EventWaitHandle.Set Method ()


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Sets the state of the event to signaled, allowing one or more waiting threads to proceed.

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

Public Function Set As Boolean

Return Value

Type: System.Boolean

true if the operation succeeds; otherwise, false.

Exception Condition

The Close method was previously called on this System.Threading.EventWaitHandle.

For an EventWaitHandle with EventResetMode.AutoReset (including AutoResetEvent), the Set method releases a single thread. If there are no waiting threads, the wait handle remains signaled until a thread attempts to wait on it, or until its Reset method is called.


There is no guarantee that every call to the Set method will release a thread from an EventWaitHandle whose reset mode is EventResetMode.AutoReset. If two calls are too close together, so that the second call occurs before a thread has been released, only one thread is released. It is as if the second call did not happen. Also, if Set is called when there are no threads waiting and the EventWaitHandle is already signaled, the call has no effect.

For an EventWaitHandle with EventResetMode.ManualReset (including ManualResetEvent), calling the Set method leaves the wait handle in a signaled state until its Reset method is called.

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, _

    <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)
        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
        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.")

            ' 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

        ' 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)
        Next i

        ' Wait until all the threads have started and blocked.
        While Interlocked.Read(threadCount) < 5
        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.")

    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.

        ' Wait on the EventWaitHandle.

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

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

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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