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EventWaitHandle Class

Represents a thread synchronization event.

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

[ComVisibleAttribute(true)]
public class EventWaitHandle : WaitHandle

The EventWaitHandle type exposes the following members.

  NameDescription
Public propertySupported by Silverlight for Windows PhoneSupported by Xbox 360SafeWaitHandleGets or sets the native operating-system handle. (Inherited from WaitHandle.)
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  NameDescription
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360CloseWhen overridden in a derived class, releases all resources held by the current WaitHandle. (Inherited from WaitHandle.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360Dispose()Releases all resources used by the current instance of the WaitHandle class. (Inherited from WaitHandle.)
Protected methodSupported by Silverlight for Windows PhoneSupported by Xbox 360Dispose(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 Silverlight for Windows PhoneSupported by Xbox 360Equals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows PhoneSupported by Xbox 360FinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360GetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360GetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows PhoneSupported by Xbox 360MemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360ResetSets the state of the event to non-signaled, which causes threads to block.
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360SetSets the state of the event to signaled, which allows one or more waiting threads to proceed.
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360ToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360WaitOne()Blocks the current thread until the current WaitHandle receives a signal. (Inherited from WaitHandle.)

In Silverlight for Windows Phone, this member is overridden by WaitOne().


In XNA Framework, this member is overridden by WaitOne().
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360WaitOne(Int32)Blocks the current thread until the current WaitHandle receives a signal, using 32-bit signed integer to specify the time interval. (Inherited from WaitHandle.)
Public methodSupported by Silverlight for Windows PhoneSupported by Xbox 360WaitOne(TimeSpan)Blocks the current thread until the current instance receives a signal, using a TimeSpan to specify the time interval. (Inherited from WaitHandle.)
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EventWaitHandle is the base class for AutoResetEvent and ManualResetEvent.

Thread synchronization events enable threads to communicate with each other by signaling. Typically, one or more threads block on an event object until an unblocked thread calls the Set method, releasing one or more of the blocked threads.

NoteNote:

Thread synchronization events are not .NET Framework events. Thread synchronization events have no event handlers, for example. For historical reasons, the term "event" has been applied to two completely different technologies.

The behavior of an event that has been signaled depends on its reset mode. An AutoResetEvent resets automatically when signaled, after releasing a single waiting thread. By contrast, a ManualResetEvent 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 tries to wait for 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 for 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 example demonstrates the use of both kinds of EventWaitHandle: AutoResetEvent and ManualResetEvent. The example shows several uses of each.

The example is run on a separate thread from the main UI thread, because event wait handles synchronize threads by blocking them. The UI thread should not be blocked, because that makes it unresponsive to user actions. Blocking the UI thread also blocks any cross-thread calls to UI elements that you make using the Dispatcher, a technique that this example relies on.

The example begins with a Demo method that does the following:

  • Displays an introductory message.

  • Starts the thread that controls the demo.

  • Hooks up an event handler (MouseUp) for the MouseLeftButtonUp event.

The event handler for the MouseLeftButtonUp event calls the Set method on an AutoResetEvent named areSyncDemoThread, to signal the next step of the demo to proceed. An AutoResetEvent is the best choice here, because it automatically resets after it has been signaled.

The DemoThread method, which is the thread procedure that controls the demo, performs the following steps:

  1. Starts three threads and passes an AutoResetEvent to each of them. DemoThread uses the WaitHandle.WaitAll method to block until all three threads are queued waiting for mref, which is a ManualResetEvent.

    The thread procedure that is used by these threads posts a message to the TextBlock before it calls the Set method on its AutoResetEvent. The thread then calls WaitOne on mref, blocking until DemoThread releases it. The thread procedure posts an "end" message to the TextBlock and signals the AutoResetEvent again to let DemoThread know that the message has been posted. Here, too, AutoResetEvent is the best choice because it resets automatically.

    Important noteImportant Note:

    The example displays its output in a TextBlock on the UI thread. To access the TextBlock from the callback thread, the example uses the Dispatcher property to obtain a Dispatcher object for the TextBlock, and then uses the Dispatcher.BeginInvoke method to make the cross-thread call. In this example, the displayHelper delegate is used to invoke the DisplayOutput helper method.

