public void AddCustomer(Customer _customer)
        {
            try
            {
                XElement doc = XElement.Load(connectionString);

                _customer.Id = IncreaseCounterIdentificators(doc);
                XElement customer = CustomerFactory.MakeXCustomer(_customer);

                doc.Add(customer);
                doc.Save(connectionString);
            }
            catch (Exception)
            {
                throw new Exception(ErrorTypes.ERROR_INSERT_CLIENT);
            }
        }
        public void UpdateCustomer(Customer _customer)
        {
            try
            {
                XElement doc = XElement.Load(connectionString);

                XElement customer = (from node in doc.Elements("Customer")
                                     where node.Element("Id").Value == _customer.Id.ToString()
                                     select node).SingleOrDefault();

                if (customer != null)
                {
                    customer.ReplaceAll(CustomerFactory.MakeXCustomer(_customer).Elements());
                }

                doc.Save(connectionString);
            }
            catch (Exception)
            {
                throw new Exception(ErrorTypes.ERROR_UPDATE_CLIENT);
            }
        }