Unregister Method
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RegisteredWaitHandle.Unregister Method

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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 is finished. In particular, if a callback method executes Unregister, waitObject is not signaled until that callback method is finished.

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 displays its output in a TextBlock on the UI thread. To access the TextBlock from the callback thread, the WaitProc method 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.

using System;
using System.Threading;

// TaskInfo contains data that will be passed to the callback 
// method.
public class TaskInfo
   public RegisteredWaitHandle Handle = null;
   public System.Windows.Controls.TextBlock OutputBlock = null;

public class Example
    // The Demo method runs the example. It sets up an event handler to 
    // signal the wait handle, saves the TextBlock that is  used for 
    // output, and finally registers the wait handle.
    public static void Demo(System.Windows.Controls.TextBlock outputBlock)
        outputBlock.Text = "Click here signal the wait handle.\r\n";

        // Create the wait handle that the example waits on.
        AutoResetEvent ev = new AutoResetEvent(false);

        // Set up an event handler to signal the wait handle when the 
        // TextBlock is clicked.
        outputBlock.MouseLeftButtonUp += (object sender, 
            System.Windows.Input.MouseButtonEventArgs e) => ev.Set();

        // Create a TaskInfo and save the TextBlock that the example uses
        // for output.
        TaskInfo ti = new TaskInfo();
        ti.OutputBlock = outputBlock;

        // Set the Handle property of the TaskInfo to the registered wait
        // handle that is returned by RegisterWaitForSingleObject. This 
        // enables the wait to be terminated when the handle has been 
        // signaled once (see WaitProc).
        ti.Handle = ThreadPool.RegisterWaitForSingleObject(
            new WaitOrTimerCallback(WaitProc),

    // 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)
        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.

        ti.OutputBlock.Dispatcher.BeginInvoke(delegate () {
            ti.OutputBlock.Text += 
                String.Format("WaitProc is running; cause = {0}.\n", cause);

/* This example produces output similar to the following:

Click here to signal the wait handle.
WaitProc is running; cause = TIMED OUT.
WaitProc is running; cause = TIMED OUT.
WaitProc is running; cause = TIMED OUT.
WaitProc is running; cause = SIGNALED.

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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