private void login_Click(object sender, RoutedEventArgs e)
 {
     if (username.Text == "admin" && mdp.Password == "123456")
     {
         MenuWindow menu = new MenuWindow();
         menu.Show();
         this.Close();
     }
     else
     {
         error.Content = "*Username or Password INCORRECT ! ";
     }
 }
 private void Window_KeyDown(object sender, KeyEventArgs e)
 {
     if (e.Key == Key.Return)
     {
         if (username.Text == "admin" && mdp.Password == "123456")
         {
             MenuWindow menu = new MenuWindow();
             menu.Show();
             this.Close();
         }
         else
         {
             error.Content = "*Username or Password INCORRECT ! ";
         }
     }
 }