WaitHandle.WaitOne Method

Blocks the current thread until the current WaitHandle receives a signal.

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

'Declaration
Public Overridable Function WaitOne As Boolean

Return Value

Type: System.Boolean
true if the current instance receives a signal. If the current instance is never signaled, WaitOne never returns.

ExceptionCondition
ObjectDisposedException

The current instance has already been disposed.

The caller of this method blocks indefinitely until the current instance receives a signal. Use this method to block until a WaitHandle receives a signal from another thread, such as the signal that is generated when an asynchronous operation completes. For more information, see the IAsyncResult interface.

Calling this method overload is equivalent to calling the WaitOne(Int32) method overload and specifying -1 or Timeout.Infinite for millisecondsTimeout.

Override this method to customize the behavior of derived classes.

The following example demonstrates features of AutoResetEvent, ManualResetEvent, and EventWaitHandle, which are all derived from WaitHandle. The WaitOne method is used in several places. For example, the control thread calls WaitOne to block at the end of each step of the demonstration. Many of the threads used in the demonstration call WaitOne to block until the controlling thread releases them.

For a full description of this code, see the introduction to the example for the ManualResetEvent class.


Imports System.Threading

' The following Imports are not required for ManualResetEvent or
' AutoResetEvent; they merely simplify the code.
Imports System.Windows.Controls
Imports System.Windows.Input

Public Class Example

   ' mre is used to block and release threads manually.
   Private Shared mre As New ManualResetEvent(False)

   ' The DemoThread method waits on this AutoResetEvent before each step of the
   ' demo. The MouseLeftButtonUp event handler calls Set to allow each step to
   ' run.
   Private Shared areSyncDemoThread As New AutoResetEvent(False)

   ' All output is displayed here.
   Private Shared outputBlock As TextBlock

   ' This array of AutoResetEvent objects is used to ensure that all the threads
   ' created for the first step of the demo are waiting on the ManualResetEvent
   ' before the step is executed, and to ensure that all the threads are complete
   ' before going on to the next step.
   Private Shared autoResets() As AutoResetEvent = { New AutoResetEvent(False), _
                          New AutoResetEvent(False), New AutoResetEvent(False) }


   ' The Shared Demo method starts the thread that controls the demo and hooks 
   ' up the handler for the MouseLeftButtonUp event.
   Public Shared Sub Demo(ByVal outputBlock As TextBlock)

      Example.outputBlock = outputBlock
      outputBlock.Text &= "Click here to begin the demo." & vbLf & vbLf

      Dim t As New Thread(AddressOf DemoThread)
      t.Start()

      AddHandler outputBlock.MouseLeftButtonUp, AddressOf MouseUp

   End Sub


   ' Each time the TextBlock is clicked, the mouse event handler calls Set() on 
   ' the AutoResetEvent, to signal DemoThread to execute the next step of the 
   ' demo. Optionally, it clears the TextBlock.
   '
   Private Shared clear As Boolean = False
   Private Shared Sub MouseUp(ByVal sender As Object, ByVal e As MouseButtonEventArgs)

      ' If the clear flag has been set, clear the contents of the TextBlock.
      If clear Then
         outputBlock.Text = ""
         clear = False
      End If
      'outputBlock.Text &= "Click." & vbLf

      ' Signal the next step of the demo to proceed.
      areSyncDemoThread.Set()

   End Sub


   ' Before each step of the demo, DemoThread waits on areSyncDemoThread. When
   ' the MouseLeftButtonUp event handler signals areSyncDemoThread, DemoThread
   ' executes the step. Because areSyncDemoThread is an AutoResetEvent, it 
   ' immediately resets after DemoThread is released.
   Private Shared Sub DemoThread()

      ' Wait for a mouse click.
      areSyncDemoThread.WaitOne()


      ' Step 1: Start 3 named threads that block on a ManualResetEvent.

      For i As Integer = 0 To 2
         Dim t As New Thread(AddressOf ThreadProc)
         t.Name = "Thread_" & i
         t.Start(autoResets(i))
      Next i

      ' Wait until all three threads have finished displaying their "start" 
      ' messages and called mre.WaitOne().
      WaitHandle.WaitAll(autoResets)

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "3 threads are queued, waiting for the ManualResetEvent. Click to signal" & vbLf & _
         "the ManualResetEvent by calling its Set() method." & vbLf & vbLf)

      ' Wait for a mouse click.
      areSyncDemoThread.WaitOne()


      ' Step 2: Call mre.Set() to release the threads.

      mre.Set()

      ' Wait until all three threads have finished displaying their "end" 
      ' messages.
      WaitHandle.WaitAll(autoResets)

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "All the threads were released, and the ManualResetEvent remains in the" & vbLf & _
         "signaled state. Click to start more threads." & vbLf & vbLf) 

      ' Wait for a mouse click.
      areSyncDemoThread.WaitOne()


      ' Step 3: Show that mre remains signaled by starting more threads. These
      '         threads will not block, so there is no reason to pass them an
      '         AutoResetEvent.

