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 await until the first task completes successfully, or until all the tasks have failed.

How can I implement an overload of the Task.WhenAny method that accepts a predicate, so that I can exclude the non-successfully completed tasks?

Answer 1

public static Task<T> GetFirstResult<T>(
    ICollection<Func<CancellationToken, Task<T>>> taskFactories, 
    Predicate<T> predicate) where T : class
{
    var tcs = new TaskCompletionSource<T>();
    var cts = new CancellationTokenSource();

    int completedCount = 0;
    // in case you have a lot of tasks you might need to throttle them 
    //(e.g. so you don't try to send 99999999 requests at the same time)
    // see: http://stackoverflow.com/a/25877042/67824
    foreach (var taskFactory in taskFactories)
    {
        taskFactory(cts.Token).ContinueWith(t => 
        {
            if (t.Exception != null)
            {
                Console.WriteLine($"Task completed with exception: {t.Exception}");
            }
            else if (predicate(t.Result))
            {
                cts.Cancel();
                tcs.TrySetResult(t.Result);
            }

            if (Interlocked.Increment(ref completedCount) == taskFactories.Count)
            {
                tcs.SetException(new InvalidOperationException("All tasks failed"));
            }

        }, cts.Token);
    }

    return tcs.Task;
}

Sample usage:

using System.Net.Http;
var client = new HttpClient();
var response = await GetFirstResult(
    new Func<CancellationToken, Task<HttpResponseMessage>>[] 
    {
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
        ct => client.GetAsync("http://microsoft123456.com", ct),
    }, 
    rm => rm.IsSuccessStatusCode);
Console.WriteLine($"Successful response: {response}");