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Timer.Stop Method

Stops raising the Elapsed event by setting Enabled to false.

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

public void Stop()

You can also stop timing by setting Enabled to false.


The signal to raise the Elapsed event is always queued for execution on a ThreadPool thread, so the event-handling method might run on one thread at the same time that a call to the Stop method runs on another thread. This might result in the Elapsed event being raised after the Stop method is called. The code example in the next section shows one way to work around this race condition.

The following code example shows one way to prevent the thread that calls the Stop method from continuing until a currently executing Elapsed event ends, and also to prevent two Elapsed events from executing the event handler at the same time (often referred to as reentrancy).

The example executes 100 test runs. Each time the test is run, the timer is started with an interval of 150 milliseconds. The event handler uses the Thread.Sleep method to simulate a task that randomly varies in length from 50 to 200 milliseconds. The test method also starts a control thread that waits for a second and then stops the timer. If an event is being handled when the control thread stops the timer, the control thread must wait until the event is finished before proceeding.

The Interlocked.CompareExchange(Int32, Int32, Int32) method overload is used to avoid reentrancy and to prevent the control thread from continuing until an executing event ends. The event handler uses the CompareExchange(Int32, Int32, Int32) method to set a control variable to 1, but only if the value is currently zero. This is an atomic operation. If the return value is zero, the control variable has been set to 1 and the event handler proceeds. If the return value is non-zero, the event is simply discarded to avoid reentrancy. (If it were necessary to execute every event, the Monitor class would be a better way to synchronize the events.) When the event handler ends, it sets the control variable back to zero. The example records the total number of events that executed, that were discarded because of reentrancy, and that occurred after the Stop method was called.

The control thread uses the CompareExchange(Int32, Int32, Int32) method to set the control variable to -1 (minus one), but only if the value is currently zero. If the atomic operation returns non-zero, an event is currently executing. The control thread waits and tries again. The example records the number of times the control thread had to wait for an event to finish.

using System;
using System.Timers;
using System.Threading;

public class Test
    // Change these values to control the behavior of the program.
    private static int testRuns = 100;
    // Times are given in milliseconds:
    private static int testRunsFor = 500;
    private static int timerIntervalBase = 100;
    private static int timerIntervalDelta = 20;

    // Timers.
    private static System.Timers.Timer Timer1 = new System.Timers.Timer();
    private static System.Timers.Timer Timer2 = new System.Timers.Timer();
    private static System.Timers.Timer currentTimer = null;

    private static Random rand = new Random();

    // This is the synchronization point that prevents events
    // from running concurrently, and prevents the main thread 
    // from executing code after the Stop method until any 
    // event handlers are done executing.
    private static int syncPoint = 0;

    // Count the number of times the event handler is called,
    // is executed, is skipped, or is called after Stop.
    private static int numEvents = 0;
    private static int numExecuted = 0;
    private static int numSkipped = 0;
    private static int numLate = 0;

    // Count the number of times the thread that calls Stop
    // has to wait for an Elapsed event to finish.
    private static int numWaits = 0;

    public static void Main()
        Timer1.Elapsed += new ElapsedEventHandler(Timer1_ElapsedEventHandler);
        Timer2.Elapsed += new ElapsedEventHandler(Timer2_ElapsedEventHandler);

        for(int i = 1; i <= testRuns; i++)
            Console.Write("\rTest {0}/{1}    ", i, testRuns);

        Console.WriteLine("{0} test runs completed.", testRuns);
        Console.WriteLine("{0} events were raised.", numEvents);
        Console.WriteLine("{0} events executed.", numExecuted);
        Console.WriteLine("{0} events were skipped for concurrency.", numSkipped);
        Console.WriteLine("{0} events were skipped because they were late.", numLate);
        Console.WriteLine("Control thread waited {0} times for an event to complete.", numWaits);

    public static void TestRun()
        // Set syncPoint to zero before starting the test 
        // run. 
        syncPoint = 0;

        // Test runs alternate between Timer1 and Timer2, to avoid
        // race conditions between tests, or with very late events.
        if (currentTimer == Timer1)
            currentTimer = Timer2;
            currentTimer = Timer1;

        currentTimer.Interval = timerIntervalBase 
            - timerIntervalDelta + rand.Next(timerIntervalDelta * 2);
        currentTimer.Enabled = true;

        // Start the control thread that shuts off the timer.
        Thread t = new Thread(ControlThreadProc);

        // Wait until the control thread is done before proceeding.
        // This keeps the test runs from overlapping.


    private static void ControlThreadProc()
        // Allow the timer to run for a period of time, and then 
        // stop it.

        // The 'counted' flag ensures that if this thread has
        // to wait for an event to finish, the wait only gets 
        // counted once.
        bool counted = false;

        // Ensure that if an event is currently executing,
        // no further processing is done on this thread until
        // the event handler is finished. This is accomplished
        // by using CompareExchange to place -1 in syncPoint,
        // but only if syncPoint is currently zero (specified
        // by the third parameter of CompareExchange). 
        // CompareExchange returns the original value that was
        // in syncPoint. If it was not zero, then there's an
        // event handler running, and it is necessary to try
        // again.
        while (Interlocked.CompareExchange(ref syncPoint, -1, 0) != 0)
            // Give up the rest of this thread's current time
            // slice. This is a naive algorithm for yielding.

            // Tally a wait, but don't count multiple calls to
            // Thread.Sleep.
            if (!counted)
                numWaits += 1;
                counted = true;

        // Any processing done after this point does not conflict
        // with timer events. This is the purpose of the call to
        // CompareExchange. If the processing done here would not
        // cause a problem when run concurrently with timer events,
        // then there is no need for the extra synchronization.

    // Event-handling methods for the Elapsed events of the two
    // timers.
    private static void Timer1_ElapsedEventHandler(object sender, 
        ElapsedEventArgs e)
        HandleElapsed(sender, e);

    private static void Timer2_ElapsedEventHandler(object sender, 
        ElapsedEventArgs e)
        HandleElapsed(sender, e);

    private static void HandleElapsed(object sender, ElapsedEventArgs e)
        numEvents += 1;

        // This example assumes that overlapping events can be
        // discarded. That is, if an Elapsed event is raised before 
        // the previous event is finished processing, the second
        // event is ignored. 
        // CompareExchange is used to take control of syncPoint, 
        // and to determine whether the attempt was successful. 
        // CompareExchange attempts to put 1 into syncPoint, but
        // only if the current value of syncPoint is zero 
        // (specified by the third parameter). If another thread
        // has set syncPoint to 1, or if the control thread has
        // set syncPoint to -1, the current event is skipped. 
        // (Normally it would not be necessary to use a local 
        // variable for the return value. A local variable is 
        // used here to determine the reason the event was 
        // skipped.)
        int sync = Interlocked.CompareExchange(ref syncPoint, 1, 0);
        if (sync == 0)
            // No other event was executing.
            // The event handler simulates an amount of work
            // lasting between 50 and 200 milliseconds, so that
            // some events will overlap.
            int delay = timerIntervalBase 
                - timerIntervalDelta / 2 + rand.Next(timerIntervalDelta);
            numExecuted += 1;

            // Release control of syncPoint.
            syncPoint = 0;
            if (sync == 1) { numSkipped += 1; } else { numLate += 1; }

/* On a dual-processor computer, this code example produces
   results similar to the following:

Test 100/100    100 test runs completed.
436 events were raised.
352 events executed.
84 events were skipped for concurrency.
0 events were skipped because they were late.
Control thread waited 77 times for an event to complete.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 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.