Possible Duplicate:
Windows Phone 7 close application
How do I programmatically close a WP7 application?
You can always call an exit by doing this at your landing page use this code on click of your application back button:
if (NavigationService.CanGoBack)
{
while (NavigationService.RemoveBackEntry() != null)
{
NavigationService.RemoveBackEntry();
}
}
This will remove back entries from the stack, and you will press a back button it will close the application without any exception.