ForEachConcurrentAsync.cs

    public static class TPLExtensions
    {
        public static async Task<TResult[]> ForEachConcurrentAsync<TItem, TResult>(this IEnumerable<TItem> enumerable, Func<TItem, Task<TResult>> action, int? maxDegreeOfParallelism = null)
        {
            if (maxDegreeOfParallelism.HasValue)
            {
                using (var semaphoreSlim = new SemaphoreSlim(maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
                {
                    var tasksWithThrottler = new List<Task<TResult>>();

                    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 () =>
                        {
                            var result = await action(item);

                            // action is completed, so decrement the number of currently running tasks
                            semaphoreSlim.Release();

                            return result;
                        }));
                    }

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