DispatcherTimer will increasing Interval for every time it's used

Question

I followed an example from Head First C# on DispatcherTimer.

First time I press the button the ticker will increase by 1 second, but the next time I click on the button the ticker will increase by 2 seconds for every second/tick. Third time ticker increases with 3 seconds and so on (1 second is added for every button press).

Why is that and how to i "reset" the ticker Interval so it will only increase by 1 second every time?

Here is code:

DispatcherTimer timer = new DispatcherTimer();
    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
        timer.Tick += timer_Tick;
        timer.Interval = TimeSpan.FromMilliseconds(1000);
        timer.Start();
        CheckHappiness();
    }
    int i = 0;
    void timer_Tick(object sender, object e)
    {
        ticker.Text = "Tick #" + i++;
    }

    private async void CheckHappiness()
    {
        ... code ..
        timer.Stop();
    }
   }
  }

Cheers!

Answer 1

timer.Tick += timer_Tick;

This line adds the method to the eventhandler everytime you press the button; in which you do an i++ which increases i by one. When you have two methods doing that at the same time (since the timer ticks on your interval) then you get an increase by two every tick of the timer.