private void QuantityPlusButton_Click(object sender, RoutedEventArgs e)
 {
     if (!ValidationAssist.GetHasError(QuantityTextBox))
     {
         Value++;
         QuantityTextBox.Text = Value.ToString();
     }
 }
 private void NumberMinusButton_Click(object sender, RoutedEventArgs e)
 {
     if (Value > 0 && !ValidationAssist.GetHasError(QuantityTextBox))
     {
         Value--;
         QuantityTextBox.Text = Value.ToString();
     }
 }