How to implement an efficient WhenEach that streams an IAsyncEnumerable of task results?

Question

I am trying to update my toolset with the new tools offered by C# 8, and one method that seems particularly useful is a version of Task.WhenAll that returns an IAsyncEnumera

Answer 1

By using code from this article, you can implement the following:

public static Task>[] Interleaved(IEnumerable> tasks)
{
   var inputTasks = tasks.ToList();

   var buckets = new TaskCompletionSource>[inputTasks.Count];
   var results = new Task>[buckets.Length];
   for (int i = 0; i < buckets.Length; i++)
   {
       buckets[i] = new TaskCompletionSource>();
       results[i] = buckets[i].Task;
   }

   int nextTaskIndex = -1;
   Action> continuation = completed =>
   {
       var bucket = buckets[Interlocked.Increment(ref nextTaskIndex)];
       bucket.TrySetResult(completed);
   };

   foreach (var inputTask in inputTasks)
       inputTask.ContinueWith(continuation, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);

   return results;
}

Then change your WhenEach to call the Interleaved code

public static async IAsyncEnumerable WhenEach(Task[] tasks)
{
    foreach (var bucket in Interleaved(tasks))
    {
        var t = await bucket;
        yield return await t;
    }
}

Then you can call your WhenEach as per usual

await foreach (int result in WhenEach(tasks))
{
    Console.WriteLine($"Processed: {result}");
}

I did some rudimentary benchmarking with 10k tasks and performed 5 times better in terms of speed.