Run N threads and know when all finished running

Question

I have the following problem, I need to run the same method N times in a for and know when all the threads they’ve finished running it.

I needed something like this:

foreach (MeuObjeto obj in lstObj)
{
    new Thread(delegate() { MetodoX(obj ); }).Start();             
}  
metodoY(); // só rodar depois que todas as thread  acima já tiverem terminado.

Answer 1

It is possible to do this using Parallels (i.e. PLINQ) or Tasks. I recommend using Tasks, as PLINQ will probably eventually become obsolete in favor of the previous.

See how to do:

• using the method ForAll of the PLINQ... is the simplest of all:

  lstObj.AsParallel().ForAll(MetodoX);
  metodoY();
  

• using Task.WhenAll: simple call, modified method

  var manyTasks = lstObj.Select(MetodoX); // a chamada fica símples
  Task.WhenAll(manyTasks).Wait();
  metodoY();
  

  Whereas the MetodoX would have to be changed to use async and await:

  private async Task MetodoX(MeuObjeto obj)
  {
      await Task.Yield(); // primeira linha do método deve ser um Yield
  

• using Task.WhenAll and Task.Run: less simple call, original method

  var manyTasks = lstObj.Select(x => Task.Run(() => MetodoX(x)));
  Task.WhenAll(manyTasks).Wait();
  metodoY();
  

If you still don’t want to use any of these alternatives, don’t want to use LINQ, PLINQ, or Tasks, and want to handle Threads by hand, then you can do so:

var threads = new List<Thread>();
foreach (string obj in lstObj)
    threads.Add(new Thread(delegate() { MetodoX(obj); }));

foreach (var thread in threads)
    thread.Start();

foreach (var thread in threads)
    thread.Join();

Answer 2

It is possible to use Tasks as recommended by Miguel Anelo and do in a simplified way without worrying about the mechanism:

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
                    
public class Program {
    public static void Main() {
        var lstObj = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 };
        Parallel.ForEach(lstObj, obj => MetodoX(obj));
        Console.WriteLine("Fim"); //aqui poderia ser o MetodoY()
    }
    public static void MetodoX(int obj) {
        Console.WriteLine($"ThreadID: {Thread.CurrentThread.ManagedThreadId, 3}, Valor: {obj}");
    }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

In general it is not recommended to use Threads rough until there’s a good reason to use.