How to Pass an Argument into ElapsedEventHandler

Question

I am trying to pass an integer value into a ElapsedEventHandler so I can do logic based on the integer passed in. However, when the Event is raised, the value coming in is not the value I initialized it to. I may not be understanding completely how the delegate works.

class Example
{
    Dictionary<int, Timer> timer;

    public Example()
    {
        timer = new Dictionary<int, timer>();
        for(int i = 0; i < 12; ++i)
        {
            timer.Add(i, new Timer(5000));
            timer[i].Elapsed += delegate { TimerTickCustom(i); };
        }
    }
    public void Process() // called each cycle
    {
        for(int i = 0; i < 12; ++i)
        {
            timer[i].Start();
        }
    }
    private void TimerTickCustom(int i)
    {
        // The value of i coming in does not match the dictionary key.
    }
}

Try making a new object that extends `Timer` and creating a field there! — Ilan, Jan 12 '17 at 21:47
@Ilan321 You don't create a new timer every time you want to use data in the `tick` handler. That doesn't make conceptual sense (it's not a different kind of timer) it's way more work, and it's simply unnecessary. — Servy, Jan 12 '17 at 22:26

score 1 · Accepted Answer · edited May 23 '17 at 12:07

It depends on where the local value i is for the delegate to regard it as "specific" to it's scope. As i is defined outside the loop, the delegate doesn't define it's own "copy" of this variable - as all the delegates expect the same i.

What you need to do is assign it to a variable that's "on the same level" as the delegate itself.

Not sure if I'm using the right language here to explain it. But this should work:

class Example
{
    Dictionary<int, Timer> timer;

    public Example()
    {
        timer = new Dictionary<int, Timer>();
        for(int i = 0; i < 12; ++i)
        {
            int iInScopeOfDelegate = i;
            timer.Add(i, new Timer(5000));
            timer[i].Elapsed += delegate { TimerTickCustom(iLocalToDelegate ); };
        }
    }
    public void Process() // called each cycle
    {
        for(int i = 0; i < 12; ++i)
        {
            timer[i].Start();
        }
    }
    private void TimerTickCustom(int i)
    {
        // The value of i coming in does not match the dictionary key.
    }
}

There's some interesting discussion if you know which words to type into the search engine (which of course, you can't really know until someone tells you).

How to Pass an Argument into ElapsedEventHandler

1 Answers1