Why am I getting error when I write a code for timer in WPF?

I wrote a code for using timer in WPF. It was working in windows form but it is not working in WPF. How can I fix the problem?

Here is the code:

timerFPS = new Timer(1000);  //The error is :"The name timeFPS is does not exist in the context"
timerFPS.Elapsed += TimerFPS_Elapsed;
timerFPS.Start();