When two or more threads need to access a shared resource at the same time, the system needs a synchronization mechanism to ensure that only one thread at a time uses the resource. System.Threading.Mutex is a synchronization primitive that grants exclusive access to the shared resource to only one thread. If a thread acquires a mutex, the second thread that wants to acquire that mutex is suspended until the first thread releases the mutex.

You can use the WaitHandle.WaitOne(int, bool) method to request ownership of a mutex. The thread that owns a mutex can request the same mutex in repeated calls to erload:System.Threading.WaitHandle.WaitOne without blocking its execution. However, the thread must call the Mutex.ReleaseMutex method the same number of times to release ownership of the mutex. The System.Threading.Mutex class enforces thread identity, so a mutex can be released only by the thread that acquired it. By contrast, the System.Threading.Semaphore class does not enforce thread identity.

If a thread terminates while owning a mutex, the mutex is said to be abandoned. The state of the mutex is set to signaled, and the next waiting thread gets ownership. Beginning in version 2.0 of the .NET Framework, an System.Threading.AbandonedMutexException is thrown in the next thread that acquires the abandoned mutex. Before version 2.0 of the .NET Framework, no exception was thrown.

In the case of a system-wide mutex, an abandoned mutex might indicate that an application has been terminated abruptly (for example, by using Windows Task Manager).

Mutexes are of two types: local mutexes, which are unnamed, and named system mutexes. A local mutex exists only within your process. It can be used by any thread in your process that has a reference to the System.Threading.Mutex object that represents the mutex. Each unnamed System.Threading.Mutex object represents a separate local mutex.

Named system mutexes are visible throughout the operating system, and can be used to synchronize the activities of processes. You can create a System.Threading.Mutex object that represents a named system mutex by using a constructor that accepts a name. The operating-system object can be created at the same time, or it can exist before the creation of the System.Threading.Mutex object. You can create multiple System.Threading.Mutex objects that represent the same named system mutex, and you can use the erload:System.Threading.Mutex.OpenExisting method to open an existing named system mutex.