Get the result of multiple tasks in a ValueTuple and WhenAll

  • Gérald Barré

In .NET, you can use Task.WhenAll to wait for multiple tasks. Once the tasks are completed, you can get the results using .Result or by awaiting them.

Task<int> task1 = Task.Run(() => 1);
Task<string> task2 = Task.Run(() => "meziantou");

await Task.WhenAll(task1, task2);
var task1Result = task1.Result; // or await task1
var task2Result = task2.Result; // or await task2

I don't really want write this kind of code. Instead, I would like to get the results directly from the WhenAll method. Something like the following:

Task<int> task1 = Task.Run(() => 1);
Task<string> task2 = Task.Run(() => "meziantou");

// This doesn't work
var (task1Result, task2Result) = await Task.WhenAll(task1, task2);

You can write custom WhenAll methods that return a ValueTuple with the results of the tasks.

public static class TaskEx
{
    public static async Task<(T0, T1)> WhenAll<T0, T1>(Task<T0> task0, Task<T1> task1)
    {
        await Task.WhenAll(task0, task1).ConfigureAwait(false);
        // It's ok to use task.Result here as the task is completed
        return (task0.Result, task1.Result);
    }

    public static async Task<(T0, T1, T2)> WhenAll<T0, T1, T2>(Task<T0> task0, Task<T1> task1, Task<T2> task2)
    {
        await Task.WhenAll(task0, task1, task2).ConfigureAwait(false);
        return (task0.Result, task1.Result, task2.Result);
    }

    public static async Task<(T0, T1, T2, T3)> WhenAll<T0, T1, T2, T3>(Task<T0> task0, Task<T1> task1, Task<T2> task2, Task<T3> task3)
    {
        await Task.WhenAll(task0, task1, task2, task3).ConfigureAwait(false);
        return (task0.Result, task1.Result, task2.Result, task3.Result);
    }

    public static async Task<(T0, T1, T2, T3, T4)> WhenAll<T0, T1, T2, T3, T4>(Task<T0> task0, Task<T1> task1, Task<T2> task2, Task<T3> task3, Task<T4> task4)
    {
        await Task.WhenAll(task0, task1, task2, task3, task4).ConfigureAwait(false);
        return (task0.Result, task1.Result, task2.Result, task3.Result, task4.Result);
    }

    public static async Task<(T0, T1, T2, T3, T4, T5)> WhenAll<T0, T1, T2, T3, T4, T5>(Task<T0> task0, Task<T1> task1, Task<T2> task2, Task<T3> task3, Task<T4> task4, Task<T5> task5)
    {
        await Task.WhenAll(task0, task1, task2, task3, task4, task5).ConfigureAwait(false);
        return (task0.Result, task1.Result, task2.Result, task3.Result, task4.Result, task5.Result);
    }

    public static async Task<(T0, T1, T2, T3, T4, T5, T6)> WhenAll<T0, T1, T2, T3, T4, T5, T6>(Task<T0> task0, Task<T1> task1, Task<T2> task2, Task<T3> task3, Task<T4> task4, Task<T5> task5, Task<T6> task6)
    {
        await Task.WhenAll(task0, task1, task2, task3, task4, task5, task6).ConfigureAwait(false);
        return (task0.Result, task1.Result, task2.Result, task3.Result, task4.Result, task5.Result, task6.Result);
    }
}

You can now use it this way:

Task<int> task1 = Task.Run(() => 1);
Task<string> task2 = Task.Run(() => "meziantou");
Task<string> task2 = Task.Run(() => 'm');

// This works :)
var (t1Result, t2Result, t3Result) = await TaskEx.WhenAll(task1, task2, task3);

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