Firing off multiple Tasks asynchronously and waiting for them to complete

Question

I have a situation where I want to fire off a user-defined number of tasks asynchronously and wait for all of them to complete. Simplified, here's what I'm dealing with:

[TestMethod]
public async Task Start() {
        var numDrivers = 2;
        while (numDrivers != 0) {
            var rnd = new Random();
            var r = rnd.Next(itemArray.Count);
            var target = itemArray[r];
            var proxyDriver = GetProxiedDriver();
            Task.Run(() => HandleIntro(proxyDriver, target));
            numDrivers--;
        }
 }

For some context - these are Selenium webdrivers getting spun up to run some UI tests. I see the browsers pop up, but as soon as the last Task.Run() completes, all execution stops. How do I fire off n drivers to run asynchronously while waiting for them all to complete before stopping execution?

I've tried await Task.Run(() => HandleIntro(proxyDriver, target)); but this awaits the task and they don't run simultaneously.

HandleIntro:

private async Task HandleIntro(FirefoxDriver driver, string target) {
    // do stuff
}

You can use `Task.WhenAll` on an array of `Task`s, and `await` that. — Luaan, Feb 16 '15 at 08:30
Look into `Task.WaitAll()`/`WhenAll()`. https://msdn.microsoft.com/en-us/library/dd270695%28v=vs.110%29.aspx — Jeroen Vannevel, Feb 16 '15 at 08:30

score 7 · Accepted Answer · answered Feb 16 '15 at 08:33

7

It's rather simple with async-await. You have Task.WhenAll which itself returns an awaitable which can be asynchronously waited for all tasks to complete:

[TestMethod]
public async Task Start() 
{
   var numDrivers = 2;
   List<Task> tasks = new List<Task>();

   while (numDrivers != 0) 
   {
       var rnd = new Random();
       var r = rnd.Next(itemArray.Count);

       var target = itemArray[r];
       var proxyDriver = GetProxiedDriver();

       tasks.Add(Task.Run(() => HandleIntro(proxyDriver, target)));
       numDrivers--;
   }

   await Task.WhenAll(tasks);
}

answered Feb 16 '15 at 08:33

Yuval Itzchakov

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Hmm... execution stops immediately with this too, test passes as soon as the first one starts up – SB2055 Feb 16 '15 at 08:40
Perhaps an exception is thrown inside `HandleIntro`? Set CLR Exceptions on and see. – Yuval Itzchakov Feb 16 '15 at 08:41
1

I'm a dummy, mistyped something. Thanks so much :) – SB2055 Feb 16 '15 at 08:46

Firing off multiple Tasks asynchronously and waiting for them to complete

1 Answers1