using System;

using System.Threading;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {   

        var processTimeSpan = TimeSpan.FromSeconds(1);

        // not working (current state, where sum((number of concurrent threads - 1) * processTimeSpan) > waitTimeSpan)

        //var waitTimeSpan = TimeSpan.FromSeconds(processTimeSpan.Seconds / 2); // 3 > 0.5

        //var waitTimeSpan = TimeSpan.FromSeconds(processTimeSpan.Seconds * 2); // 3 > 2

        // working (current state, where sum((number of concurrent threads - 1) * processTimeSpan) < waitTimeSpan)

        //var waitTimeSpan = TimeSpan.FromSeconds(processTimeSpan.Seconds * 10); // 3 < 10

        // working (future state)

        var waitTimeSpan = Timeout.InfiniteTimeSpan;

        Worker worker = new Worker();

        // start all threads approximately at the same time

        new Thread(async () => await worker.Run(0, processTimeSpan, waitTimeSpan)).Start();

        new Thread(async () => await worker.Run(1, processTimeSpan, waitTimeSpan)).Start();

        new Thread(async () => await worker.Run(2, processTimeSpan, waitTimeSpan)).Start();

        new Thread(async () => await worker.Run(3, processTimeSpan, waitTimeSpan)).Start();

        Thread.Sleep(TimeSpan.FromSeconds(7));

        Console.WriteLine("Done");

    }

    class Worker

    {

        private readonly SemaphoreSlim _semaphoreSlim;

        public Worker()

        {

            _semaphoreSlim = new SemaphoreSlim(1, 1);

        }

        public async Task Run(int id, TimeSpan processTimeSpan, TimeSpan waitTimeSpan)

        {

            Console.WriteLine(id + " start wait");

            await _semaphoreSlim.WaitAsync(waitTimeSpan);

            Console.WriteLine(id + " end wait");

            try

            {

                Console.WriteLine(id + " start process");

                Thread.Sleep(processTimeSpan);

                Console.WriteLine(id + " end process");

            }

            finally

            {

                Console.WriteLine(id + " start semaphore release");

                _semaphoreSlim.Release();

                Console.WriteLine(id + " end semaphore release");

            }

        }

    }

}