ParallelForEachAsync.cs

public static Task ParallelForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
{
    async Task AwaitPartition(IEnumerator<T> partition)
    {
        using (partition)
        {
            while (partition.MoveNext())
            { await body(partition.Current); }
        }
    }
    return Task.WhenAll(
        Partitioner
            .Create(source)
            .GetPartitions(dop)
            .AsParallel()
            .Select(p => AwaitPartition(p)));
}