Export (0) Print
Expand All

EventWaitHandle.Set Method

Updated: September 2010

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

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

[SecuritySafeCriticalAttribute]
public bool Set()

Return Value

Type: System.Boolean
true if the operation succeeds; otherwise, false.

ExceptionCondition
ObjectDisposedException

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

For an AutoResetEvent, the Set method releases a single thread. If there are no waiting threads, the wait handle remains signaled until a thread tries to wait for it, or until its Reset method is called.

Important noteImportant Note:

There is no guarantee that every call to the Set method will release a thread from an AutoResetEvent. If two calls are so close together 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 occur. Also, if Set is called when there are no threads waiting and the AutoResetEvent is already signaled, the call has no effect.

For a ManualResetEvent, calling the Set method leaves the wait handle in a signaled state until its Reset method is called.

The following example demonstrates the use of the Set method with both kinds of EventWaitHandle: AutoResetEvent and ManualResetEvent.

NoteNote:

For a more detailed description of this example, see the comments in the code or the full introduction to the example that is provided for the EventWaitHandle class.

The Set method is used to signal an AutoResetEvent in the MouseUp method, which handles the MouseLeftButtonUp event. The thread that runs the example blocks on this AutoResetEvent between steps, so that clicking the mouse runs the next step. AutoResetEvent is a good choice because it automatically resets to the non-signaled state.

The Set method is also used with AutoResetEvent in the ThreadProc and ThreadProcARE thread procedures. In these cases, the AutoResetEvent is passed to the thread procedure as a parameter. The main thread, DemoThread, waits for the AutoResetEvent until ThreadProc or ThreadProcARE signals it to proceed by calling Set. This ensures that the threads in the example are coordinated, particularly with respect to the order in which they append messages to the TextBlock control that displays the output from the example.

The Set method is used with ManualResetEvent to release several threads at the same time, in step 2 and step 5b.

In step 6, the example shows one way to ensure that two calls to Set do not occur too close together, when you release multiple threads that are blocked on an AutoResetEvent. In this example, WaitHandle.WaitAny is used to ensure that the first thread is released and finishes running before Set is called again. If you comment out WaitHandle.WaitAny and run the program several times, you will very likely see cases in which only one thread is released.


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.

Date

History

Reason

September 2010

Added missing exception.

Customer feedback.

Community Additions

ADD
Show:
© 2014 Microsoft