C# Waiting for multiple threads to finish

Question

I have a windows forms app that I am checking all the serial ports to see if a particular device is connected.

This is how I spin off each thread. The below code is already spun off the main gui thread.

foreach (cpsComms.cpsSerial ser in availPorts)
{
    Thread t = new Thread(new ParameterizedThreadStart(lookForValidDev));
    t.Start((object)ser);//start thread and pass it the port
}

I want the next line of code to wait until all the threads have finished. I've tried using a t.join in there, but that just processes them linearly.

Strictly as a side note and not that you asked about it, but you can put IsBackground = true on the thread to not have it block the main thread if you exit the application. — Patrick, Feb 17 '10 at 15:43

Yuriy Faktorovich · Accepted Answer · 2010-07-22T15:14:48.133

48

List<Thread> threads = new List<Thread>();
foreach (cpsComms.cpsSerial ser in availPorts)
{
    Thread t = new Thread(new ParameterizedThreadStart(lookForValidDev));
    t.Start((object)ser);//start thread and pass it the port
    threads.Add(t);
}
foreach(var thread in threads)
{
    thread.Join();
}

Edit

I was looking back at this, and I like the following better

availPorts.Select(ser =>
      {
          Thread thread = new Thread(lookForValidDev);
          thread.Start(ser);
          return thread;
      }).ToList().ForEach(t => t.Join());

edited Jul 22 '10 at 15:14

answered Feb 17 '10 at 15:31

Yuriy Faktorovich

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This is good and simple solution. But in my case System.Threading.Thread.Join() is visible in instrumentation profiling report with high exclusive time value. Despite of this, that is a really great way. – Raphoq Dec 22 '16 at 09:04

Islam Yahiatene · Answer 2 · 2010-03-05T15:21:46.860

Use the AutoResetEvent and ManualResetEvent Classes:

private ManualResetEvent manual = new ManualResetEvent(false);
void Main(string[] args)
{
    AutoResetEvent[] autos = new AutoResetEvent[availPorts.Count];

    manual.Set();

    for (int i = 0; i < availPorts.Count - 1; i++)
        {

        AutoResetEvent Auto = new AutoResetEvent(false);
        autos[i] = Auto;

        Thread t = new Thread(() => lookForValidDev(Auto, (object)availPorts[i]));
        t.Start();//start thread and pass it the port  

    }
    WaitHandle.WaitAll(autos);
    manual.Reset();

}


void lookForValidDev(AutoResetEvent auto, object obj)
{
    try
    {
         manual.WaitOne();
         // do something with obj 
    }
    catch (Exception)
    {

    }
    finally
    {
        auto.Set();
    }


}

auto.Set() should be in a finally block – Robert Davis Feb 17 '10 at 22:43 — Robert Davis, Feb 17 '10 at 22:43

IamIC · Answer 3 · 2013-07-12T16:24:26.057

10

The simplest and safest way to do this is to use a CountdownEvent. See Albahari.

edited Jul 12 '13 at 16:24

answered Jul 12 '13 at 08:46

IamIC

17,747
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Ah, I didn't know that if the thread has terminated, the Join would return. Thanks for the correction. – IamIC Jul 12 '13 at 16:24

score 2 · Answer 4 · answered Feb 17 '10 at 15:31

2

Store the Thread results in a list after they were spawned and iterate the list - during iteration call join then. You still join linearly, but it should do what you want.

answered Feb 17 '10 at 15:31

NG.

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Kiril · Answer 5 · 2010-03-05T17:25:27.717

You can use a CountDownLatch:

public class CountDownLatch
{
    private int m_remain;
    private EventWaitHandle m_event;

    public CountDownLatch(int count)
    {
        Reset(count);
    }

    public void Reset(int count)
    {
        if (count < 0)
            throw new ArgumentOutOfRangeException();
        m_remain = count;
        m_event = new ManualResetEvent(false);
        if (m_remain == 0)
        {
            m_event.Set();
        }
    }

    public void Signal()
    {
        // The last thread to signal also sets the event.
        if (Interlocked.Decrement(ref m_remain) == 0)
            m_event.Set();
    }

    public void Wait()
    {
        m_event.WaitOne();
    }
}

Example how to use it:

void StartThreads
{
    CountDownLatch latch = new CountDownLatch(availPorts.Count);

    foreach (cpsComms.cpsSerial ser in availPorts)
    {
        Thread t = new Thread(new ParameterizedThreadStart(lookForValidDev));

        //start thread and pass it the port and the latch
        t.Start((object)new Pair(ser, latch));

    }

    DoSomeWork();

    // wait for all the threads to signal
    latch.Wait();

    DoSomeMoreWork();
}

// In each thread
void NameOfRunMethod
{
    while(running)
    {
        // do work
    }

    // Signal that the thread is done running
    latch.Signal();
}

Isn't this included in .NET as CountdownEvent? https://msdn.microsoft.com/en-us/library/system.threading.countdownevent(v=vs.110).aspx — Redwood, Dec 03 '15 at 00:09

C# Waiting for multiple threads to finish

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