How to implement Task.WhenAny() with a predicate

Question

I want to execute several asynchronous tasks concurrently. Each task will run an HTTP request that can either complete successfully or throw an exception. I need to aw

Answer 1

public static Task> WhenFirst(IEnumerable> tasks, Func, bool> predicate)
{
    if (tasks == null) throw new ArgumentNullException(nameof(tasks));
    if (predicate == null) throw new ArgumentNullException(nameof(predicate));

    var tasksArray = (tasks as IReadOnlyList>) ?? tasks.ToArray();
    if (tasksArray.Count == 0) throw new ArgumentException("Empty task list", nameof(tasks));
    if (tasksArray.Any(t => t == null)) throw new ArgumentException("Tasks contains a null reference", nameof(tasks));

    var tcs = new TaskCompletionSource>();
    var count = tasksArray.Count;

    Action> continuation = t =>
        {
            if (predicate(t))
            {
                tcs.TrySetResult(t);
            }
            if (Interlocked.Decrement(ref count) == 0)
            {
                tcs.TrySetResult(null);
            }
        };

    foreach (var task in tasksArray)
    {
        task.ContinueWith(continuation);
    }

    return tcs.Task;
}

Sample usage:

var task = await WhenFirst(tasks, t => t.Status == TaskStatus.RanToCompletion);

if (task != null)
    var value = await task;

Note that this doesn't propagate exceptions of failed tasks (just as WhenAny doesn't).

You can also create a version of this for the non-generic Task.