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ThreadPool.RegisterWaitForSingleObject Method (WaitHandle, WaitOrTimerCallback, Object, UInt32, Boolean)

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

Registers a delegate to wait for a WaitHandle, specifying a 32-bit unsigned integer for the time-out in milliseconds.

This API is not CLS-compliant. 

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

public static RegisteredWaitHandle RegisterWaitForSingleObject(
	WaitHandle waitObject,
	WaitOrTimerCallback callBack,
	Object state,
	uint millisecondsTimeOutInterval,
	bool executeOnlyOnce


Type: System.Threading.WaitHandle
The wait handle to register.
Type: System.Threading.WaitOrTimerCallback
The delegate to call when the waitObject parameter is signaled.
Type: System.Object
The object passed to the delegate.
Type: System.UInt32
The time-out in milliseconds. If the millisecondsTimeOutInterval parameter is 0 (zero), the function tests the object's state and returns immediately. If millisecondsTimeOutInterval is -1, the function's time-out interval never elapses.
Type: System.Boolean
true to indicate that the thread will no longer wait on the waitObject parameter after the delegate has been called; false to indicate that the timer is reset every time the wait operation completes until the wait is unregistered.

Return Value

Type: System.Threading.RegisteredWaitHandle
The registered wait handle.


The millisecondsTimeOutInterval parameter is less than -1.

When you are finished using the RegisteredWaitHandle that is returned by this method, call its RegisteredWaitHandle.Unregister method to release references to the wait handle. We recommend that you always call the RegisteredWaitHandle.Unregister method, even if you specify true for executeOnlyOnce. Garbage collection works more efficiently if you call the RegisteredWaitHandle.Unregister method instead of depending on the registered wait handle's finalizer.

The RegisterWaitForSingleObject method queues the specified delegate to the thread pool. A worker thread will execute the delegate when one of the following occurs:

  • The specified object is in the signaled state.

  • The time-out interval elapses.

The RegisterWaitForSingleObjectmethod checks the current state of the specified object's WaitHandle. If the object's state is unsignaled, the method registers a wait operation. The wait operation is performed by a thread from the thread pool. The delegate is executed by a worker thread when the object's state becomes signaled or the time-out interval elapses. If the timeOutInterval parameter is not 0 (zero) and the executeOnlyOnce parameter is false, the timer is reset every time the event is signaled or the time-out interval elapses.

To cancel the wait operation, call the RegisteredWaitHandle.Unregister method.

Before returning, the method modifies the state of some types of synchronization objects. Modification occurs only for the object whose signaled state caused the wait condition to be satisfied.

Version Notes

Windows Phone

When a user navigates away from a Windows Phone application, the application is typically put into a dormant state. When the user returns to a dormant application, the application automatically resumes. If the application is put into a dormant state while this API is being used, the API will not complete as expected. Applications should be designed to handle this possibility. For more information about the Windows Phone execution model, see Execution Model for Windows Phone.

The following example 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 displaying a message.

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.

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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