Export (0) Print
Expand All

EventWaitHandle Class

Represents a thread synchronization event.

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

'Declaration
<ComVisibleAttribute(True)> _
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,  _
	ExternalThreading := True)> _
Public Class EventWaitHandle _
	Inherits WaitHandle

The EventWaitHandle type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkEventWaitHandle(Boolean, EventResetMode)Initializes a new instance of the EventWaitHandle class, specifying whether the wait handle is initially signaled, and whether it resets automatically or manually.
Public methodEventWaitHandle(Boolean, EventResetMode, String)Initializes a new instance of the EventWaitHandle class, specifying whether the wait handle is initially signaled if created as a result of this call, whether it resets automatically or manually, and the name of a system synchronization event.
Public methodEventWaitHandle(Boolean, EventResetMode, String, Boolean)Initializes a new instance of the EventWaitHandle class, specifying whether the wait handle is initially signaled if created as a result of this call, whether it resets automatically or manually, the name of a system synchronization event, and a Boolean variable whose value after the call indicates whether the named system event was created.
Public methodEventWaitHandle(Boolean, EventResetMode, String, Boolean, EventWaitHandleSecurity)Initializes a new instance of the EventWaitHandle class, specifying whether the wait handle is initially signaled if created as a result of this call, whether it resets automatically or manually, the name of a system synchronization event, a Boolean variable whose value after the call indicates whether the named system event was created, and the access control security to be applied to the named event if it is created.
Top

  NameDescription
Public propertySupported by the XNA FrameworkHandle Obsolete. Gets or sets the native operating system handle. (Inherited from WaitHandle.)
Public propertySafeWaitHandleGets or sets the native operating system handle. (Inherited from WaitHandle.)
Top

  NameDescription
Public methodSupported by the XNA FrameworkCloseWhen overridden in a derived class, releases all resources held by the current WaitHandle. (Inherited from WaitHandle.)

In XNA Framework, this member is overridden by Close.
Public methodCreateObjRefCreates an object that contains all the relevant information required to generate a proxy used to communicate with a remote object. (Inherited from MarshalByRefObject.)
Public methodSupported by Portable Class LibraryDisposeReleases all resources used by the current instance of the WaitHandle class. (Inherited from WaitHandle.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryDispose(Boolean)When overridden in a derived class, releases the unmanaged resources used by the WaitHandle, and optionally releases the managed resources. (Inherited from WaitHandle.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)

In XNA Framework, this member is overridden by Finalize.
Public methodGetAccessControlGets an EventWaitHandleSecurity object that represents the access control security for the named system event represented by the current EventWaitHandle object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetLifetimeServiceRetrieves the current lifetime service object that controls the lifetime policy for this instance. (Inherited from MarshalByRefObject.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodInitializeLifetimeServiceObtains a lifetime service object to control the lifetime policy for this instance. (Inherited from MarshalByRefObject.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Protected methodMemberwiseClone(Boolean)Creates a shallow copy of the current MarshalByRefObject object. (Inherited from MarshalByRefObject.)
Public methodStatic memberOpenExisting(String)Opens an existing named synchronization event.
Public methodStatic memberOpenExisting(String, EventWaitHandleRights)Opens an existing named synchronization event, specifying the desired security access.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryResetSets the state of the event to nonsignaled, causing threads to block.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySetSets the state of the event to signaled, allowing one or more waiting threads to proceed.
Public methodSetAccessControlSets the access control security for a named system event.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkWaitOneBlocks the current thread until the current WaitHandle receives a signal. (Inherited from WaitHandle.)

In XNA Framework, this member is overridden by WaitOne.
Public methodWaitOne(Int32)Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval. (Inherited from WaitHandle.)
Public methodWaitOne(TimeSpan)Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval. (Inherited from WaitHandle.)
Public methodSupported by the XNA FrameworkWaitOne(Int32, Boolean)Blocks the current thread until the current WaitHandle receives a signal, using a 32-bit signed integer to specify the time interval and specifying whether to exit the synchronization domain before the wait. (Inherited from WaitHandle.)