  2. Calls the Set method on mref. This demonstrates that all the waiting threads are released at the same time.

  3. Starts two more threads to demonstrate that threads will not block on a ManualResetEvent as long as it remains signaled.

    NoteNote:

    When these threads are created, they are not passed an AutoResetEvent because they do not need to notify DemoThread when they call WaitOne. Instead, DemoThread uses the Thread.Join method to wait until both threads are finished.

  4. Calls the Reset method on mref, and then starts another thread to demonstrate that threads once again block.

  5. There are two parts to this step. First, the thread from step 4 is released and the TextBlock contents are cleared. Second, an AutoResetEvent (in the variable named wait) is signaled when there are no threads waiting for it. It remains in the signaled state while DemoThread queues and releases three more threads, this time using the ThreadProcARE method for the thread procedure. The first thread to wait for the signaled AutoResetEvent is the only one that does not block, because the AutoResetEvent returns to the non-signaled state as soon as it releases a thread.

    The ThreadProcARE method receives an array that contains two AutoResetEvent objects. It uses the first AutoResetEvent to signal DemoThread when it has posted its messages to the TextBlock, and the second AutoResetEvent to signal that it is ending. DemoThread uses the WaitHandle.WaitAny method to detect that at least one thread has ended.

  6. Releases the threads that are still waiting on the AutoResetEvent in the variable wait by calling Set, and disposes of the static wait handles. If the threads are not released before DemoThread ends and the variable wait goes out of scope, they will never be released, they will never run, and their resources will not be reclaimed until the application ends. Whether this is important to you depends on the nature of your application.

    Important noteImportant Note:

    When you release multiple threads that are blocked on an AutoResetEvent, remember that if two calls to Set occur too close together, the second call might not release a thread. In this example, WaitHandle.WaitAny is used to ensure that the first thread is released before Set is called again. If you comment out WaitHandle.WaitAny and run the program several times, you might observe this behavior.


using System;
using System.Threading;

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

public class Example
{
   // mre is used to block and release threads manually.
   private static ManualResetEvent mre = 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 static AutoResetEvent areSyncDemoThread = new AutoResetEvent(false);

   // All output is displayed here.
   private static TextBlock outputBlock;

   // 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 static AutoResetEvent[] autoResets = { new AutoResetEvent(false), 
                       new AutoResetEvent(false), new AutoResetEvent(false)};


   // The static Demo method starts the thread that controls the demo and hooks 
   // up the handler for the MouseLeftButtonUp event.
   public static void Demo(TextBlock outputBlock)
   {
      Example.outputBlock = outputBlock;
      outputBlock.Text += "Click here to begin the demo.\n\n";

      Thread t = new Thread(DemoThread);
      t.Start();

      outputBlock.MouseLeftButtonUp += new MouseButtonEventHandler(MouseUp);
   }


   // 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 static bool clear = false;
   private static void MouseUp(object sender, MouseButtonEventArgs e)
   {
      // If the clear flag has been set, clear the contents of the TextBlock.
      if (clear)
      {
         outputBlock.Text = "";
         clear = false;
      }
      //outputBlock.Text += "Click.\n"

      // Signal the next step of the demo to proceed.
      areSyncDemoThread.Set();
   }


   // 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 static void DemoThread()
   {
      // Wait for a mouse click.
      areSyncDemoThread.WaitOne();


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

      for(int i = 0; i < 3; i++)
      {
         Thread t = new Thread(ThreadProc);
         t.Name = "Thread_" + i;
         t.Start(autoResets[i]);
      }

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

      outputBlock.Dispatcher.BeginInvoke(displayHelper, 
         "\n3 threads are queued, waiting for the ManualResetEvent. Click to signal\n" + 
         "the ManualResetEvent by calling its Set() method.\n\n");

      // 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, 
         "\nAll the threads were released, and the ManualResetEvent remains in the\n" + 
         "signaled state. Click to start more threads.\n\n");

      // 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.

