How to run multiple tasks in c# and get an event on complete of these tasks?

Question

I am re-running a Task when its completed. Below is the function I call in the Application_Start of my application.

private void Run()
{
    Task t = new Task(() => new XyzServices().ProcessXyz());
    t.Start();
    t.ContinueWith((x) =>
    {
        Thread.Sleep(ConfigReader.CronReRunTimeInSeconds);
        Run();
    });
}

I want to run multiple tasks, number which will be read from web.config app setttings.

I am trying something like this,

private void Run()
{
    List<Task> tasks = new List<Task>();
    for (int i = 0; i < ConfigReader.ThreadCount - 1; i++)
    {
        tasks.Add(Task.Run(() => new XyzServices().ProcessXyz()));
    }

    Task.WhenAll(tasks);

    Run();
}

Whats the correct way to do this ?

if you don't want to dig into building your own taskrunner/extending that class I'd probably wait for the first task to end by using `tasks.First().Wait();` and then use a while loop with a Thread.Sleep to observe the status of the remaining tasks. — Steffen Winkler, Jul 29 '15 at 07:54
You want to run a piece of code repeatedly in predetermined time intervals? Sounds like the exact reason `Timer` exists, really. And of course, if you want to stick with this, just bind your continuation to the result of `Task.WhenAll` and everything will work the same as before. — Luaan, Jul 29 '15 at 08:27
You either need `await Task.WhenAll(tasks);` or `Task.WaitAll(tasks);`. One is a async call and should be awaited, the other is a normal function that can be called in a synchronous context. Of course you can also go for the ugly `Task.WhenAll(tasks).Wait();` — Dorus, Jul 29 '15 at 12:49

score 13 · Accepted Answer · edited Jul 29 '15 at 08:00

13

I believe you are looking for:

Task.WaitAll(tasks.ToArray());

https://msdn.microsoft.com/en-us/library/dd270695(v=vs.110).aspx

edited Jul 29 '15 at 08:00

Yasser Shaikh

46,934
46
204
281

answered Jul 29 '15 at 07:52

Mark Jansen

1,491
12
24

1

You're missing a call to `ToArray()` – Amit Jul 29 '15 at 07:57
Shoot ! Is just what I needed. – Yasser Shaikh Jul 29 '15 at 08:00
1

@Amit a discrepancy between the list used here, and the array used in the example, thanks :) – Mark Jansen Jul 29 '15 at 08:07
`WaitAll` is the pre `async` `await` method that unnecessarily ties up the main thread. http://stackoverflow.com/questions/6123406/waitall-vs-whenall – Jodrell Jul 29 '15 at 08:09
2

@Yasser, this will work but you should use `Task.WhenAll`, look it up. – Jodrell Jul 29 '15 at 08:17
I prefer `await Task.WhenAll(tasks)` – tymtam Jun 10 '16 at 03:04

Jodrell · Answer 2 · 2015-07-29T14:25:07.783

9

if you want to run the tasks one after the other,

await Task.Run(() => new XyzServices().ProcessXyz());
await Task.Delay(ConfigReader.CronReRunTimeInSeconds * 1000);

if you want to run them concurrently, as the task scheduler permits,

await Task.WhenAll(new[]
    {
        Task.Run(() => new XyzServices().ProcessXyz()),
        Task.Run(() => new XyzServices().ProcessXyz())
    });

So, your method should be something like,

private async Task Run()
{
    var tasks =
        Enumerable.Range(0, ConfigReader.ThreadCount)
        .Select(i => Task.Run(() => new XyzServices().ProcessXyz()));

    await Task.WhenAll(tasks); 
}

edited Jul 29 '15 at 14:25

answered Jul 29 '15 at 08:03

Jodrell

34,946
5
87
124

I read earlier that await the tasks is not what starts their execution, that they actually start immediately or thereabouts. Is this correct? It would complicate your first statements. – D. Ben Knoble Jul 29 '15 at 14:12
If the functions you are calling are async themself, you do not need to wrap them in a `Task.Run`. However, it is important to realize a async task will start right away, on the current thread, and only switch context once it runs into a async call itself. For CPU heavy functions you should certainly wrap them in a `Task.Run`, for functions that quickly switch to a async call (usually IO), you get better performance by not wrapping them. – Dorus Jul 30 '15 at 10:04

score 1 · Answer 3 · answered Jul 29 '15 at 08:03

1

If you want to wait all tasks to finish and then restart them, Marks's answer is correct.

But if you want ThreadCount tasks to be running at any time (start a new task as soon as any one of them ends), then

void Run()
{
    SemaphoreSlim sem = new SemaphoreSlim(ConfigReader.ThreadCount);

    Task.Run(() =>
    {
        while (true)
        {
            sem.Wait();
            Task.Run(() => { /*Your work*/  })
                .ContinueWith((t) => { sem.Release(); });
        }
    });
}

answered Jul 29 '15 at 08:03

EZI

15,209
2
27
33

Why the inner `Task.Run`? You're taking up one thread blocking while another is used to run something synchronously. Just use `sem.Wait(); try { RunWhatever(); } finally { sem.Release(); }`. – Luaan Jul 29 '15 at 08:26
@Luaan No It is not what I want to do with my code. I am not waiting one task to finish then start a new one. I just create a new task whenever one of them finishes. So there will be always *ThreadCount* tasks running.. – EZI Jul 29 '15 at 08:32
Doesn't Tasks.WhenAll uses the ThreadPool which automatically runs the 'best' number of tasks at once, while queuing the others? – Roy T. Jul 29 '15 at 10:03
In a async context I would strongly prefer to use `WaitAsync()` on the semaphore. And there is no reason at all to not switch to a async context here. – Dorus Jul 29 '15 at 12:45
@RoyT. Not sure, but [Parallel.ForEach](https://msdn.microsoft.com/en-us/library/dd460720%28v=vs.110%29.aspx) would make so much more sense in that case. [TPL](https://msdn.microsoft.com/en-us/library/dd460717%28v=vs.110%29.aspx) is build around running tasks in parallel, no need to mix that with async methods like `Tasks.WhenAll` unless your work is async. – Dorus Jul 29 '15 at 13:06

How to run multiple tasks in c# and get an event on complete of these tasks?

3 Answers3