(string text, Action OncallbackDone)
{
    threadCount++;
    var t = new Thread(new ThreadStart(() =>
    {
        lock (_lock) // make sure that this code is only accessed by one thread
        {
            if (threadCount > 1) // if a new thread got in here return and let the last one execute
            {
                threadCount--;
                return;
            }
            // do some work in here
            Thread.Sleep(1000);
            OncallbackDone();
            threadCount--;
        }
    }));
    t.Start();
}

if I fire that method 5 times then all the threads will be waiting for the lock until the lock is released. I want to make sure that I execute the last thread though. when the threads are waiting to be the owner of the lock how can I determine which will be the next one owning the lock. I want them to own the resource in the order that I created the threads…

EDIT

I am not creating this application with .net 4.0 . Sorry for not mentioning what I was trying to accomplish. I am creating an autocomplete control where I am filtering a lot of data. I don’t want the main window to freeze eveytime I want to filter results. also I want to filter results as the user types. If the user types 5 letters at once I want to stop all threads and I will just be interested in the last one. because the lock blocks all the threads sometimes the last thread that I created may own the lock first.

Answer 1

I think you are overcomplicating this. If you are able to use 4.0, then just use the Task Parallel Library. With it, you can just set up a ContinueWith function so that threads that must happen in a certain order are done in the order you dictate. If this is NOT what you are looking for, then I actually would suggest that you not use threading, as this sounds like a synchronous action that you are trying to force into parallelism.

If you are just looking to cancel tasks: then here is a SO question on how to cancel TPL tasks. Why waste the resources if you are just going to dump them all except for the last one.

If you are not using 4.0, then you can accomplish the same thing with a Background Worker. It just takes more boilerplate code to accomplish the same thing 🙂