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TimerCallback Delegate

Represents the method that handles calls from a Timer.

[Visual Basic]
<Serializable>
Public Delegate Sub TimerCallback( _
   ByVal state As Object _
)
[C#]
[Serializable]
public delegate void TimerCallback(
   object state
);
[C++]
[Serializable]
public __gc __delegate void TimerCallback(
   Object* state
);

[JScript] In JScript, you can use the delegates in the .NET Framework, but you cannot define your own.

Parameters [Visual Basic, C#, C++]

The declaration of your callback method must have the same parameters as the TimerCallback delegate declaration.

state
An object containing application-specific information relevant to the method invoked by this delegate, or a null reference (Nothing in Visual Basic).

Remarks

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.

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.

Example

[Visual Basic, C#, C++] The following code example shows how to create the delegate used with the Timer class.

[Visual Basic] 
Imports Microsoft.VisualBasic
Imports System
Imports System.Threading

Public Class TimerExample

    Shared Sub Main()
    
        Dim autoEvent As New AutoResetEvent(False)
        Dim statusChecker As New StatusChecker(10)

        ' Create the delegate that invokes methods for the timer.
        Dim timerDelegate 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(timerDelegate, 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

[C#] 
using System;
using System.Threading;

class TimerExample
{
    static void Main()
    {
        AutoResetEvent autoEvent     = new AutoResetEvent(false);
        StatusChecker  statusChecker = new StatusChecker(10);

        // Create the delegate that invokes methods for the timer.
        TimerCallback timerDelegate = 
            new TimerCallback(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(timerDelegate, 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
{
    int invokeCount, 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();
        }
    }
}

[C++] 
#using <mscorlib.dll>
using namespace System;
using namespace System::Threading;

__gc class StatusChecker
{
    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(S"{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();
        }
    }
};

void main()
{
    AutoResetEvent* autoEvent     = new AutoResetEvent(false);
    StatusChecker*  statusChecker = new StatusChecker(10);

    // Create the delegate that invokes methods for the timer.
    TimerCallback* timerDelegate = 
        new 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(S"{0} Creating timer.\n", 
        DateTime::Now.ToString("h:mm:ss.fff"));
    Timer* stateTimer = 
        new Timer(timerDelegate, autoEvent, 1000, 250);

    // When autoEvent signals, change the period to every 1/2 second.
    autoEvent->WaitOne(5000, false);
    stateTimer->Change(0, 500);
    Console::WriteLine(S"\nChanging period.\n");

    // When autoEvent signals the second time, dispose of the timer.
    autoEvent->WaitOne(5000, false);
    stateTimer->Dispose();
    Console::WriteLine(S"\nDestroying timer.");
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Threading

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework

Assembly: Mscorlib (in Mscorlib.dll)

See Also

System.Threading Namespace | Timer | ThreadPool | Timer

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