Getting return values from Task.WhenAll

Question

Hopefully a fairly simple one here. I have a collection of objects, each of which has an async method that I want to call and collect values from. I'd like them to run in parallel. What I'd like to achieve can be summed up in one broken line of code:

IEnumerable<TestResult> results = await Task.WhenAll(myCollection.Select(v => v.TestAsync()));

I've tried various ways of writing this without success. Any thoughts?

This line of code looks OK. What doesn't work exactly? Any compiler error, run-time exception, etc.? — Dennis, Nov 10 '14 at 09:04
I guess his `TestAsync()` signature was looking like this: `public async TestResult TestAsync()`, instead of `public async Task TestAsync()`. — AlexMelw, Sep 29 '17 at 20:19
Somewhat related: [ForEachAsync with Result](https://stackoverflow.com/questions/30907650/foreachasync-with-result). — Theodor Zoulias, Aug 11 '23 at 17:55

score 18 · Accepted Answer · answered Nov 10 '14 at 08:51

18

If the tasks you're awaiting have a result of the same type Task.WhenAll returns an array of them. For example for this class:

public class Test
{
    public async Task<TestResult> TestAsync()
    {
        await Task.Delay(1000); // Imagine an I/O operation.
        return new TestResult();
    }
}

We get these results:

var myCollection = new List<Test>();
myCollection.Add(new Test());

IEnumerable<TestResult> results = await Task.WhenAll(myCollection.Select(v => v.TestAsync()));

answered Nov 10 '14 at 08:51

i3arnon

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4

@I3arnon: this is the same code, which OP had written. It's better to clarify, what doesn't work for him. – Dennis Nov 10 '14 at 08:59
I've explained what should work and why. I'm waiting for the OP to clarify what doesn't work for him. – i3arnon Nov 10 '14 at 09:04
4

I'm an idiot - My TestAsync method was returning a Task rather than a Task - as soon as I changed that, it worked. Thanks for pointing me in the right direction - I couldn't see what was wrong even though it was riht in front of me! – wwarby Nov 10 '14 at 12:16
Could you supply an example where `TestAsync` accepts parameters, and is a `static` method ? – Kraang Prime Jul 30 '16 at 22:56
What worked for me was replacing `Task.WaitAll` with `await Task.WhenAll` in [this answer](https://stackoverflow.com/a/25319520/1684720) – gattsbr Oct 21 '20 at 03:15

score 0 · Answer 2 · answered Aug 10 '23 at 09:20

Do it like this:

public async Task RunMultipleTasksInParallelAsync()
{
    var tasks = new List<Task<MyResultType>>(); //<---- Add the return type here

    foreach (var item in items)
    {
        var task = DoSomethingAsync();
        tasks.Add(task);
    }

    var myResults = await Task.WhenAll(tasks);
}

public async Task<MyResultType> DoSomethingAsync()
{
    return new MyResultType();
}

Recogize that you have to add the return type into the task list, otherwhise Task.WhenAll will have the return type void.

Getting return values from Task.WhenAll

2 Answers2

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