EventWaitHandle
The EventWaitHandle class allows threads to communicate with each other by signaling and by waiting for signals. Event wait handles (also referred to simply as events) are wait handles that can be signaled in order to release one or more waiting threads. After it is signaled, an event wait handle is reset either manually or automatically. The EventWaitHandle class can represent either a local event wait handle (local event) or a named system event wait handle (named event or system event, visible to all processes).
Note
Event wait handles are not events in the sense usually meant by that word in the .NET Framework. There are no delegates or event handlers involved. The word "event" is used to describe them because they have traditionally been referred to as operating-system events, and because the act of signaling the wait handle indicates to waiting threads that an event has occurred.
Both local and named event wait handles use system synchronization objects, which are protected by SafeWaitHandle wrappers to ensure that the resources are released. You can use the IDisposable.Dispose method to free the resources immediately when you have finished using the object,
Event Wait Handles That Reset Automatically
You create an automatic reset event by specifying EventResetMode.AutoReset when you create the EventWaitHandle object. As its name implies, this synchronization event resets automatically when signaled, after releasing a single waiting thread. Signal the event by calling its Set method.
Automatic reset events are usually used to provide exclusive access to a resource for a single thread at a time. A thread requests the resource by calling the WaitOne method. If no other thread is holding the wait handle, the method returns true and the calling thread has control of the resource.
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As with all synchronization mechanisms, you must ensure that all code paths wait on the appropriate wait handle before accessing a protected resource. Thread synchronization is cooperative. |
If an automatic reset event is signaled when no threads are waiting, it remains signaled until a thread attempts to wait on it. The event releases the thread and immediately resets, blocking subsequent threads.
Event Wait Handles That Reset Manually
You create a manual reset event by specifying EventResetMode.ManualReset when you create the EventWaitHandle object. As its name implies, this synchronization event must be reset manually after it has been signaled. Until it is reset, by calling its Reset method, threads that wait on the event handle proceed immediately without blocking.
A manual reset event acts like the gate of a corral. When the event is not signaled, threads that wait on it block, like horses in a corral. When the event is signaled, by calling its Set method, all waiting threads are free to proceed. The event remains signaled until its Reset method is called. This makes the manual reset event an ideal way to hold up threads that need to wait until one thread finishes a task.
Like horses leaving a corral, it takes time for the released threads to be scheduled by the operating system and to resume execution. If the Reset method is called before all the threads have resumed execution, the remaining threads once again block. Which threads resume and which threads block depends on random factors like the load on the system, the number of threads waiting for the scheduler, and so on. This is not a problem if the thread that signals the event ends after signaling, which is the most common usage pattern. If you want the thread that signaled the event to begin a new task after all the waiting threads have resumed, you must block it until all the waiting threads have resumed. Otherwise, you have a race condition, and the behavior of your code is unpredictable.
Features Common to Automatic and Manual Events
Typically, one or more threads block on an EventWaitHandle until an unblocked thread calls the Set method, which releases one of the waiting threads (in the case of automatic reset events) or all of them (in the case of manual reset events). A thread can signal an EventWaitHandle and then block on it, as an atomic operation, by calling the static WaitHandle.SignalAndWait method.
EventWaitHandle objects can be used with the static WaitHandle.WaitAll and WaitHandle.WaitAny methods. Because the EventWaitHandle and Mutex classes both derive from WaitHandle, you can use both classes with these methods.
Named Events
The Windows operating system allows event wait handles to have names. A named event is system wide. That is, once the named event is created, it is visible to all threads in all processes. Thus, named events can be used to synchronize the activities of processes as well as threads.
You can create an EventWaitHandle object that represents a named system event by using one of the constructors that specifies an event name.
Note
Because named events are system wide, it is possible to have multiple EventWaitHandle objects that represent the same named event. Each time you call a constructor, or the OpenExisting method, a new EventWaitHandle object is created. Specifying the same name repeatedly creates multiple objects that represent the same named event.
Caution is advised in using named events. Because they are system wide, another process that uses the same name can block your threads unexpectedly. Malicious code executing on the same computer could use this as the basis of a denial-of-service attack.
Use access control security to protect an EventWaitHandle object that represents a named event, preferably by using a constructor that specifies an EventWaitHandleSecurity object. You can also apply access control security using the SetAccessControl method, but this leaves a window of vulnerability between the time the event wait handle is created and the time it is protected. Protecting events with access control security helps prevent malicious attacks, but it does not solve the problem of unintentional name collisions.
Note
Unlike the EventWaitHandle class, the derived classes AutoResetEvent and ManualResetEvent can represent only local wait handles. They cannot represent named system events.