How to create a FIFO/strong semaphore

Question

I need to code my own FIFO/strong semaphore in C#, using a semaphore class of my own as a base. I found this example, but it's not quite right since I'm not supposed to be using Monitor.Enter/Exit yet.

These are the methods for my regular semaphore, and I was wondering if there was a simple way to adapt it to be FIFO.

public virtual void Acquire()
{

    lock (this)
    {

        while (uintTokens == 0)
        {

            Monitor.Wait(this);

        }

        uintTokens--;

    }

}

public virtual void Release(uint tokens = 1)
{

    lock (this)
    {

        uintTokens += tokens;
        Monitor.PulseAll(this);

    }

}

Answer 1

So SemaphoreSlim gives us a good starting place, so we'll begin by wrapping one of those in a new class, and directing everything but the wait method to that semaphore.

To get a queue like behavior we'll want a queue object, and to make sure it's safe in the face of multithreaded access, we'll use a ConcurrentQueue.

In this queue we'll put TaskCompletionSource objects. When we want to have something start waiting it can create a TCS, add it to the queue, and then inform the semaphore to asynchronously pop the next item off of the queue and mark it as "completed" when the wait finishes. We'll know that there will always be an equal or lesser number of continuations as there are items in the queue.

Then we just wait on the Task from the TCS.

We can also trivially create a WaitAsync method that returns a task, by just returning it instead of waiting on it.

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();
    }
}