How to get notification that a System.Threading.Tasks.Task has completed

user_401743 0
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I am currently replacing some home baked task functionality with a new implementation using the new System.Threading.Tasks functionality found in .net 4.

I have a slight issue though, and although I can think of some solutions I would like some advice on which is generally the best way to do it, and if I am missing a trick somewhere.

What I need is for an arbitrary process to be able to start a Task but then carry on and not wait for the Task to finish. Not a problem, but when I then need to do something with the result of a task i'm not quite sure the best way of doing it.

All the examples I have seen use either Wait() on the task until it completes or references the Result parameter on the task. Both of these will block the thread which started the Task, which I don't want.

Some solutions I have thought of:

  1. Create a new thread and start the task on that, then use Wait() or .Result to block the new thread and sync the result back to the caller somehow, possibly with polling to the tasks IsCompleted parameter.

  2. Have a 'Notify Completed' task which I can start after completion of the task I want to run which then raises a static event or something.

  3. Pass a delegate into the input of the task and call that to notify that the task is finished.

I can think or pros and cons to all of them, but I especially don't like the idea of having to explicitly create a new thread to start the task on when the one of the aims of using the Task class in the first place is to abstract away from direct Thread usage.

Any thoughts about the best way? Am I missing something simple? Would a 'Completed' event be too much to ask for :)? (Sure there is a good reason why there isn't one!)

.net-4.0 .net c#
6 Answers
 Task task = Task.Run ( () => { Thread.Sleep ( 2000 ); } );

 task.GetAwaiter ().OnCompleted ( () => 
   {
     MessageBox.Show ( "the task completed in the main thread", "");
   } );
user_75640130

I created a small example illustrating Jon Skeet's answer, which I'd like to share with you:

using System;
using System.Threading;
using System.Threading.Tasks;

public class Program
{
    static void Main(string[] args)
    {
        for (int cnt = 0; cnt < NumTasks; cnt++)
        {
            var task = new Task<int>(DoSomething); // any other type than int is possible
            task.ContinueWith(t => Console.WriteLine($"Waited for {t.Result} milliseconds."));
            task.Start(); // fire and forget
        }

        PlayMelodyWhileTasksAreRunning();       
    }
    static int NumTasks => Environment.ProcessorCount;

    static int DoSomething()
    {
        int milliSeconds = random.Next(4000) + 1000;
        Console.WriteLine($"Waiting for {milliSeconds} milliseconds...");
        Thread.Sleep(milliSeconds);

        return milliSeconds; // make available to caller as t.Result
    }
    static Random random = new Random();

    static void PlayMelodyWhileTasksAreRunning()
    {
        Console.Beep(587, 200); // D
        Console.Beep(622, 200); // D#
        Console.Beep(659, 200); // E
        Console.Beep(1047, 400); // C
        Console.Beep(659, 200); // E
        Console.Beep(1047, 400); // C
        Console.Beep(659, 200); // E
        Console.Beep(1047, 1200); // C
        Console.Beep(1047, 200); // C
        Console.Beep(1175, 200); // D
        Console.Beep(1245, 200); // D#
        Console.Beep(1319, 200); // E
        Console.Beep(1047, 200); // C
        Console.Beep(1175, 200); // D
        Console.Beep(1319, 400); // E
        Console.Beep(988, 200); // H
        Console.Beep(1175, 400); // D
        Console.Beep(1047, 1600); // C
    }
}
user_57737330

You can use the ContinueWith function with your routine as a first argument, and a task scheduler as the second argument given by TaskScheduler.FromCurrentSynchronizationContext().

It goes like this:

var task1 = new Task(() => {do_something_in_a_remote_thread();} );

task1.ContinueWith(() =>  {do_something_in_the_ui_thread();},
TaskScheduler.FromCurrentSynchronizationContext());
user_50748510
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