AllScreens gets an array of all displays on the system.
Example
Display window in the second display WPF
private void Window_Loaded(object sender, RoutedEventArgs e)
{
   var screen = System.Windows.Forms.Screen.AllScreens.Where(s
=> !s.Primary).First();
   var area = screen.WorkingArea;
   Left = area.Left;
   Top = area.Top;
   Width = area.Width;
   Height = area.Height;
}
