using System;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Concurrent;

​

public class Program

{

    public static async Task Main()

    {

        var q = new SemaphoreQueue(1,1);

        var tasks = Enumerable.Range(0,5).Select(i => Start(q, i));

        await Task.WhenAll(tasks);

        Console.WriteLine("Done");

    }

    public static async Task Start(SemaphoreQueue q, int i)

    {

        var t = new Random().Next(100);

        Console.WriteLine($"Starting {i}");

        await q.WaitAsync();

        await Task.Delay(t);

        Console.WriteLine($"Started {i}");

        q.Release();

    }

}

​

public class SemaphoreQueue

{

    private SemaphoreSlim semaphore;

    private ConcurrentQueue<TaskCompletionSource<bool>> queue =

        new ConcurrentQueue<TaskCompletionSource<bool>>();

    public SemaphoreQueue(int initialCount)

    {

        semaphore = new SemaphoreSlim(initialCount);

    }

    public SemaphoreQueue(int initialCount, int maxCount)

    {

        semaphore = new SemaphoreSlim(initialCount, maxCount);

    }

    public void Wait()

    {

        WaitAsync().Wait();

    }

    public Task WaitAsync()

    {

        var tcs = new TaskCompletionSource<bool>();

        queue.Enqueue(tcs);

        semaphore.WaitAsync().ContinueWith(t =>

        {

            TaskCompletionSource<bool> popped;

            if (queue.TryDequeue(out popped))

                popped.SetResult(true);

        });

        return tcs.Task;

    }

    public void Release()

    {

        semaphore.Release();

    }

}