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?

Answer 1

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()));