private void extra_Click(object sender, RoutedEventArgs e)
        {
            if (this.dueDateBox.SelectedDate == null || this.amountBox.Text == "" || this.interestBox.Text == "" || this.frequencyBox.SelectedIndex == -1 || this.rb1Box.Text == "" && this.rb2Box.Text == "")
            {
                string msgtext = "Cannot apply extra principle.\n\nEnter mortgage information for all required fields.";
                string txt     = "Error";

                MessageBoxButton button = MessageBoxButton.OKCancel;
                MessageBoxResult result = MessageBox.Show(msgtext, txt, button);

                switch (result)
                {
                case MessageBoxResult.OK:
                    break;

                case MessageBoxResult.Cancel:
                    break;
                }
            }

            else
            {
                ExtraPrinciple extraForm = new ExtraPrinciple(this.amountBox.Text, this.interestBox.Text, this.frequencyBox.SelectionBoxItem, this.rb1Box.Text, this.rb2Box.Text, this.dueDateBox.SelectedDate);
                this.NavigationService.Navigate(extraForm);
            }
        }
        private void extra_Click(object sender, RoutedEventArgs e)
        {
            ExtraPrinciple extraForm = new ExtraPrinciple();

            this.NavigationService.Navigate(extraForm);
        }