AsyncHelper

  internal static class AsyncHelper
    {
        public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
        {
            return Task.WhenAll(
                Partitioner.Create(source)
                    .GetPartitions(dop)
                    .Select(partition => Task.Run(async delegate
                    {
                        using (partition)
                            while (partition.MoveNext())
                                await body(partition.Current).ContinueWith(t =>
                                {
                                    //observe exceptions
                                });
                    })));
        }
        public static async Task<IEnumerable<T2>> SelectParallel<T, T2>(this IEnumerable<T> source, int dop, Func<T, Task<T2>> action)
        {
            var concurrentTasks = Partitioner.Create(source)
                    .GetPartitions(dop)
                    .Select(partition => Task.Run(async delegate
                    {
                        using (partition)
                        {
                            var concurrentBag = new List<T2>();
                            while (partition.MoveNext())
                                concurrentBag.Add(await action(partition.Current));
                            return concurrentBag;
                        }
                    }));
            var lists = await Task.WhenAll(concurrentTasks);
            return lists.SelectMany(bag => bag);
        }
    }