TaskCompletionExtensions.cs

public static class TaskCompletionExtensions
{
    public static Task<T> FirstCompletedTask<T>(this IEnumerable<Task<T>> source)
    {
        foreach (var completedTask in tasks.InCompletionOrder())
        {
            if (completedTask.Status == TaskStatus.RanToCompletion)
            {
                return completedTask;
            }
        }

        throw new Exception("None of the given tasks completed successfully.");
        // alternatively: return Task.FromResult(default(T));
    }

    // Sourced from Jon Skeet
    public static IEnumerable<Task<T>> InCompletionOrder<T>(this IEnumerable<Task<T>> source)
    {
        var inputs = source.ToList();
        var boxes  = inputs.Select(x => new TaskCompletionSource<T>()).ToList();
        int currentIndex = -1;

        foreach (var task in inputs)
        {
            task.ContinueWith(completed =>
            {
                var nextBox = boxes[Interlocked.Increment(ref currentIndex)];
                PropagateResult(completed, nextBox);
            }, TaskContinuationOptions.ExecuteSynchronously);
        }

        return boxes.Select(box => box.Task);
    }

    // Sourced from Jon Skeet
    private static void PropagateResult<T>(Task<T> completedTask,
          TaskCompletionSource<T> completionSource)
    {
        switch(completedTask.Status)
        {
          case TaskStatus.Canceled:
              completionSource.TrySetCanceled();
              break;
          case TaskStatus.Faulted:
              completionSource.TrySetException(completedTask.Exception.InnerExceptions);
              break;
          case TaskStatus.RanToCompletion:
              completionSource.TrySetResult(completedTask.Result);
              break;
          default:
              throw new ArgumentException ("Task was not completed.");
        }
    }
}