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 ?

Answer 1

I believe you are looking for:

Task.WaitAll(tasks.ToArray());

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

Answer 2

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); 
}

Answer 3

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(); });
        }
    });
}