private void TxtBFirstname_LostFocus(object sender, RoutedEventArgs e)
 {
     if (ValidatorUtils.PropertyErrorAction("Firstname", this.User, (String errorMessage) =>
     {
         this.lblError.Content += errorMessage;
         this.lblError.Visibility = Visibility.Visible;
     }))
     {
         this.lblError.Visibility = Visibility.Collapsed;
     }
 }