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Timer.Elapsed Event

Occurs when the interval elapses.

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

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

The Elapsed event is raised if the Enabled property is true and the time interval (in milliseconds) defined by the Interval property elapses. If the AutoReset property is true, the event is raised repeatedly at an interval defined by the Interval property; otherwise, the event is raised only once, the first time the Interval value 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 example instantiates a Timer object that fires its Timer.Elapsed event every two seconds (2000 milliseconds), sets up an event handler for the event, and starts the timer. The event handler displays the value of the ElapsedEventArgs.SignalTime property each time it is raised.

// Use this code inside a project created with the Visual C# > Windows Desktop > Console Application template.  
// Replace the code in Program.cs with this code.  

using System;

// To avoid confusion with other Timer classes, this sample always uses the fully-qualified 
// name of System.Timers.Timer instead of a using statement for System.Timers. 

public class Example
{
    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 and set a two second interval.
        aTimer = new System.Timers.Timer();
        aTimer.Interval = 2000;

        // Alternate method: create a Timer with an interval argument to the constructor. 
        //aTimer = new System.Timers.Timer(2000); 

        // Create a timer with a two second interval.
        aTimer = new System.Timers.Timer(2000);

        // Hook up the Elapsed event for the timer. 
        aTimer.Elapsed += OnTimedEvent;

        // Have the timer fire repeated events (true is the default)
        aTimer.AutoReset = true;

        // Start the timer
        aTimer.Enabled = true;

        Console.WriteLine("Press the Enter key to exit the program at any time... ");
        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) 
    }

    private static void OnTimedEvent(Object source, System.Timers.ElapsedEventArgs e)
    {
        Console.WriteLine("The Elapsed event was raised at {0}", e.SignalTime);
    }
}

// This example displays output like the following:  
//       Press the Enter key to exit the program at any time...  
//       The Elapsed event was raised at 5/20/2015 8:48:58 PM  
//       The Elapsed event was raised at 5/20/2015 8:49:00 PM  
//       The Elapsed event was raised at 5/20/2015 8:49:02 PM  
//       The Elapsed event was raised at 5/20/2015 8:49:04 PM  
//       The Elapsed event was raised at 5/20/2015 8:49:06 PM 

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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