//Random rnd = new Random(Guid.NewGuid().GetHashCode());

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            //get data from storage
            _customer = XMLCustStorage.ReadXML <ObservableCollection <Customer> >("customerfile.xml");
            _cars     = XMLCustStorage.ReadXML <ObservableCollection <CarStorage> >("carStorage.xml");
            _history  = XMLCustStorage.ReadXML <ObservableCollection <HistoryRecord> >("historyRecord.xml");

            _custhistory = XMLCustStorage.ReadXML <ObservableCollection <Customer> >("custhistoryRecord.xml");

            if (_customer == null)
            {
                _customer = new ObservableCollection <Customer>();
            }
            if (_cars == null)
            {
                _cars = new ObservableCollection <CarStorage>();
            }
            if (_history == null)
            {
                _history = new ObservableCollection <HistoryRecord>();
            }
            if (_custhistory == null)
            {
                _custhistory = new ObservableCollection <Customer>();
            }
        }
        private void Application_Exit(object sender, ExitEventArgs e)
        {
            XMLCustStorage.WriteXml <ObservableCollection <Customer> >(_customer, "customerfile.xml");
            XMLCustStorage.WriteXml <ObservableCollection <CarStorage> >(_cars, "carStorage.xml");
            XMLCustStorage.WriteXml <ObservableCollection <HistoryRecord> >(_history, "historyRecord.xml");

            XMLCustStorage.WriteXml <ObservableCollection <Customer> >(_custhistory, "custhistoryRecord.xml");
        }