if (App.Current.MainWindow != null && App.Current.MainWindow.GetType() == typeof(MainWindow))
{
    this.Close();
}
else
{
        //main Window hasn't been created yet so create it!
        MainWindow main = new MainWindow();
        App.Current.MainWindow = main;
        this.Close();
        main.Show();
        wndw = main;
    }

MainWindow main;
if
{...
}
else
{...
}
main= App.Current.MainWindow;

var window = Application.Current.Windows.OfType<Window>().SingleOrDefault(w => w.IsActive);

var main = App.Current.MainWindow as MainWindow

var main = (MainWindow)App.Current.MainWindow

public static Window ActivatedWindow {get;set;}

public class MetroToolWindowBase
   {
     public MetroToolWindowBase()
     {
        Activated += new EventHandler(MakeActive);
     }
     private void MakeActive(object sender, EventArgs e)
     {
    App.ActivatedWindow= this;
     }
   }

private void XWindow_Activated(object sender,EventArgs e)
  {
  App.ActivatedWindow= this;
  }