private void BuyTicketPage_Loaded(object sender, RoutedEventArgs e)
 {
     foreach (string namePerformance in cashbox.GetPerformancesName())
     {
         comboPerformanceBox.Items.Add(namePerformance);
     }
 }
        private void Page_Loaded(object sender, RoutedEventArgs e)
        {
            foreach (Client client in cashbox.GetClients())
            {
                clientsComboBox.Items.Add(client.fullName);
            }

            foreach (string namePerformnce in cashbox.GetPerformancesName())
            {
                performancesComboBox.Items.Add(namePerformnce);
            }
        }