using System;

using System.Threading;

using System.Threading.Tasks;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        RateLimiter rateLimiter = new(2, TimeSpan.FromMilliseconds(500));

        Task<int>[] tasks = Enumerable.Range(1, 10).Select(async item =>

        {

            Thread.Sleep(100); // Each task is created 100 msec after the previous task.

            Print($"Item #{item} before rateLimiter.WaitAsync");

            await rateLimiter.WaitAsync();

            Print($"Processing #{item}");

            await Task.Delay(200);

            return item;

        }).ToArray();

        int[] results = Task.WhenAll(tasks).Result;

        Print($"Results: {String.Join(", ", results)}");

        Thread.Sleep(500);

        Print($"Finished");

    }

​

    /// <summary>

    /// Limits the number of workers that can access a resource, during the specified

    /// time span.

    /// </summary>

    public class RateLimiter

    {

        private readonly SemaphoreSlim _semaphore;

        private readonly TimeSpan _timeUnit;

​

        public RateLimiter(int maxActionsPerTimeUnit, TimeSpan timeUnit)

        {

            if (maxActionsPerTimeUnit < 1)

                throw new ArgumentOutOfRangeException(nameof(maxActionsPerTimeUnit));

            if (timeUnit < TimeSpan.Zero || timeUnit.TotalMilliseconds > Int32.MaxValue)

                throw new ArgumentOutOfRangeException(nameof(timeUnit));

            _semaphore = new SemaphoreSlim(maxActionsPerTimeUnit, maxActionsPerTimeUnit);

            _timeUnit = timeUnit;

        }

​

        public async Task WaitAsync(CancellationToken cancellationToken = default)

        {

            await _semaphore.WaitAsync(cancellationToken).ConfigureAwait(false);

            // Schedule the release of the semaphore on the ThreadPool.

            // Use the newly created Timer object as the state object, to prevent GC.

            System.Threading.Timer timer = new(_ => _semaphore.Release());

            try { timer.Change(_timeUnit, Timeout.InfiniteTimeSpan); }

            catch { _semaphore.Release(); throw; }

        }

    }

​

    private static void Print(object value)

    {

        Console.WriteLine($@"{DateTime.Now:HH:mm:ss.fff} [{Thread.CurrentThread.ManagedThreadId}] > {value}");

    }

}