c# event handler being added twice

Question

This is a fictional example but I was wandering what happens if the InitialiseTimer function gets called twice. Does the timer elapsed function get triggered twice. Will this change if the functions are made static?

    private static void InitialiseTimer()
    {
            TheTimer = new System.Timers.Timer();
            TheTimer.Interval = 400;
            TheTimer.Elapsed += new ElapsedEventHandler(TheTimer_Elapsed);
            TheTimer.AutoReset = false;
    }   

    public void TheTimer_Elapsed(object sender, ElapsedEventArgs e)
    {
        //Do stuff in here
    }

I was going to use below to prevent this

Has an event handler already been added?

Thanks, Richard

score 5 · Answer 1 · answered Mar 03 '11 at 12:32

If you register the event handler twice, it will be invoked twice every time the event is raised.

This won't change if you make TheTimer_Elapsed static, because you'll still hold two references to this static method.

In most cases there's no need to write compicated things like what Blair Conrad posted in the question you linked to. Just don't forget to use -= every time you have += and you'll be safe.

score 4 · Accepted Answer · answered Mar 03 '11 at 12:54

I think the following demonstrates the scenario and does indeed fire twice, also propose a simple change (commented code) to the Init method that should fix the behavior. (Not thread safe btw, additional locks would be required)

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        var counter = 0;
        var ts = new ThreadStart(() =>
            {

                Foo.Fired += (o, e) =>
                    {
                        counter++;
                    };
                Foo.InitialiseTimer();
                Foo.InitialiseTimer();
            });
        var t = new Thread(ts);
        t.Start();

        Thread.Sleep(30);
        Assert.AreEqual(1, counter);
    }
}

public class Foo
{
    private static System.Timers.Timer TheTimer = null;

    public static event EventHandler Fired;

    public static void InitialiseTimer()
    {
        //if (TheTimer != null)
        //{
        //    TheTimer.Stop();
        //    TheTimer = null;
        //}
        TheTimer = new System.Timers.Timer();
        TheTimer.Interval = 10;
        TheTimer.Elapsed += new ElapsedEventHandler(TheTimer_Elapsed);
        TheTimer.AutoReset = false;
        TheTimer.Start();
    }

    public static void TheTimer_Elapsed(object sender, ElapsedEventArgs e)
    {
        //Do stuff in here
        if (Fired != null)
        {
            Fired(null, null);
        }
    }
}

score 2 · Answer 3 · answered Mar 03 '11 at 12:35

if you call the method InitialiseTimer twice you will create two Timers each of them will have only one event handler attached but they might elapse both. It's not really about having the method static or not, it's more about the method itself, you could check if TheTimer is null and do the rest only if it's null so you assign it only once.

score 1 · Answer 4 · answered Mar 03 '11 at 12:33

1

If event is registered twice you will have two executions.

You can check if event is null, and the problem will be solved.

answered Mar 03 '11 at 12:33

buda

2,372
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How does checking for null solve this? What if other handlers have been registered for this event? – Ilya Kogan Mar 03 '11 at 12:34
That would be a null-check on the newly created `Timer` instance, not the one that was created in the previous call, so that does not really solve anything. However, null checking the field `TheTimer` would solve the problem. – Fredrik Mörk Mar 03 '11 at 12:38
Of course, i thought that you will have to check if event is null on same timer, not on newly created. – buda Mar 03 '11 at 14:45

score 1 · Answer 5 · answered Mar 03 '11 at 12:36

1

Static or not, you are recreating the Timer. So you can invoke the InitialiseTimer many, many times without adding more than a single handler. You will end up with many timers though...

answered Mar 03 '11 at 12:36

Emond

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c# event handler being added twice

5 Answers5