The System.Threading.Semaphore class represents a named (systemwide) or local semaphore. It's a thin wrapper around the Win32 semaphore object. Win32 semaphores are counting semaphores that control access to a pool of resources.
The System.Threading.SemaphoreSlim class represents a lightweight, fast semaphore that can be used for waiting within a single process when wait times are expected to be short. During the spin-wait phase, the CPU is actively spinning—it's not idle. How short "short" needs to be depends on the nature of the wait: if threads are competing for CPU resources, the spinning thread is consuming CPU time, so the wait should be very brief, measured in microseconds. If the wait is for a non-CPU-bound resource (such as I/O), the spin-wait overhead is less of a concern and the acceptable wait can be somewhat longer, measured in milliseconds. When wait times are unpredictable or expected to be significant, use System.Threading.Semaphore instead, which doesn't spin. System.Threading.SemaphoreSlim relies as much as possible on synchronization primitives provided by the common language runtime (CLR). However, it also provides lazily initialized, kernel-based wait handles as necessary to support waiting on multiple semaphores. System.Threading.SemaphoreSlim also supports the use of cancellation tokens, but it doesn't support named semaphores or the use of a wait handle for synchronization.
Managing a limited resource
Threads enter the semaphore by calling different methods depending on the semaphore type. For a System.Threading.Semaphore object, call the System.Threading.WaitHandle.WaitOne method (inherited from System.Threading.WaitHandle). For a System.Threading.SemaphoreSlim object, call the System.Threading.SemaphoreSlim.Wait or System.Threading.SemaphoreSlim.WaitAsync method. When the call returns, the count on the semaphore is decremented. When a thread requests entry and the count is zero, the thread blocks. As threads release the semaphore by calling the System.Threading.Semaphore.Release or System.Threading.SemaphoreSlim.Release method, blocked threads can enter. No guaranteed order—such as first-in, first-out (FIFO) or last-in, first-out (LIFO)—governs which blocked thread enters the semaphore next.
A thread can enter the semaphore multiple times by calling the System.Threading.Semaphore object's System.Threading.WaitHandle.WaitOne method or the System.Threading.SemaphoreSlim object's System.Threading.SemaphoreSlim.Wait method repeatedly. To release the semaphore, call the System.Threading.Semaphore.Release or System.Threading.SemaphoreSlim.Release method the same number of times the thread entered. Alternatively, call the System.Threading.Semaphore.Release%28System.Int32%29 or System.Threading.SemaphoreSlim.Release%28System.Int32%29 overload and specify the number of entries to release.
Semaphores and thread identity
The two semaphore types don't enforce thread identity on calls to the System.Threading.WaitHandle.WaitOne, System.Threading.SemaphoreSlim.Wait, System.Threading.Semaphore.Release, and System.Threading.SemaphoreSlim.Release methods. For example, a common usage scenario for semaphores involves a producer thread and a consumer thread, with one thread always incrementing the semaphore count and the other always decrementing it.
Ensure that a thread doesn't release the semaphore too many times. For example, suppose a semaphore has a maximum count of two, and that thread A and thread B both enter the semaphore. If a programming error in thread B causes it to call Release twice, both calls succeed. The count on the semaphore is full, and when thread A eventually calls Release, a System.Threading.SemaphoreFullException is thrown.
Named semaphores
The Windows operating system allows semaphores to have names. A named semaphore is system wide—once created, it's visible to all threads in all processes. Named semaphores can therefore synchronize the activities of processes as well as threads.
Create a System.Threading.Semaphore object that represents a named system semaphore by using one of the constructors that specifies a name.