Wait for threads to complete

Question

    private void button1_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < 15; i++)
        {
            Thread nova = new Thread(Method);
            nova.Start();
        }
        listBox1.Items.Add("Some text");
    }

    private void Method()
    {
        for (int i = 0; i < 15; i++)
        {
            Console.WriteLine(i);
        }

    }

This code does write: Some text and then numbers 111222333..... I would like that it writes 111122223333.... and then on the end Some text. is it possible to do that with threads (parent thread wait for child threads)? or do i have to use something else?

Why are you using threads for this? I assume this is a contrived example, but I need more information to give you an accurate answer. — Matthew Brubaker, Jun 22 '12 at 16:10
possible duplicate of [C#: Waiting for all threads to complete](http://stackoverflow.com/questions/263116/c-waiting-for-all-threads-to-complete) — Alan, Jun 22 '12 at 16:13
No this is just an example. i need to access unknown number of web pages. and all of them read same info and store it in database. so when all threads are finished i need to do something with that info. i can't do anything until i have all the info i needed — Car90, Jun 22 '12 at 16:14

score 16 · Answer 1 · answered Jun 22 '12 at 16:10

You need to keep track of all the threads, and use Thread.Join on each one. This waits until the specified thread terminates, and then continues executing. Like this:

var threads = new List<Thread>();
for (int i = 0; i < 15; i++)
{
    Thread nova = new Thread(Method);
    nova.Start();
    threads.Add(nova);
}
foreach (var thread in threads)
    thread.Join();
listBox1.Items.Add("Some text");

score 10 · Accepted Answer · answered Jun 22 '12 at 16:23

I suggest to use TPL to get this done. You won't need to spawn so many threads. By default TPL will use thread pool for that:

using System;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        private const int TaskCount = 15;

        static void Main(string[] args)
        {
            var tasks = new Task[TaskCount];
            for (var index = 0; index < TaskCount; index++)
            {
                tasks[index] = Task.Factory.StartNew(Method);
            }
            Task.WaitAll(tasks);

            Console.WriteLine("Some text");
        }

        private static void Method()
        {
            for (int i = 0; i < 15; i++)
            {
                Console.WriteLine(i);
            }
        }
    }
}

payo · Answer 3 · 2017-03-21T04:25:18.047

You could have two threads, the 2nd one waiting on a signal to fire from the first.

private void Foo()
{
  var signals = new List<ManualResetEvent>();

  for (int i = 0; i < 15; i++)
  {
    var signal = new ManualResetEvent(false);
    signals.Add(signal);
    var thread = new Thread(() => { Method(); signal.Set(); });
    thread.Start();
  }

  var completionTask = new Thread(() =>
  {
    WaitHandle.WaitAll(signals.ToArray());
    CompletionWork();
  });
  completionTask.Start();

}

private void Method()
{
}

private void CompletionWork()
{
}

The better solution now with .Net 4.0 (and later) is to use Tasks, and invoke a method using ContinueWith

private void Foo()
{
  var childThreads = new List<Task>();
  for (int i = 0; i < 15; i++)
  {
    var task = new Task(Method);
    task.Start();
    childThreads.Add(task);
  }

  var completionTask = new Task(() => 
  { 
    Task.WaitAll(childThreads.ToArray());
  }).ContinueWith(t => CompletionWork());

}

private void Method()
{
}

private void CompletionWork()
{
}

The join answers also work, but require the containing thread to block. If you don't want the GUI to block, Spawn an additional thread around the 1st.

in the ManualResetEvent example, you don't seem to add anything to the signals list. It will be empty. — Garr Godfrey, Mar 20 '17 at 20:33

score 0 · Answer 4 · answered Jun 22 '12 at 16:11

0

You can use thread.join to wait for threads to end. See http://www.dotnetperls.com/thread-join for an page on how to use it.

answered Jun 22 '12 at 16:11

Andrew Cox

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Wait for threads to complete

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