Question

I’m trying to implement a multithread in c#.

The basic idea is:

1. There will be a lot of thread process calling a function and every thread doesn’t end in order they are called.

2. I want to limit maximum number of thread running.

3. When a thread is finished, I want to call another thread until everything is done.

So to implement this, I use a list of thread. For every second, I check if there the list is already full and if there’s any thread finished its job.

Here is my code:

List<Thread> threadCompany = new List<Thread>();
List<Thread> fireThisGuy = new List<Thread>();
while (i < _finish) {
    if (threadCompany.Count < _MAX_THREAD) {
        Thread worker = new Thread(delegate() {
            CallFunction(i);
        });

        threadCompany.Add(worker);
        i++;
        worker.Start();
    }

    Thread.Sleep(1000); //Wait for a while instead of keep calling if

    foreach (Thread worker in threadCompany) { 
        if (!worker.IsAlive) {
            fireThisGuy.Add(worker); //because threadCompany may not be 
                                     //modified in iteration.
        }
    }

    foreach (Thread worker in fireThisGuy) {
        threadCompany.Remove(worker);
    }
    fireThisGuy.Clear();
}

This works, but I don’t think I’m being elegant and efficient here, how can I improve my code?

Answer 1

This is not the right way to solve the problem.
You don’t need to keep a list of your threads, you just need to notify the application when all thread finish running.

This is a possible way to handle the problem using SynchronizationContext to notify the main thread when execution completes, without any kind of wait cycle.

public class OwnerClass
{
    private SynchronizationContext syncContext;
    private int count;
    private int completedCount;

    // This event will be raised when all thread completes
    public event EventHandler Completed;

    public OwnerClass() :
        this(SynchronizationContext.Current)
    {
    }

    public OwnerClass(SynchronizationContext context)
    {
        if (context == null)
            throw new ArgumentNullException("context");
        this.syncContext = context;
    }

    // Call this method to start running
    public void Run(int threadsCount)
    {
        this.count = threadsCount; 
        for (int i = 0; i < threadsCount; ++i)
        {
            ThreadPool.QueueUserWorkItem(this.ThreadFunc, null);
        }
    }

    private void ThreadFunc(object threadContext)
    {
        Thread.Sleep(1000); /// my long and complicated function

        if (Interlocked.Increment(ref this.completedCount) >= this.count)
        {
            this.syncContext.Post(OnCompleted, null);
        }
    }

    protected virtual void OnCompleted(object state)
    {
        var handler = this.Completed;
        if (handler != null)
            handler(this, EventArgs.Empty);
    }
}

If you just want to you run in a multiprocessor machne assigning a thread for each processor you can just use Parallel.For