using System;

using System.Threading;

using System.Threading.Tasks;

​

// https://docs.microsoft.com/en-us/dotnet/api/system.threading.semaphoreslim

public class Example

{

    private static SemaphoreSlim semaphore;

    // A padding interval to make the output more orderly.

    private static int padding;

​

    public static void Main()

    {

        // Create the semaphore.

        semaphore = new SemaphoreSlim(0, 3);

        Console.WriteLine("{0} tasks can enter the semaphore.", semaphore.CurrentCount);

        Task[] tasks = new Task[5];

​

        // Create and start five numbered tasks.

        for (int i = 0; i < 5; i++)

        {

            tasks[i] = Task.Run(() =>

            {

                // Each task begins by requesting the semaphore.

                Console.WriteLine("Task {0} begins and waits for the semaphore.", Task.CurrentId);

                int semaphoreCount;

                semaphore.Wait();

                try

                {

                    Interlocked.Add(ref padding, 100);

​

                    Console.WriteLine("Task {0} enters the semaphore.", Task.CurrentId);

​

                    // The task just sleeps for 1+ seconds.

                    Thread.Sleep(1000 + padding);

                }

                finally

                {

                    semaphoreCount = semaphore.Release();

                }

                Console.WriteLine("Task {0} releases the semaphore; previous count: {1}.", Task.CurrentId, semaphoreCount);

            });

        }

​

        // Wait for half a second, to allow all the tasks to start and block.

        Thread.Sleep(500);

​

        // Restore the semaphore count to its maximum value.

        Console.WriteLine("Main thread calls Release(3) --> 3 tasks can enter Semaphore");

        semaphore.Release(3);

​

        // Main thread waits for the tasks to complete.

        Task.WaitAll(tasks);

​

        Console.WriteLine("Main thread exits.");

    }

}

// The example displays output like the following:

//       0 tasks can enter the semaphore.

//       Task 1 begins and waits for the semaphore.

//       Task 5 begins and waits for the semaphore.

//       Task 2 begins and waits for the semaphore.

//       Task 4 begins and waits for the semaphore.

//       Task 3 begins and waits for the semaphore.

//       Main thread calls Release(3) --> 3 tasks can enter the semaphore.

//       Task 4 enters the semaphore.

//       Task 1 enters the semaphore.

//       Task 3 enters the semaphore.

//       Task 4 releases the semaphore; previous count: 0.

//       Task 2 enters the semaphore.

//       Task 1 releases the semaphore; previous count: 0.

//       Task 3 releases the semaphore; previous count: 0.

//       Task 5 enters the semaphore.

//       Task 2 releases the semaphore; previous count: 1.

//       Task 5 releases the semaphore; previous count: 2.

//       Main thread exits.