Async Parallel.ForEach

ParallelAsync.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace MyNamespace
{
    public static class ParallelAsync
    {
        public static Task ForEachAsync<T>( this IEnumerable<T> source, Func<T, Task> body )
        {
            Task completedTask = Task.WhenAll(
                source.Select( item => body( item ) )
                );

            return ( completedTask );
        }

        public static Task ForEachAsync<T>( this IEnumerable<T> source, ParallelOptions parallelOptions, Func<T, Task> body )
        {
            Task completedTask = Task.WhenAll(
                Partitioner.Create( source )
                .GetPartitions( parallelOptions.MaxDegreeOfParallelism )
                .Select( partition => Task.Run( async delegate
                {
                    using ( partition )
                    {
                        while ( partition.MoveNext() )
                        {
                            await body( partition.Current );
                        }
                    }
                }, parallelOptions.CancellationToken ) )
                );

            return ( completedTask );
        }
    }
}