      Dim twoMoreThreads() As Thread = { New Thread(AddressOf ThreadProc), _
                                         New Thread(AddressOf ThreadProc) }
      For i As Integer = 0 To 1
         twoMoreThreads(i).Name = "Thread_" & (i + 3)
         twoMoreThreads(i).Start(Nothing)
      Next i

      ' Wait until the threads have displayed their messages and finished 
      ' executing.
      For Each t As Thread In twoMoreThreads
         t.Join()
      Next t

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "As long as the ManualResetEvent remains in the signaled state, threads" & vbLf & _
         "that wait on it do not block. Click to reset the ManualResetEvent." & vbLf) 

      ' Wait for a mouse click.
      areSyncDemoThread.WaitOne()


      ' Step 4: Demonstrate that Reset puts the ManualResetEvent back into the
      '         unsignaled state.

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
                                           "Calling mre.Reset()." & vbLf & vbLf)
      mre.Reset()

      ' Start a thread that waits on the ManualResetEvent.
      Dim t5 As New Thread(AddressOf ThreadProc)
      t5.Name = "Thread_5"
      Dim wait As New AutoResetEvent(False)
      t5.Start(wait)

      ' Wait until the thread has displayed its message and is waiting.
      wait.WaitOne()

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "With the ManualResetEvent in the unsignaled state, threads once again block." & vbLf & _
         "Click to release the thread." & vbLf & vbLf)

      ' Signal the MouseUp event to clear the screen, and wait for a click.
      clear = True
      areSyncDemoThread.WaitOne()

      mre.Set()
      t5.Join()


      ' Step 5a: Signal an AutoResetEvent that does not have a thread waiting on
      '          it. Note that you can create an AutoResetEvent in the signaled
      '          state by using New AutoResetEvent(True).

      ' Put the AutoResetEvent into the signaled state.
      wait.Set()

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "If an AutoResetEvent is signaled when there is no thread waiting on it, " & vbLf & _
         "the AutoResetEvent remains in the signaled state until a thread waits on" & vbLf & _
         "it. That thread is immediately released, and the AutoResetEvent returns" & vbLf & _
         "to the unsignaled state. Click here to demonstrate this." & vbLf & vbLf)

      ' Wait for a click.
      areSyncDemoThread.WaitOne()


      ' Step 5b: Create and release three threads that all wait on the signaled 
      '          AutoResetEvent. Each thread receives an array that contains two
      '          AutoResetEvent objects, one to signal when the thread is ready
      '          for release and one to wait on.

      ' Reset the ManualResetEvent that will synchronize the release of the three
      ' threads.
      mre.Reset()

      For i As Integer = 0 To 2
         Dim t As New Thread(AddressOf ThreadProcARE)
         t.Name = "Thread_" & (i + 6)
         t.Start(New AutoResetEvent() { autoResets(i), wait })
      Next i

      ' Wait until all three threads are queued, then release them all at once. 
      WaitHandle.WaitAll(autoResets)
      mre.Set()

      ' Wait until one thread has been released by the signaled AutoResetEvent and 
      ' has posted its message to the TextBlock.
      Dim winner As Integer = WaitHandle.WaitAny(autoResets)

      Dim name As String = "Thread_" & (winner + 6)
      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & name & _
         " was the first thread to wait on the signaled AutoResetEvent. As soon" & vbLf & _
         "as " & name & " was released, the AutoResetEvent was reset, blocking the other two" _
         & vbLf & "threads. Click to release the waiting threads." & vbLf & vbLf)

      ' Wait for a click.
      areSyncDemoThread.WaitOne()


      ' Step 6: Release threads and dispose of the Shared resources.

      outputBlock.Dispatcher.BeginInvoke(displayHelper, _
         "Set() is called twice on the AutoResetEvent, releasing one waiting thread" & vbLf & _ 
         "each time. In order to ensure that both threads are released, a suitable delay" & vbLf & _
         "must elapse between calls to Set(). This is accomplished by calling WaitAny()" & vbLf & _
         "on the autoResets array." & vbLf & vbLf)
      wait.Set()
      WaitHandle.WaitAny(autoResets)
      wait.Set()

      ' To ensure that both threads have ended before DemoThread ends, WaitAny()  
      ' is called to wait for the last thread to end. When DemoThread ends, the
      ' variable 'wait' goes out of scope and the AutoResetEvent it holds is reclaimed
      ' by garbage collection. If the thread is not released before this, it will not
      ' be released (and therefore will not run) and it will not be reclaimed by 
      ' collection until the application ends. You might think that the end of
      ' DemoThread and the end of the application are synonymous, but in fact the
      ' application is still running as long as the browser window remains active.
      ' The questions of whether it is important for the last thread to run before 
      ' the application ends, or whether it is a problem to temporarily leak the
      ' thread, will have different answers from application to application.
      WaitHandle.WaitAny(autoResets)

      ' Dispose of the shared (class-level) wait handles. This is important only
      ' if the program will go on running, and the wait handles will not be used.
      mre.Close()
      areSyncDemoThread.Close()
      For Each are As AutoResetEvent In autoResets
         are.Close()
      Next

