Executes a collection of functions, in parallel managed by a semaphore.

ManagedForEachAsync.cs

/// <summary>
/// Executes a list of functions, in parallel managed by a semaphore.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="values">The values to act on.</param>
/// <param name="maxDegreeOfParallelism">Maximum degree of parallelism when executing functions.</param>
/// <param name="fn">Function to call using value.</param>
/// <returns></returns>
public static async Task ManagedForEachAsync<T>(this IEnumerable<T> values, int maxDegreeOfParallelism, Func<T, Task> fn)
{
    // initialize manager(semaphore) 
    var resourceManager = new SemaphoreSlim(maxDegreeOfParallelism, maxDegreeOfParallelism);

    // list of tasks to throttle
    var throttledTasks = new List<Task>();

    // iterate values
    foreach (var value in values)
    {
        // await resource available signal
        await resourceManager.WaitAsync();

        // add task to run
        throttledTasks.Add(Task.Run(async () =>
        {
            // await finish task function
            await fn(value).ContinueWith(res =>
            {
                // release resource
                resourceManager.Release();
            });
        }));
    }

    // Wait for all tasks to complete.
    await Task.WhenAll(throttledTasks.ToArray()).ContinueWith(x =>
    {
        // dispose resourceManager
        resourceManager.Dispose();
    });

}