In XNA Framework, this member is overridden by WaitOne(Int32, Boolean).
Public methodWaitOne(TimeSpan, Boolean)Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval and specifying whether to exit the synchronization domain before the wait. (Inherited from WaitHandle.)
Top

  NameDescription
Explicit interface implemetationPrivate methodSupported by the XNA FrameworkIDisposable.DisposeInfrastructure. Releases all resources used by the WaitHandle. (Inherited from WaitHandle.)
Top

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.

NoteNote

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, CountdownEvent, and ManualResetEvent.

NoteNote

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 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.


Imports System
Imports System.Threading

Public Class Example

    ' The EventWaitHandle used to demonstrate the difference
    ' between AutoReset and ManualReset synchronization events.
    '
    Private Shared ewh As EventWaitHandle

    ' A counter to make sure all threads are started and
    ' blocked before any are released. A Long is used to show
    ' the use of the 64-bit Interlocked methods.
    '
    Private Shared threadCount As Long = 0

    ' An AutoReset event that allows the main thread to block
    ' until an exiting thread has decremented the count.
    '
    Private Shared clearCount As New EventWaitHandle(False, _
        EventResetMode.AutoReset)

    <MTAThread> _
    Public Shared Sub Main()

        ' Create an AutoReset EventWaitHandle.
        '
        ewh = New EventWaitHandle(False, EventResetMode.AutoReset)

        ' Create and start five numbered threads. Use the
        ' ParameterizedThreadStart delegate, so the thread
        ' number can be passed as an argument to the Start 
        ' method.
        For i As Integer = 0 To 4
            Dim t As New Thread(AddressOf ThreadProc)
            t.Start(i)
        Next i

        ' Wait until all the threads have started and blocked.
        ' When multiple threads use a 64-bit value on a 32-bit
        ' system, you must access the value through the
        ' Interlocked class to guarantee thread safety.
        '
        While Interlocked.Read(threadCount) < 5
            Thread.Sleep(500)
        End While

        ' Release one thread each time the user presses ENTER,
        ' until all threads have been released.
        '
        While Interlocked.Read(threadCount) > 0
            Console.WriteLine("Press ENTER to release a waiting thread.")
            Console.ReadLine()

            ' SignalAndWait signals the EventWaitHandle, which
            ' releases exactly one thread before resetting, 
            ' because it was created with AutoReset mode. 
            ' SignalAndWait then blocks on clearCount, to 
            ' allow the signaled thread to decrement the count
            ' before looping again.
            '
            WaitHandle.SignalAndWait(ewh, clearCount)
        End While
        Console.WriteLine()

        ' Create a ManualReset EventWaitHandle.
        '
        ewh = New EventWaitHandle(False, EventResetMode.ManualReset)

        ' Create and start five more numbered threads.
        '
        For i As Integer = 0 To 4
            Dim t As New Thread(AddressOf ThreadProc)
            t.Start(i)
        Next i

        ' Wait until all the threads have started and blocked.
        '
        While Interlocked.Read(threadCount) < 5
            Thread.Sleep(500)
        End While

        ' Because the EventWaitHandle was created with
        ' ManualReset mode, signaling it releases all the
        ' waiting threads.
        '
        Console.WriteLine("Press ENTER to release the waiting threads.")
        Console.ReadLine()
        ewh.Set()

    End Sub

    Public Shared Sub ThreadProc(ByVal data As Object)
        Dim index As Integer = CInt(data)

        Console.WriteLine("Thread {0} blocks.", data)
        ' Increment the count of blocked threads.
        Interlocked.Increment(threadCount)

        ' Wait on the EventWaitHandle.
        ewh.WaitOne()

        Console.WriteLine("Thread {0} exits.", data)
        ' Decrement the count of blocked threads.
        Interlocked.Decrement(threadCount)

        ' After signaling ewh, the main thread blocks on
        ' clearCount until the signaled thread has 
        ' decremented the count. Signal it now.
        '
        clearCount.Set()
    End Sub
End Class


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

This type is thread safe.

Community Additions

ADD
Show:
© 2014 Microsoft