c# - Need to understand the usage of SemaphoreSlim

Question

Here is the code I have but I don't understand what SemaphoreSlim is doing.

async Task WorkerMainAsync()
{
    SemaphoreSlim ss = new SemaphoreSlim(10);
    List<Task> trackedTasks = new List<Task>();
    while (DoMore())
    {
        await ss.WaitAsync();
        trackedTasks.Add(Task.Run(() =>
        {
            DoPollingThenWorkAsync();
            ss.Release();
        }));
    }
    await Task.WhenAll(trackedTasks);
}

void DoPollingThenWorkAsync()
{
    var msg = Poll();
    if (msg != null)
    {
        Thread.Sleep(2000); // process the long running CPU-bound job
    }
}

What does await ss.WaitAsync(); and ss.Release(); do?

I guess that if I run 50 threads at a time then write code like SemaphoreSlim ss = new SemaphoreSlim(10); then it will be forced to run 10 active thread at time.

When one of 10 thread completes then another thread will start. If I am not right then help me to understand with sample situation.

Why is await needed along with ss.WaitAsync();? What does ss.WaitAsync(); do?

Answer 1

In the kindergarden around the corner they use a SemaphoreSlim to control how many kids can play in the PE room.

They painted on the floor, outside of the room, 5 pairs of footprints.

As the kids arrive, they leave their shoes on a free pair of footprints and enter the room.

Once they are done playing they come out, collect their shoes and "release" a slot for another kid.

If a kid arrives and there are no footprints left, they go play elsewhere or just stay around for a while and check every now and then (i.e., no FIFO priorities).

When a teacher is around, she "releases" an extra row of 5 footprints on the other side of the corridor such that 5 more kids can play in the room at the same time.

It also has the same "pitfalls" of SemaphoreSlim...

If a kid finishes playing and leaves the room without collecting the shoes (does not trigger the "release") then the slot remains blocked, even though there is theoretically an empty slot. The kid usually gets told off, though.

Sometimes one or two sneaky kid hide their shoes elsewhere and enter the room, even if all footprints are already taken (i.e., the SemaphoreSlim does not "really" control how many kids are in the room).

This does not usually end well, since the overcrowding of the room tends to end in kids crying and the teacher fully closing the room.