private void btnExport_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                switch (cboData.SelectedValue)
                {
                case "Customer":
                    CustomerManager.ExportExcel(customers);
                    lblStatus.Content = "Successfully Exported Customers";
                    break;

                case "Employee":
                    EmployeeManager.ExportExcel(employees);
                    lblStatus.Content = "Successfully Exported Employees";
                    break;

                case "Appointment":
                    AppointmentManager.ExportExcel(appointments);
                    lblStatus.Content = "Successfully Exported Appointments";
                    break;

                case "Service Type":
                    ServiceTypeManager.ExportExcel(serviceTypes);
                    lblStatus.Content = "Successfully Exported Service Types";
                    break;

                case "Invoice":
                    if (grdData.SelectedValue == null)
                    {
                        //export the whole grid to excel
                        InvoiceManager.ExportExcel(invoices);
                        lblStatus.Content = "Successfully Exported Invoices";
                    }
                    else
                    {
                        //export a selected item to PDF
                        string invoicePDF = "Invoice" + "-" + invoice.ServiceDate.ToString("MM-dd-yyyy") + "-" + invoice.CustomerFullName;
                        InvoiceManager.InvoicePDF(invoicePDF, invoice);
                    }
                    break;

                case "User":
                    UserManager.ExportExcel(users);
                    lblStatus.Content = "Successfully Exported Users";
                    break;
                }
            }
            catch (Exception ex)
            {
                lblStatus.Foreground = new SolidColorBrush(Colors.Red);
                lblStatus.Content    = ex.Message;
            }
        }