AsyncParallelForEach.cs

public static async Task AsyncParallelForEach<T>(this IAsyncEnumerable<T> source, Func<T, Task> body, int maxDegreeOfParallelism = DataflowBlockOptions.Unbounded, TaskScheduler scheduler = null)
{
    var options = new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = maxDegreeOfParallelism
    };
    if (scheduler != null)
        options.TaskScheduler = scheduler;

    var block = new ActionBlock<T>(body, options);

    await foreach (var item in source)
        block.Post(item);

    block.Complete();
    await block.Completion;
}