      Thread[] twoMoreThreads = {new Thread(ThreadProc), new Thread(ThreadProc)};
      for(int i = 0; i < 2; i++)
      {
         twoMoreThreads[i].Name = "Thread_" + (i + 3);
         twoMoreThreads[i].Start(null);
      }

      // Wait until the threads have displayed their messages and finished 
      // executing.
      foreach (Thread t in twoMoreThreads)
      {
         t.Join();
      }

      outputBlock.Dispatcher.BeginInvoke(displayHelper, 
         "\nAs long as the ManualResetEvent remains in the signaled state, threads\n" + 
         "that wait on it do not block. Click to reset the ManualResetEvent.\n");

      // Wait for a mouse click.
      areSyncDemoThread.WaitOne();


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

      outputBlock.Dispatcher.BeginInvoke(displayHelper, "\nCalling mre.Reset().\n\n");
      mre.Reset();

      // Start a thread that waits on the ManualResetEvent.
      Thread t5 = new Thread(ThreadProc);
      t5.Name = "Thread_5";
      AutoResetEvent wait = new AutoResetEvent(false);
      t5.Start(wait);

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

      outputBlock.Dispatcher.BeginInvoke(displayHelper, 
         "\nWith the ManualResetEvent in the unsignaled state, threads once again block.\n" + 
         "Click to release the thread.\n\n");

      // Signal the MouseUp event to clear the screen, and wait for a click.
      clear = true;
      areSyncDemoThread.WaitOne();

      // Release the waiting thread, and block until it ends.
      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, 
         "\nIf an AutoResetEvent is signaled when there is no thread waiting on it, \n" + 
         "the AutoResetEvent remains in the signaled state until a thread waits on\n" + 
         "it. That thread is immediately released, and the AutoResetEvent returns\n" + 
         "to the unsignaled state. Click here to demonstrate this.\n\n");

      // 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(int i=0; i<=2; i++)
      {
         Thread t = new Thread(ThreadProcARE);
         t.Name = "Thread_"+ (i + 6);
         t.Start(new AutoResetEvent[]{ autoResets[i], wait });
      }

      // 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.
      int winner = WaitHandle.WaitAny(autoResets);

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

      // 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\n" + 
         "each time. In order to ensure that both threads are released, a suitable delay\n" +
         "must elapse between calls to Set(). This is accomplished by calling WaitAny()\n" +
         "on the autoResets array.\n\n");
      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 static (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();
      foreach (AutoResetEvent are in autoResets)
      {
         are.Close();
      }

      // Unhook the mouse button event.
      outputBlock.Dispatcher.BeginInvoke(delegate () {
         outputBlock.MouseLeftButtonUp -= new MouseButtonEventHandler(MouseUp); });

      outputBlock.Dispatcher.BeginInvoke(displayHelper, 
         "\nTo run the demo again, refresh the page.\n");
   }


   // Thread Procedures:

   // This thread procedure is executed by most of the named threads created in 
   // this example.
   private static void ThreadProc(object state)
   {
      AutoResetEvent are = (AutoResetEvent) state;
      string name = Thread.CurrentThread.Name;

      outputBlock.Dispatcher.BeginInvoke(displayHelper, 
                                   name + " starts and calls mre.WaitOne()\n");

      // Signal that the thread is about to wait.
      if (are != null) { are.Set(); }

      // Wait until the ManualResetEvent is signaled.
      mre.WaitOne();

      outputBlock.Dispatcher.BeginInvoke(displayHelper, name + " ends.\n");

      // Signal that the thread is about to exit.
      if (are != null) { are.Set(); }
   }


   // 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 static void ThreadProcARE(object state)
   {
      // Get two AutoResetEvent objects, one to signal when this thread is ready
      // for release, and one to wait on. The second AutoResetEvent is already
      // in the signaled state.
      AutoResetEvent[] are = (AutoResetEvent[]) state;
      string name = 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.\n");

      // Wait on the previously signaled AutoResetEvent.
      are[1].WaitOne();

      outputBlock.Dispatcher.BeginInvoke(displayHelper, name + " ends.\n");

      // Signal that this thread is ready to end.
      are[0].Set();
   }


   // 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.
   //
   private static Action<string> displayHelper = new Action<string>(DisplayOutput);
   private static void DisplayOutput(string msg)
   {
      outputBlock.Text += msg;
   }
}

/* 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.
 */


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

This type is thread safe.

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