ForEachAsync.cs

    using System;
    using System.Collections.Concurrent;
    using System.Collections.Generic;
    using System.Linq;
    using System.Threading.Tasks;
    namespace Extensions
    {
        public static class Extensions
        {
            public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
            {
                return Task.WhenAll(
                    from partition in Partitioner.Create(source).GetPartitions(dop)
                    select Task.Run(async delegate
                    {
                        using (partition)
                            while (partition.MoveNext())
                                await body(partition.Current);
                    }));
            }
        }
    }