Running async methods in parallel

Task.WhenAll() has a tendency to become unperformant with large scale/amount of tasks firing simultaneously - without moderation/throttling.

If you are doing a lot of tasks in a list and wanting to await the final outcome, then I propose using a partition with a limit on the degree of parallelism.

I have modified Stephen Toub's blog elegant approach to modern LINQ:

public static Task ParallelForEachAsync<T>(this IEnumerable<T> source, Func<T, Task> funcBody, int maxDoP = 4)
{
    async Task AwaitPartition(IEnumerator<T> partition)
    {
        using (partition)
        {
            while (partition.MoveNext())
            {
                 await Task.Yield(); // prevents a sync/hot thread hangup
                 await funcBody(partition.Current);
            }
        }
    }

    return Task.WhenAll(
        Partitioner
            .Create(source)
            .GetPartitions(maxDoP)
            .AsParallel()
            .Select(p => AwaitPartition(p)));
}

How it works is simple, take an IEnumerable - dissect it into evenish partitions and the fire a function/method against each element, in each partition, at the same time. No more than one element in each partition at anyone time, but n Tasks in n partitions.

Extension Usage:

await myList.ParallelForEachAsync(myFunc, Environment.ProcessorCount);

Edit: I now keep some overloads in a repository on Github if you need more options. It's in a NuGet too for NetStandard.

Edit 2: Thanks to comments from Theodor below, I was able to mitigate poorly written Async Tasks from blocking parallelism by using await Task.Yield();.

Is there a better to run async methods in parallel, or are tasks a good approach?

Yes, the "best" approach is to utilize the Task.WhenAll method. However, your second approach should have ran in parallel. I have created a .NET Fiddle, this should help shed some light. Your second approach should actually be running in parallel. My fiddle proves this!

Consider the following:

public Task<Thing[]> GetThingsAsync()
{
    var first = GetExpensiveThingAsync();
    var second = GetExpensiveThingAsync();

    return Task.WhenAll(first, second);
}

Note

It is preferred to use the "Async" suffix, instead of GetThings and GetExpensiveThing - we should have GetThingsAsync and GetExpensiveThingAsync respectively - source.