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TimerCallback Delegate

Represents the method that handles calls from a Timer.

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

<ComVisibleAttribute(True)> _
Public Delegate Sub TimerCallback ( _
	state As Object _


Type: System.Object

An object containing application-specific information relevant to the method invoked by this delegate, or Nothing.

Use a TimerCallback delegate to specify the method that is called by a Timer. This method does not execute in the thread that created the timer; it executes in a separate thread pool thread that is provided by the system. The TimerCallback delegate invokes the method once after the start time elapses, and continues to invoke it once per timer interval until the Dispose method is called, or until the Timer.Change method is called with the interval value Infinite.


Callbacks can occur after the Dispose method overload has been called, because the timer queues callbacks for execution by thread pool threads. You can use the Dispose(WaitHandle) method overload to wait until all callbacks have completed.

The timer delegate is specified when the timer is constructed, and cannot be changed. The start time for a Timer is passed in the dueTime parameter of the Timer constructors, and the period is passed in the period parameter. For an example that demonstrates creating and using a TimerCallback delegate, see System.Threading.Timer.

The following code example shows how to create the delegate used with the Timer class.

Imports Microsoft.VisualBasic
Imports System
Imports System.Threading

Public Class TimerExample

    <MTAThread> _
    Shared Sub Main()

        ' Create an event to signal the timeout count threshold in the 
        ' timer callback. 
        Dim autoEvent As New AutoResetEvent(False)

        Dim statusChecker As New StatusChecker(10)

        ' Create an inferred delegate that invokes methods for the timer. 
        Dim tcb As TimerCallback = AddressOf statusChecker.CheckStatus

        ' Create a timer that signals the delegate to invoke 
        ' CheckStatus after one second, and every 1/4 second 
        ' thereafter.
        Console.WriteLine("{0} Creating timer." & vbCrLf, _
        Dim stateTimer As Timer = New Timer(tcb, autoEvent, 1000, 250)

        ' When autoEvent signals, change the period to every  
        ' 1/2 second.
        autoEvent.WaitOne(5000, False)
        stateTimer.Change(0, 500)
        Console.WriteLine(vbCrLf & "Changing period." & vbCrLf)

        ' When autoEvent signals the second time, dispose of  
        ' the timer.
        autoEvent.WaitOne(5000, False)
        Console.WriteLine(vbCrLf & "Destroying timer.")

    End Sub 
End Class 

Public Class StatusChecker

    Dim invokeCount, maxCount As Integer  

    Sub New(count As Integer)
        invokeCount  = 0
        maxCount = count
    End Sub 

    ' This method is called by the timer delegate. 
    Sub CheckStatus(stateInfo As Object)
        Dim autoEvent As AutoResetEvent = _
            DirectCast(stateInfo, AutoResetEvent)
        invokeCount += 1
        Console.WriteLine("{0} Checking status {1,2}.", _
            DateTime.Now.ToString("h:mm:ss.fff"), _

        If invokeCount = maxCount Then 
            ' Reset the counter and signal to stop the timer.
            invokeCount  = 0
        End If 
    End Sub 

End Class

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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