Need to understand the usage of SemaphoreSlim

c#, multithreading, semaphore, task-parallel-library

Solution

i guess that if i run 50 thread at a time then code like SemaphoreSlim ss = new SemaphoreSlim(10); will force to run 10 active thread at time

That is correct; the use of the semaphore ensures that there won't be more than 10 workers doing this work at the same time.

Calling `WaitAsync` on the semaphore produces a task that will be completed when that thread has been given "access" to that token. `await`-ing that task lets the program continue execution when it is "allowed" to do so. Having an asynchronous version, rather than calling `Wait`, is important both to ensure that the method stays asynchronous, rather than being synchronous, as well as deals with the fact that an `async` method can be executing code across several threads, due to the callbacks, and so the natural thread affinity with semaphores can be a problem.

A side note: `DoPollingThenWorkAsync` shouldn't have the `Async` postfix because it's not actually asynchronous, it's synchronous. Just call it `DoPollingThenWork`. It will reduce confusion for the readers.

Problem

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 do `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 threads 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?

Original source