      ' Unhook the mouse button event.
      outputBlock.Dispatcher.BeginInvoke(unhookHelper)

      outputBlock.Dispatcher.BeginInvoke(displayHelper, vbLf & _
         "To run the demo again, refresh the page." & vbLf)

   End Sub


   ' Thread Procedures:

   ' This thread procedure is executed by most of the named threads created in 
   ' this example.
   Private Shared Sub ThreadProc(ByVal state As Object)

      Dim are As AutoResetEvent = CType(state, AutoResetEvent)
      Dim name As String = Thread.CurrentThread.Name

      outputBlock.Dispatcher.BeginInvoke(displayHelper, _
                                name & " starts and calls mre.WaitOne()" & vbLf)

      ' Signal that the thread is about to wait.
      If are IsNot Nothing Then are.Set()

      ' Wait until the ManualResetEvent is signaled.
      mre.WaitOne()

      outputBlock.Dispatcher.BeginInvoke(displayHelper, name & " ends." & vbLf)

      ' Signal that the thread is about to exit.
      If are IsNot Nothing Then are.Set()
   End Sub


   ' This thread procedure is used by the last step, which shows the behavior
   ' of an AutoResetEvent that has been left in the signaled state.
   Private Shared Sub ThreadProcARE(ByVal state As Object)

      ' Get two AutoResetEvent objects, one to signal when this thread is ready
      ' for release, and one to wait on. The second AutoResetEvent may already
      ' be in the signaled state.
      Dim are() As AutoResetEvent = CType(state, AutoResetEvent())
      Dim name As String = Thread.CurrentThread.Name

      ' Signal that this thread is ready for release.
      are(0).Set()

      ' All threads are started at once, when mre is signaled.
      mre.WaitOne()

      outputBlock.Dispatcher.BeginInvoke(displayHelper, _
                         name & " waits on the AutoResetEvent" & vbLf)

      ' Wait on the previously signaled AutoResetEvent.
      are(1).WaitOne()

      outputBlock.Dispatcher.BeginInvoke(displayHelper, name & " ends." & vbLf)

      ' Signal that this thread is ready to end.
      are(0).Set()

   End Sub


   ' Helper methods:

   ' In order to update the TextBlock object, which is on the UI thread, you must
   ' make a cross-thread call by using the Dispatcher object that is associated 
   ' with the TextBlock. The DisplayOutput helper method and its delegate, 
   ' displayHelper, are used by the BeginInvoke method of the Dispatcher object
   ' to append text to the TextBlock. UnhookMouseUp and its delegate, unhookHelper,
   ' unhook the event handler for the MouseLeftButtonUp event.
   '
   Private Shared displayHelper As New Action(Of String)(AddressOf DisplayOutput)
   Private Shared Sub DisplayOutput(ByVal msg As String)
      outputBlock.Text &= msg 
   End Sub

   Private Shared unhookHelper As New Action(AddressOf UnhookMouseUp)
   Private Shared Sub UnhookMouseUp()
      RemoveHandler outputBlock.MouseLeftButtonUp, AddressOf MouseUp
   End Sub

End Class

' This example produces output similar to the following:
'
'Click here to begin the demo.
'
'Thread_0 starts and calls mre.WaitOne()
'Thread_1 starts and calls mre.WaitOne()
'Thread_2 starts and calls mre.WaitOne()
'
'3 threads are queued, waiting for the ManualResetEvent. Click to signal
'the ManualResetEvent by calling its Set() method.
'
'Thread_2 ends.
'Thread_1 ends.
'Thread_0 ends.
'
'All the threads were released, and the ManualResetEvent remains in the
'signaled state. Click to start more threads.
'
'Thread_3 starts and calls mre.WaitOne()
'Thread_3 ends.
'Thread_4 starts and calls mre.WaitOne()
'Thread_4 ends.
'
'As long as the ManualResetEvent remains in the signaled state, threads
'that wait on it do not block. Click to reset the ManualResetEvent.
'
'Calling mre.Reset()
'
'Thread_5 starts and calls mre.WaitOne()
'
'With the ManualResetEvent in the unsignaled state, threads once again block.
'Click to release the thread.
'
'Thread_5 ends.
'
'If an AutoResetEvent is signaled when there is no thread waiting on it, 
'the AutoResetEvent remains in the signaled state until a thread waits on
'it. That thread is immediately released, and the AutoResetEvent returns
'to the unsignaled state. Click here to demonstrate this.
'
'Thread_7 waits on the AutoResetEvent.
'Thread_7 ends.
'Thread_8 waits on the AutoResetEvent.
'Thread_6 waits on the AutoResetEvent.
'
'Thread_7 was the first thread to wait on the signaled AutoResetEvent. As soon
'as Thread_7 was released, the AutoResetEvent was reset, blocking the other
'two threads. Click to release the waiting threads.
'
'Set() is called twice on the AutoResetEvent, releasing one waiting thread 
'each time. In order to ensure that both threads are released, a suitable delay
'must elapse between calls to Set(). This is accomplished by calling WaitAny()
'on the autoResets array.
'
'Thread_8 ends.
'Thread_6 ends.
'
'To run the demo again, refresh the page.


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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