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EventWaitHandle Class

Updated: April 2011

Represents a thread synchronization event.

Namespace:  System.Threading
Assembly:  mscorlib (in mscorlib.dll)

public class EventWaitHandle extends WaitHandle


The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: Synchronization | ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

The EventWaitHandle class allows threads to communicate with each other by signaling. Typically, one or more threads block on an EventWaitHandle until an unblocked thread calls the Set method, releasing one or more of the blocked threads. A thread can signal an EventWaitHandle and then block on it, by calling the static (Shared in Visual Basic) WaitHandle.SignalAndWait method.


The EventWaitHandle class provides access to named system synchronization events.

The behavior of an EventWaitHandle that has been signaled depends on its reset mode. An EventWaitHandle created with the EventResetMode.AutoReset flag resets automatically when signaled, after releasing a single waiting thread. An EventWaitHandle created with the EventResetMode.ManualReset flag remains signaled until its Reset method is called.

Automatic reset events provide exclusive access to a resource. 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.

Manual reset events are like gates. When the event is not signaled, threads that wait on it will block. When the event is signaled, all waiting threads are released, and the event remains signaled (that is, subsequent waits do not block) until its Reset method is called. Manual reset events are useful when one thread must complete an activity before other threads can proceed.

EventWaitHandle objects can be used with the static (Shared in Visual Basic) WaitHandle.WaitAll and WaitHandle.WaitAny methods.

For more information about thread synchronization mechanisms, see EventWaitHandle, AutoResetEvent, and ManualResetEvent.

The following code example uses the SignalAndWait(WaitHandle, WaitHandle) method overload to allow the main thread to signal a blocked thread and then wait until the thread finishes a task.

The example starts five threads and allows them to block on an EventWaitHandle created with the EventResetMode.AutoReset flag, then releases one thread each time the user presses the ENTER key. The example then queues another five threads and releases them all using an EventWaitHandle created with the EventResetMode.ManualReset flag.

No code example is currently available or this language may not be supported.

This type is thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0, 2.0, 1.0




April 2011

Correction: The WaitHandle.SignalAndWait method is not atomic.

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