Represents the method that handles calls from a Timer.
Assembly: mscorlib (in mscorlib.dll)
<ComVisibleAttribute(True)> _ Public Delegate Sub TimerCallback ( _ state As Object _ )
[ComVisibleAttribute(true)] public delegate void TimerCallback( Object state )
[ComVisibleAttribute(true)] public delegate void TimerCallback( Object^ state )
[<ComVisibleAttribute(true)>] type TimerCallback = delegate of state:Object -> unit
Parameters
- state
- Type: System.Object
An object containing application-specific information relevant to the method invoked by this delegate, or null.
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.
Note
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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, _ DateTime.Now.ToString("h:mm:ss.fff")) 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) stateTimer.Dispose() 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"), _ invokeCount.ToString()) If invokeCount = maxCount Then ' Reset the counter and signal to stop the timer. invokeCount = 0 autoEvent.Set() End If End Sub End Class
using System; using System.Threading; class TimerExample { static void Main() { // Create an event to signal the timeout count threshold in the // timer callback. AutoResetEvent autoEvent = new AutoResetEvent(false); StatusChecker statusChecker = new StatusChecker(10); // Create an inferred delegate that invokes methods for the timer. TimerCallback tcb = 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.\n", DateTime.Now.ToString("h:mm:ss.fff")); Timer stateTimer = 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("\nChanging period.\n"); // When autoEvent signals the second time, dispose of // the timer. autoEvent.WaitOne(5000, false); stateTimer.Dispose(); Console.WriteLine("\nDestroying timer."); } } class StatusChecker { private int invokeCount; private int maxCount; public StatusChecker(int count) { invokeCount = 0; maxCount = count; } // This method is called by the timer delegate. public void CheckStatus(Object stateInfo) { AutoResetEvent autoEvent = (AutoResetEvent)stateInfo; Console.WriteLine("{0} Checking status {1,2}.", DateTime.Now.ToString("h:mm:ss.fff"), (++invokeCount).ToString()); if(invokeCount == maxCount) { // Reset the counter and signal Main. invokeCount = 0; autoEvent.Set(); } } }
using namespace System; using namespace System::Threading; ref class StatusChecker { private: int invokeCount, maxCount; public: StatusChecker(int count) { invokeCount = 0; maxCount = count; } // This method is called by the timer delegate. void CheckStatus(Object^ stateInfo) { AutoResetEvent^ autoEvent = dynamic_cast<AutoResetEvent^>(stateInfo); Console::WriteLine("{0} Checking status {1,2}.", DateTime::Now.ToString("h:mm:ss.fff"), (++invokeCount).ToString()); if (invokeCount == maxCount) { // Reset the counter and signal Main. invokeCount = 0; autoEvent->Set(); } } }; ref class TimerExample { public: static void Main() { // Create an event to signal the timeout count threshold in the // timer callback. AutoResetEvent^ autoEvent = gcnew AutoResetEvent(false); StatusChecker^ statusChecker = gcnew StatusChecker(10); // Create a delegate that invokes methods for the timer. TimerCallback^ tcb = gcnew TimerCallback(statusChecker, &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.\n", DateTime::Now.ToString("h:mm:ss.fff")); Timer^ stateTimer = gcnew 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("\nChanging period.\n"); // When autoEvent signals the second time, dispose of // the timer. autoEvent->WaitOne(5000, false); stateTimer->~Timer(); Console::WriteLine("\nDestroying timer."); } }; int main() { TimerExample::Main(); }
.NET Framework
Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0.NET Framework Client Profile
Supported in: 4, 3.5 SP1Portable Class Library
Supported in: Portable Class LibraryWindows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
Reference
Other Resources
'this timer is very accurate, but it runs on separate thread
Dim timer As System.Threading.Timer
Dim tickEventHandler As System.Threading.TimerCallback
Private Sub TimeDisplayForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
tickEventHandler = New System.Threading.TimerCallback(AddressOf timer_Tick)
timer = New System.Threading.Timer(tickEventHandler, Nothing, 1000, 1000)
End Sub
Private Sub timer_Tick(ByVal state As Object)
UpdateTimeDisplay(DateTime.Now.ToLongTimeString())
End Sub
Delegate Sub UpdateTextDelegate(ByVal text As String)
Private Sub UpdateTimeDisplay(ByVal text As String)
If (Me.InvokeRequired) Then
Dim update As New UpdateTextDelegate(AddressOf UpdateTimeDisplay)
Me.Invoke(update, New Object() {text})
End If
Me.Label1.Text = text
End Sub
Private Sub TimeDisplayForm_FormClosing(ByVal sender As System.Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles MyBase.FormClosing
Me.timer.Dispose() ' this is critical!
Me.tickEventHandler = Nothing
End Sub
End Class
----------------------------------------------------
The above is an excellent example of safe cross-thread modification of controls on a form; specifically in "UpdateTimeDisplay"
Here is the same thing in C# developed in VC# 2010 express. Pardon the formatting.
[code]
using System;
using thr=System.Threading;
using System.Windows.Forms;
namespace TimerDisplay
{
public partial class TimerDisplayForm : Form
{
public TimerDisplayForm()
{
InitializeComponent();
}
thr.Timer timer;
thr.TimerCallback tickEventHandler;
Object xstate;
private void TimerDisplayForm_Load(object sender, EventArgs e)
{
tickEventHandler = new thr.TimerCallback(timer_Tick);
xstate = new Object();
timer = new thr.Timer(tickEventHandler, null, 2000, 5000);
}
private void timer_Tick(Object state)
{
UpdateTimeDisplay(DateTime.Now.ToLongTimeString());
}
public delegate void UpdateTextDelegate(string text);
private void UpdateTimeDisplay(string text)
{/*This method is used to handle cross-thread mods
See the MSDN article "How to: Make Thread-Safe Calls to Windows Forms Controls" */
if (this.InvokeRequired) //this is true if
{
UpdateTextDelegate update = new UpdateTextDelegate( UpdateTimeDisplay);
this.Invoke(update, new object[] { text });
}
this.label1.Text = DateTime.Now.ToLongTimeString();
}
private void TimerDisplayForm_FormClosing(object sender, FormClosingEventArgs e)
{
this.timer.Dispose(); //' this is critical!
this.tickEventHandler = null;
}
}
}
[/code]
Note