InvokeAsync.cs

/// <summary>
/// Executes the given async delegates in parallel,
/// up to the given maximum degree of parallelism.
/// </summary>
public static async Task InvokeAsync(IEnumerable<Func<Task>> taskFactories, int maxDegreeOfParallelism)
{
    if (taskFactories == null) throw new ArgumentNullException(nameof(taskFactories));
    if (maxDegreeOfParallelism <= 0) throw new ArgumentException(nameof(maxDegreeOfParallelism));

    // Defensive copy. Similar to what Task.WhenAll/WhenAny does.
    Func<Task>[] queue = taskFactories.ToArray();

    if (queue.Length == 0) {
        return;
    }

    List<Task> tasksInFlight = new List<Task>(maxDegreeOfParallelism);
    int index = 0;

    do
    {
        while (tasksInFlight.Count < maxDegreeOfParallelism && index < queue.Length)
        {
            Func<Task> taskFactory = queue[index++];

            tasksInFlight.Add(taskFactory());
        }

        Task completedTask = await Task.WhenAny(tasksInFlight).ConfigureAwait(false);

        await completedTask.ConfigureAwait(false); // Check result.

        tasksInFlight.Remove(completedTask);
    }
    while (index < queue.Length || tasksInFlight.Count != 0);
}