Timer Constructor (TimerCallback, Object, Int32, Int32)

Initializes a new instance of the Timer class, using a 32-bit signed integer to specify the time interval.

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

<SecuritySafeCriticalAttribute> _
Public Sub New ( _
	callback As TimerCallback, _
	state As Object, _
	dueTime As Integer, _
	period As Integer _


Type: System.Threading.TimerCallback
A delegate that represents a method to be executed.
Type: System.Object
An object that contains information to be used by the callback method, or Nothing.
Type: System.Int32
The amount of time to delay before callback is invoked, in milliseconds. Specify Timeout.Infinite to prevent the timer from starting. Specify 0 (zero) to start the timer immediately.
Type: System.Int32
The time interval between invocations of callback, in milliseconds. Specify Timeout.Infinite to disable periodic signaling.


The dueTime or period parameter is negative and is not equal to Timeout.Infinite.


The callback parameter is Nothing.

The delegate specified by the callback parameter is invoked once after dueTime elapses, and thereafter each time the period time interval elapses.


Visual Basic users can omit the TimerCallback constructor, and simply use the AddressOf operator when specifying the callback method. Visual Basic automatically calls the correct delegate constructor.

If dueTime is 0 (zero), callback is invoked immediately. If dueTime is Timeout.Infinite, callback is not invoked; the timer is disabled but can be re-enabled by calling the Change method.

If period is 0 (zero) or Timeout.Infinite, and dueTime is not Timeout.Infinite, callback is invoked once; the periodic behavior of the timer is disabled but can be re-enabled using the Change method.

The method specified for callback should be reentrant, because it is called on ThreadPool threads. The method can be executed simultaneously on two thread pool threads if the timer interval is less than the time required to execute the method, or if all thread pool threads are in use and the method is queued multiple times.

The following code example shows how to create a TimerCallback delegate and initialize a new instance of the Timer class. In this example, the delegate is created implicitly. Its type is inferred by the Visual Basic and C# compilers.

This example creates a timer, uses the Timer.Change method to change its interval, and then uses the Timer.Dispose method to destroy it.

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.

Imports System.Threading

' The following Imports are not required for the timer. They merely simplify 
' the code.
Imports System.Windows.Controls
Imports System.Windows.Input

' The Example class holds a reference to the timer, and contains the 
' event handler for the MouseLeftButtonUp events that control the demo.
Public Class Example

   ' The Shared Demo method sets the starting message and creates an 
   ' instance of Example, which hooks up the handler for the MouseLeftButtonUp
   ' event.
   Public Shared Sub Demo(ByVal outputBlock As TextBlock)

      outputBlock.Text &= "Click to create the timer." & vbLf
      Dim dummy As New Example(outputBlock)

   End Sub

   ' Instance data for the demo.
   Private phase As Integer = 0
   Private t As Timer

   Public Sub New(ByVal outputBlock As TextBlock)

      ' Hook up the mouse event when a new Example object is created. Note
      ' that this keeps garbage collection from reclaiming the Example 
      ' object.
      AddHandler outputBlock.MouseLeftButtonUp, AddressOf Me.MouseUp

   End Sub

   Private Sub MouseUp(ByVal sender As Object, ByVal e As MouseButtonEventArgs)

      Dim outputBlock As TextBlock = CType(sender, TextBlock)

      If phase = 0 Then
         ' On the first click, create the timer.
         outputBlock.Text &= vbLf & "Creating the timer at " & _
            DateTime.Now.ToString("h:mm:ss.fff") & _
            ", to start in 1 second with a half-second interval." & vbLf & _
            "Click to change the interval from 1/2 second to 1 second." & vbLf & vbLf

         ' Create a timer that invokes the callback method after one second 
         ' (1000 milliseconds) and every 1/2 second thereafter. The TextBlock
         ' that is used for output is passed as the state object. Visual Basic 
         ' infers the delegate type, as if you had typed the following:
         '     New TimerCallback(AddressOf MyTimerCallback)
         t = New Timer(AddressOf MyTimerCallback, outputBlock, 1000, 500)

      ElseIf phase = 1 Then
         ' On the next click, change the timer interval to every second.
         outputBlock.Text &= vbLf & "Changing the interval to one second." & vbLf & _
            "Click to destroy the timer." & vbLf & vbLf

         t.Change(0, 1000)

         ' On the last click, destroy the timer and shut down the demo.
         outputBlock.Text &= vbLf & "Destroying the timer." & vbLf & _
            "Refresh the page to run the demo again."
         RemoveHandler outputBlock.MouseLeftButtonUp, AddressOf Me.MouseUp


      End If

      phase += 1

   End Sub

   ' The shared callback method is invoked on a ThreadPool thread by the Timer. 
   ' The state object is passed to the callback method on each invocation. In this
   ' example, the state object is the TextBlock that displays output. In order to
   ' update the TextBlock object, which is on the UI thread, you must make the  
   ' cross-thread call by using the Dispatcher object that is associated with the 
   ' TextBlock.
   Private Shared Sub MyTimerCallback(ByVal state As Object)

      Dim outputBlock As TextBlock = CType(state, TextBlock)
      Dim msg As String = DateTime.Now.ToString("h:mm:ss.fff") & _
                          " MyTimerCallback was called." & vbLf

      outputBlock.Dispatcher.BeginInvoke(displayHelper, outputBlock, msg)

   End Sub

   ' The DisplayOutput helper method and its delegate, displayHelper, are used by
   ' the BeginInvoke method of the Dispatcher object.
   Private Shared displayHelper _
                    As New Action(Of TextBlock, String)(AddressOf DisplayOutput)
   Private Shared Sub DisplayOutput(ByVal tb As TextBlock, ByVal msg As String)
      tb.Text &= msg
   End Sub

End Class

' This example produces output similar to the following:
'Click to create the timer.
'Creating the timer at 3:17:36.980, to start in 1 second with a half-second interval.
'Click to change the interval from 1/2 second to 1 second.
'3:17:38.072 MyTimerCallback was called.
'3:17:38.586 MyTimerCallback was called.
'3:17:39.101 MyTimerCallback was called.
'3:17:39.580 MyTimerCallback was called.
'Changing the interval to one second.
'Click to destroy the timer.
'3:17:39.656 MyTimerCallback was called.
'3:17:40.689 MyTimerCallback was called.
'Destroying the timer.
'Refresh the page to run the demo again.


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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