Parallel foreach with asynchronous lambda

Question

I would like to handle a collection in parallel, but I'm having trouble implementing it and I'm therefore hoping for some help.

The trouble arises if I want to call a method marked async in C#, within the lambda of the parallel loop. For example:

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, async item =>
{
  // some pre stuff
  var response = await GetData(item);
  bag.Add(response);
  // some post stuff
}
var count = bag.Count;

The problem occurs with the count being 0, because all the threads created are effectively just background threads and the Parallel.ForEach call doesn't wait for completion. If I remove the async keyword, the method looks like this:

var bag = new ConcurrentBag<object>();
Parallel.ForEach(myCollection, item =>
{
  // some pre stuff
  var responseTask = await GetData(item);
  responseTask.Wait();
  var response = responseTask.Result;
  bag.Add(response);
  // some post stuff
}
var count = bag.Count;

It works, but it completely disables the await cleverness and I have to do some manual exception handling.. (Removed for brevity).

How can I implement a Parallel.ForEach loop, that uses the await keyword within the lambda? Is it possible?

The prototype of the Parallel.ForEach method takes an Action<T> as parameter, but I want it to wait for my asynchronous lambda.

Answer 1

My lightweight implementation of ParallelForEach async.

Features:

1. Throttling (max degree of parallelism).
2. Exception handling (aggregation exception will be thrown at completion).
3. Memory efficient (no need to store the list of tasks).

public static class AsyncEx
{
    public static async Task ParallelForEachAsync<T>(this IEnumerable<T> source, Func<T, Task> asyncAction, int maxDegreeOfParallelism = 10)
    {
        var semaphoreSlim = new SemaphoreSlim(maxDegreeOfParallelism);
        var tcs = new TaskCompletionSource<object>();
        var exceptions = new ConcurrentBag<Exception>();
        bool addingCompleted = false;

        foreach (T item in source)
        {
            await semaphoreSlim.WaitAsync();
            asyncAction(item).ContinueWith(t =>
            {
                semaphoreSlim.Release();

                if (t.Exception != null)
                {
                    exceptions.Add(t.Exception);
                }

                if (Volatile.Read(ref addingCompleted) && semaphoreSlim.CurrentCount == maxDegreeOfParallelism)
                {
                    tcs.SetResult(null);
                }
            });
        }

        Volatile.Write(ref addingCompleted, true);
        await tcs.Task;
        if (exceptions.Count > 0)
        {
            throw new AggregateException(exceptions);
        }
    }
}

Usage example:

await Enumerable.Range(1, 10000).ParallelForEachAsync(async (i) =>
{
    var data = await GetData(i);
}, maxDegreeOfParallelism: 100);

Answer 2

With SemaphoreSlim you can achieve parallelism control.

var bag = new ConcurrentBag<object>();
var maxParallel = 20;
var throttler = new SemaphoreSlim(initialCount: maxParallel);
var tasks = myCollection.Select(async item =>
{
  try
  {
     await throttler.WaitAsync();
     var response = await GetData(item);
     bag.Add(response);
  }
  finally
  {
     throttler.Release();
  }
});
await Task.WhenAll(tasks);
var count = bag.Count;

Answer 3

You can use the ParallelForEachAsync extension method from AsyncEnumerator NuGet Package:

using Dasync.Collections;

var bag = new ConcurrentBag<object>();
await myCollection.ParallelForEachAsync(async item =>
{
  // some pre stuff
  var response = await GetData(item);
  bag.Add(response);
  // some post stuff
}, maxDegreeOfParallelism: 10);
var count = bag.Count;

Answer 4

I've created an extension method for this which makes use of SemaphoreSlim and also allows to set maximum degree of parallelism

    /// <summary>
    /// Concurrently Executes async actions for each item of <see cref="IEnumerable<typeparamref name="T"/>
    /// </summary>
    /// <typeparam name="T">Type of IEnumerable</typeparam>
    /// <param name="enumerable">instance of <see cref="IEnumerable<typeparamref name="T"/>"/></param>
    /// <param name="action">an async <see cref="Action" /> to execute</param>
    /// <param name="maxDegreeOfParallelism">Optional, An integer that represents the maximum degree of parallelism,
    /// Must be grater than 0</param>
    /// <returns>A Task representing an async operation</returns>
    /// <exception cref="ArgumentOutOfRangeException">If the maxActionsToRunInParallel is less than 1</exception>
    public static async Task ForEachAsyncConcurrent<T>(
        this IEnumerable<T> enumerable,
        Func<T, Task> action,
        int? maxDegreeOfParallelism = null)
    {
        if (maxDegreeOfParallelism.HasValue)
        {
            using (var semaphoreSlim = new SemaphoreSlim(
                maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
            {
                var tasksWithThrottler = new List<Task>();

                foreach (var item in enumerable)
                {
                    // Increment the number of currently running tasks and wait if they are more than limit.
                    await semaphoreSlim.WaitAsync();

                    tasksWithThrottler.Add(Task.Run(async () =>
                    {
                        await action(item).ContinueWith(res =>
                        {
                            // action is completed, so decrement the number of currently running tasks
                            semaphoreSlim.Release();
                        });
                    }));
                }

                // Wait for all tasks to complete.
                await Task.WhenAll(tasksWithThrottler.ToArray());
            }
        }
        else
        {
            await Task.WhenAll(enumerable.Select(item => action(item)));
        }
    }

Sample Usage:

await enumerable.ForEachAsyncConcurrent(
    async item =>
    {
        await SomeAsyncMethod(item);
    },
    5);

Answer 5

If you just want simple parallelism, you can do this:

var bag = new ConcurrentBag<object>();
var tasks = myCollection.Select(async item =>
{
  // some pre stuff
  var response = await GetData(item);
  bag.Add(response);
  // some post stuff
});
await Task.WhenAll(tasks);
var count = bag.Count;

If you need something more complex, check out Stephen Toub's ForEachAsync post.