RegisteredWaitHandle.Unregister Method (WaitHandle)


Cancels a registered wait operation issued by the RegisterWaitForSingleObject method.

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

public bool Unregister(
	WaitHandle waitObject


Type: System.Threading.WaitHandle

The WaitHandle to be signaled.

Return Value

Type: System.Boolean

true if the function succeeds; otherwise, false.

If waitObject is specified, it is signaled only if the RegisteredWaitHandle is successfully unregistered. If a callback method is in progress when Unregister executes, waitObject is not signaled until the callback method completes. In particular, if a callback method executes Unregister, waitObject is not signaled until that callback method completes.

The following example shows how to use the Unregister method to unregister a task if a callback occurred because the wait handle was signaled.

The example also shows how to use the RegisterWaitForSingleObject method to execute a specified callback method when a specified wait handle is signaled. In this example, the callback method is WaitProc, and the wait handle is an AutoResetEvent.

The example defines a TaskInfo class to hold the information that is passed to the callback when it executes. The example creates a TaskInfo object and assigns it some string data. The RegisteredWaitHandle that is returned by the RegisterWaitForSingleObject method is assigned to the Handle field of the TaskInfo object so that the callback method has access to the RegisteredWaitHandle.

In addition to specifying TaskInfo as the object to pass to the callback method, the call to the RegisterWaitForSingleObject method specifies the AutoResetEvent that the task will wait for, a WaitOrTimerCallback delegate that represents the WaitProc callback method, a one second time-out interval, and multiple callbacks.

When the main thread signals the AutoResetEvent by calling its Set method, the WaitOrTimerCallback delegate is invoked. The WaitProc method tests RegisteredWaitHandle to determine whether a time-out occurred. If the callback was invoked because the wait handle was signaled, the WaitProc method unregisters the RegisteredWaitHandle, stopping additional callbacks. In the case of a time-out, the task continues to wait. The WaitProc method ends by printing a message to the console.

using System;
using System.Threading;

// TaskInfo contains data that will be passed to the callback
// method.
public class TaskInfo {
    public RegisteredWaitHandle Handle = null;
    public string OtherInfo = "default";

public class Example {
    public static void Main(string[] args) {
        // The main thread uses AutoResetEvent to signal the
        // registered wait handle, which executes the callback
        // method.
        AutoResetEvent ev = new AutoResetEvent(false);

        TaskInfo ti = new TaskInfo();
        ti.OtherInfo = "First task";
        // The TaskInfo for the task includes the registered wait
        // handle returned by RegisterWaitForSingleObject.  This
        // allows the wait to be terminated when the object has
        // been signaled once (see WaitProc).
        ti.Handle = ThreadPool.RegisterWaitForSingleObject(
            new WaitOrTimerCallback(WaitProc),

        // The main thread waits three seconds, to demonstrate the
        // time-outs on the queued thread, and then signals.
        Console.WriteLine("Main thread signals.");

        // The main thread sleeps, which should give the callback
        // method time to execute.  If you comment out this line, the
        // program usually ends before the ThreadPool thread can execute.
        // If you start a thread yourself, you can wait for it to end
        // by calling Thread.Join.  This option is not available with 
        // thread pool threads.

    // The callback method executes when the registered wait times out,
    // or when the WaitHandle (in this case AutoResetEvent) is signaled.
    // WaitProc unregisters the WaitHandle the first time the event is 
    // signaled.
    public static void WaitProc(object state, bool timedOut) {
        // The state object must be cast to the correct type, because the
        // signature of the WaitOrTimerCallback delegate specifies type
        // Object.
        TaskInfo ti = (TaskInfo) state;

        string cause = "TIMED OUT";
        if (!timedOut) {
            cause = "SIGNALED";
            // If the callback method executes because the WaitHandle is
            // signaled, stop future execution of the callback method
            // by unregistering the WaitHandle.
            if (ti.Handle != null)

        Console.WriteLine("WaitProc( {0} ) executes on thread {1}; cause = {2}.",

.NET Framework
Available since 1.1
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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