How can I throttle async functions in .Net?

Question

I'm using async-await in .Net. How can I limit the number of concurrent asynchronous calls?

Answer 1

One relatively simple way is to (ab)use TPL Dataflow. Something like:

public IEnumerable<TOutput> AsyncThrottle<TInput, TOutput>(
    IEnumerable<TInput> inputs, Func<TInput, Task<TOutput>> asyncFunction,
    int maxDegreeOfParallelism)
{
    var outputs = new ConcurrentQueue<TOutput>();

    var block = new ActionBlock<TInput>(
        async x => outputs.Enqueue(await asyncFunction(x)),
        new ExecutionDataflowBlockOptions
        { MaxDgreeOfParallelism = maxDegreeOfParallelism });

    foreach (var input in inputs)
        block.Send(input);

    block.Complete();
    block.Completion.Wait();

    return outputs.ToArray();
}