Timer.Elapsed Event

Updated: May 2010

Occurs when the interval elapses.

Namespace:  System.Timers
Assembly:  System (in System.dll)

[TimersDescriptionAttribute("TimerIntervalElapsed")]
public event ElapsedEventHandler Elapsed

If Enabled is set to true and AutoReset is set to false, the Timer raises the Elapsed event only once, the first time the interval elapses.

If Interval is set after the Timer has started, the count is reset. For example, if you set the interval to 5 seconds and then set Enabled to true, the count starts at the time Enabled is set. If you reset the interval to 10 seconds when count is 3 seconds, the Elapsed event is raised for the first time 13 seconds after Enabled was set to true.

If the SynchronizingObject property is null, the Elapsed event is raised on a ThreadPool thread. If the processing of the Elapsed event lasts longer than Interval, the event might be raised again on another ThreadPool thread. In this situation, the event handler should be reentrant.

NoteNote:

The event-handling method might run on one thread at the same time that another thread calls the Stop method or sets the Enabled property to false. This might result in the Elapsed event being raised after the timer is stopped. The example code for the Stop method shows one way to avoid this race condition.

Even if SynchronizingObject is not null, Elapsed events can occur after the Dispose or Stop method has been called or after the Enabled property has been set to false, because the signal to raise the Elapsed event is always queued for execution on a thread pool thread. One way to resolve this race condition is to set a flag that tells the event handler for the Elapsed event to ignore subsequent events.

The Timer component catches and suppresses all exceptions thrown by event handlers for the Elapsed event. This behavior is subject to change in future releases of the .NET Framework.

The following code example sets up an event handler for the Timer.Elapsed event, creates a timer, and starts the timer. The event handler displays the SignalTime property each time it is raised.

using System;
using System.Timers;

public class Timer1
{
    private static System.Timers.Timer aTimer;

    public static void Main()
    {
        // Normally, the timer is declared at the class level, 
        // so that it stays in scope as long as it is needed. 
        // If the timer is declared in a long-running method,   
        // KeepAlive must be used to prevent the JIT compiler  
        // from allowing aggressive garbage collection to occur  
        // before the method ends. You can experiment with this 
        // by commenting out the class-level declaration and  
        // uncommenting the declaration below; then uncomment 
        // the GC.KeepAlive(aTimer) at the end of the method. 
        //System.Timers.Timer aTimer; 

        // Create a timer with a ten second interval.
        aTimer = new System.Timers.Timer(10000);

        // Hook up the Elapsed event for the timer.
        aTimer.Elapsed += new ElapsedEventHandler(OnTimedEvent);

        // Set the Interval to 2 seconds (2000 milliseconds).
        aTimer.Interval = 2000;
        aTimer.Enabled = true;

        Console.WriteLine("Press the Enter key to exit the program.");
        Console.ReadLine();

        // If the timer is declared in a long-running method, use 
        // KeepAlive to prevent garbage collection from occurring 
        // before the method ends. 
        //GC.KeepAlive(aTimer);
    }

    // Specify what you want to happen when the Elapsed event is  
    // raised. 
    private static void OnTimedEvent(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("The Elapsed event was raised at {0}", e.SignalTime);
    }
}

/* This code example produces output similar to the following:

Press the Enter key to exit the program.
The Elapsed event was raised at 5/20/2007 8:42:27 PM
The Elapsed event was raised at 5/20/2007 8:42:29 PM
The Elapsed event was raised at 5/20/2007 8:42:31 PM
...
 */

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

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

Date

History

Reason

May 2010

Corrected an error in the previous update.

Content bug fix.

January 2010

Explained why Elapsed events can occur after Dispose has been called.

Customer feedback.

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