C# TaskWhenAll on method which is returing result depending on awaited call

Question

i'm trying to make this code works in async way, but i got some doubts.

public async Task<string> GetAsync(string inputA, string inputB)
{
    var result = await AnotherGetAsync(inputA, inputB)
        .ConfigureAwait(false);

    return result.Enabled
        ? inputB
        : string.Empty;
}

I got some string input collection, and i would like to run this method on them. Then i would do Task.WhenAll, and filter for non empty strings.

But it won't be async as inside the method i'm already awaiting for the result right?

Answer 1

I assumed the real question is:

If a single item is awaited inside method A, will this run sequential if I use Task.WhenAll for a range of items calling method A?

They will be run simultaneous with Task.WhenAll. Perhaps it is best explained with an example:

void Main()
{
    Test().GetAwaiter().GetResult();
}

private async Task<bool> CheckEmpty(string input)
{
    await Task.Delay(200);
    return String.IsNullOrEmpty(input);
}

private async Task Test()
{
    var list = new List<string>
    {
        "1",
        null,
        "2",
        ""
    };

    var stopwatch = new Stopwatch();
    stopwatch.Start();

    // This takes aprox 4 * 200ms == 800ms to complete.
    foreach (var itm in list)
    {
        Console.WriteLine(await CheckEmpty(itm));
    }

    Console.WriteLine(stopwatch.Elapsed);
    Console.WriteLine();
    stopwatch.Reset();
    stopwatch.Start();

    // This takes aprox 200ms to complete.
    var tasks = list.Select(itm => CheckEmpty(itm));
    var results = await Task.WhenAll(tasks); // Runs all at once.
    Console.WriteLine(String.Join(Environment.NewLine, results));
    Console.WriteLine(stopwatch.Elapsed);
}

My test results:

False
True
False
True
00:00:00.8006182

False
True
False
True
00:00:00.2017568