/// <summary>
        /// Create Invoice
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Btn_Create_Click(object sender, RoutedEventArgs e)
        {
            MI_Search.IsEnabled   = false;
            MI_Close.IsEnabled    = true;
            Btn_Create.IsEnabled  = false;
            Btn_Edit.IsEnabled    = false;
            Btn_Delete.IsEnabled  = true;
            Btn_Edit.Visibility   = Visibility.Hidden;
            Btn_Save.Visibility   = Visibility.Visible;
            Btn_Delete.Visibility = Visibility.Hidden;
            Btn_Cancel.Visibility = Visibility.Visible;

            dp_InvoiceDate.IsEnabled        = true;
            cb_InvoiceItems.IsEnabled       = true;
            btn_AddToInvoice.IsEnabled      = true;
            btn_RemoveFromInvoice.IsEnabled = true;
            tb_InvoiceItemsCost.Text        = "";
            DG_Items.ItemsSource            = null;

            Main.Visibility    = Visibility.Hidden;
            Invoice.Visibility = Visibility.Visible;
            Search.Visibility  = Visibility.Hidden;
            Item.Visibility    = Visibility.Hidden;

            cb_InvoiceItems.ItemsSource   = mn.GetLineItems();
            cb_InvoiceItems.SelectedIndex = -1;
            dp_InvoiceDate.IsEnabled      = true;

            lbl_InvoiceNumber.Content   = "Invoice #: TBD";
            dp_InvoiceDate.SelectedDate = null;
            dp_InvoiceDate.DisplayDate  = DateTime.Today;
        }