Anonymous method for event handler in for loop

Question

Can this be done in a for loop?

        TickEventArgs targs1 = new TickEventArgs(lbl1_up_time, _elapsedTime_up1);
        timer_up1.Tick += (sender, e) => Tick(targs1);

        TickEventArgs targs2 = new TickEventArgs(lbl2_up_time, _elapsedTime_up2);
        timer_up2.Tick += (sender, e) => Tick(targs2);

        TickEventArgs targs3 = new TickEventArgs(lbl3_up_time, _elapsedTime_up3);
        timer_up3.Tick += (sender, e) => Tick(targs3);

        TickEventArgs targs4 = new TickEventArgs(lbl4_up_time, _elapsedTime_up4);
        timer_up4.Tick += (sender, e) => Tick(targs4);

        TickEventArgs targs5 = new TickEventArgs(lbl5_up_time, _elapsedTime_up5);
        timer_up5.Tick += (sender, e) => Tick(targs5);

This doesnt work because i is out of bounds (5)

        targs[0] = new TickEventArgs(lbl1_up_time, _elapsedTime_up1);
        targs[1] = new TickEventArgs(lbl2_up_time, _elapsedTime_up2);
        targs[2] = new TickEventArgs(lbl3_up_time, _elapsedTime_up3);
        targs[3] = new TickEventArgs(lbl4_up_time, _elapsedTime_up4);
        targs[4] = new TickEventArgs(lbl5_up_time, _elapsedTime_up5);

        timers[0] = timer_up1;
        timers[1] = timer_up2;
        timers[2] = timer_up3;
        timers[3] = timer_up4;
        timers[4] = timer_up5;

        int i = 0;

        for (i = 0; i <= 4; i++)
        {
            timers[i].Tick += (sender, e) => Tick(targs[i]);
        } 

Answer 1

This is coming from the lambda expression; i is shared between all of them. By the time the function is executed they're essentially being called like timers[i].Tick += (sender, e) => Tick(targs[5]).

To avoid this, create a locally scoped variable (int locali = i) and use that in your line instead. This will make sure that each lambda expression actually gets the value you expect.

for (i = 0; i <= 4; i++)
{
    int locali = i;
    timers[locali].Tick += (sender, e) => Tick(targs[locali]);
} 

i becomes 5 from the last iteration of your loop before exiting. Naturally, you don't have a targs[5] element, so it throws an IndexOutOfRangeException.

Technically, you don't need to use locali for the timers[i].Tick part since it's evaluated immediately, but I personally find it confusing to mix the two.

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Some additional reading on the concepet:

The foreach identifier and closures

Closing over the loop variable considered harmful

Answer 2

There is only one i in this case and all of the lambdas are capturing the same value. Use a local that is scoped to the loop so that each lambda has a different copy

for (i = 0; i <= 4; i++)
{
  int j = i;
  timers[j].Tick += (sender, e) => Tick(targs[j]);
}