how to wait for all background threads to finish (in C#)?

Question

how to make the foreground thread wait for all background (child) threads to finish in C#? I need to get list of pending jobs from the queue (database), start a new thread to execute each of them and finally wait for all the child threads to finish. how to do that in C#? Thanks in advance.

Answer 1

You could store each launched thread in an array. Then when you need to wait for them all, call Join method on each thread in an array in a loop.

Thread child = new Thread(...);
Threads.Add(child);
child.Start()

...

foreach(Thread t in Threads)
{
   t.Join();
}

HTH

Answer 2

Consider using ThreadPool. Most of what you want is already done. There is an example from Microsoft which does pretty much your entire task. Replace "fibonacci" with "database task" and it sounds like your problem.

Answer 3

Using dynamic data you can pass your object and the WaitHandle (ActionResetEvent) that lets you wait for all the background threads to finish without declaring an extra class:

static void Main(string[] args)
{
    List<AutoResetEvent> areList = new List<AutoResetEvent>();
    foreach (MyObject o in ListOfMyObjects)
    {
        AutoResetEvent are = new AutoResetEvent(false);
        areList.Add(are);
        ThreadPool.QueueUserWorkItem(DoWork, new { o, are });
    };

    Console.WriteLine("Time: {0}", DateTime.Now);
    WaitHandle.WaitAll(areList.ToArray());
    Console.WriteLine("Time: {0}", DateTime.Now);
    Console.ReadKey();
}

static void DoWork(object state)
{
    dynamic o = state;
    MyObject myObject = (MyObject)o.o;
    AutoResetEvent are = (AutoResetEvent)o.are;

    myObject.Execute();
    are.Set();
}

Answer 4

This is incomplete code, but ManualResetEvent works for you

var waitEvents = new List<ManualResetEvent>();
foreach (var action in actions)
{
    var evt = new ManualResetEvent(false);
    waitEvents.Add(evt);
    ThreadPool.RegisterWaitForSingleObject(asyncResult.AsyncWaitHandle, TimeoutCallback, state, 5000, true);
}

if (waitEvents.Count > 0)
    WaitHandle.WaitAll(waitEvents.ToArray());