How to stop one thread until n threads have completed their work

Question

I have an application with one main thread and N worker threads. At some point I need that the main thread waits until all the N threads have completed one section of their work.

I normally would use Monitor.Wait() and Monitor.Pulse() but this will prevent the N threads from working at the same time.

Any idea on how to do that?

Thanks in advance.

Answer 1

.NET 4.0 will include the System.Threading.Barrier class that will make synchronization between multiple threads easier. A blog post with some good example code can be found here.

Similar functionality can be achieved using multiple WaitHandles in .NET 3.0+, as demonstrated in this example on MSDN.

A brief summary of the MSDN example:

const int numberOfWorkers = 5;

static void Main()
{
    var handles = new ManualResetEvent[numberOfWorkers];

    for (int i = 0; i < numberOfWorkers; i++)
    {
        handles[i] = new ManualResetEvent(false);
        ThreadPool.QueueUserWorkItem(o => worker.Work(), null);
    }

    // Wait for all workers to finish before continuing
    WaitHandle.WaitAll(handles);
    /* continue execution... */
}

Answer 2

Do something similar to garbage collection. You'll write a ThreadManager that has a count of how many threads are running. When the main thread starts a new worker, the ThreadManager will increment its count of workers. When a worker finishes, it will inform the ThreadManager who will decrement its count of threads. When it has zero worker threads, the ThreadManager will wake the main thread.