WaitOrTimerCallback Delegate

Represents a method to be called when a WaitHandle is signaled or times out.

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

<ComVisibleAttribute(True)> _
Public Delegate Sub WaitOrTimerCallback ( _
	state As Object, _
	timedOut As Boolean _


Type: System.Object
An object that contains information to be used by the callback method every time it executes.
Type: System.Boolean
true to indicate that the WaitHandle timed out; false to indicate that it was signaled.

WaitOrTimerCallback represents a callback method that you want to execute when a registered wait handle times out or is signaled. Create the delegate by passing your callback method to the WaitOrTimerCallback constructor. Your method must have the signature shown in the Syntax section.

Create the registered wait handle by passing the WaitOrTimerCallback delegate and a WaitHandle to the ThreadPool.RegisterWaitForSingleObject method. Your callback method executes every time the WaitHandle times out or is signaled.


Visual Basic and C# users can omit the WaitOrTimerCallback constructor, because the compilers infer the delegate type automatically and supply the correct constructor. In Visual Basic, use the AddressOf operator when passing the callback method to RegisterWaitForSingleObject.

If you want to pass information to your callback method, create an object that contains the necessary information and pass it to RegisterWaitForSingleObject when you create the registered wait handle. Every time your callback method executes, the state parameter contains this object.

For more information about how to use callback methods to synchronize thread pool threads, see The Managed Thread Pool.

The following example shows how to use the WaitOrTimerCallback delegate to represent a callback method that is executed when a wait handle is 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 waiting.

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.

Imports System
Imports System.Threading

' TaskInfo contains data that will be passed to the callback 
' method.
Public Class TaskInfo
    Public Handle As RegisteredWaitHandle = Nothing
    Public OutputBlock As System.Windows.Controls.TextBlock = Nothing
End Class 

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 Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock) 

        outputBlock.Text = "Click here signal the wait handle." + vbCrLf

        ' Create the wait handle that the example waits on.
        Dim ev As New AutoResetEvent(False)

        ' Set up an event handler to signal the wait handle when the 
        ' TextBlock is clicked.
        AddHandler outputBlock.MouseLeftButtonUp, Function (sender As Object, _
             e As System.Windows.Input.MouseButtonEventArgs) ev.Set()

        ' Create a TaskInfo and save the TextBlock that the example uses
        ' for output.
        Dim ti As 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( _
            ev, _
            New WaitOrTimerCallback(AddressOf WaitProc), _
            ti, _
            1000, _

    End Sub 

    ' 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 Shared Sub WaitProc(ByVal state As Object, ByVal timedOut As Boolean) 
        Dim ti As TaskInfo = CType(state, TaskInfo)

        Dim cause As String = "TIMED OUT"
        If Not timedOut Then
            cause = "SIGNALED"

            ' If the callback method executes because the WaitHandle is
            ' signaled, stop future execution of the callback method
            ' by unregistering the WaitHandle.
        End If

        ' A UI element can be accessed safely only if the code is run on
        ' the main UI thread, by calling the BeginInvoke method for the 
        ' UI element's Dispatcher. The following code creates a generic
        ' Action delegate (with parameter types TextBlock and String) to
        ' invoke the DisplayOut helper method, and then calls the 
        ' BeginInvoke method, passing the delegate, the TextBlock, and 
        ' the reason the callback was called. 
        Dim display As New Action(Of System.Windows.Controls.TextBlock, String)( _
                AddressOf DisplayOutput)

        ti.OutputBlock.Dispatcher.BeginInvoke(display, ti.OutputBlock, cause)

    End Sub

    ' The Dispatcher.BeginInvoke method runs this helper method on the 
    ' UI thread, so it can safely access the TextBlock that is used to 
    ' display the output.
    Private Shared Sub DisplayOutput(ByVal outputBlock As _
        System.Windows.Controls.TextBlock, ByVal cause As String)

        outputBlock.Text &= _
            String.Format("WaitProc is running; cause = {0}." & vbLf, cause)
    End Sub 
End Class 

' 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.


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.

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