public static void AddCustomer(string customerID, string companyName, string contactName = null, string contactTitle = null, string address = null, string city = null, string region = null, string postalCode = null, string country = null, string phone = null, string fax = null)
        {
            using (NorthwindEntities DBcontext = new NorthwindEntities())
            {
                Customer customer = new Customer()
                {
                    CustomerID = customerID,
                    CompanyName = companyName,
                    ContactName = contactName,
                    ContactTitle = contactTitle,
                    Address = address,
                    City = city,
                    Region = region,
                    PostalCode = postalCode,
                    Country = country,
                    Phone = phone,
                    Fax = fax
                };

                DBcontext.Customers.Add(customer);

                DBcontext.SaveChanges();
                Console.WriteLine("Row is inserted");
            }
        }
        public static void DeleteCustomer(string customerID)
        {
            using (NorthwindEntities DBcontext = new NorthwindEntities())
            {
                try
                {
                    Customer customer = DBcontext.Customers.First(c => c.CustomerID == customerID);

                    DBcontext.Customers.Remove(customer);
                    DBcontext.SaveChanges();
                    Console.WriteLine("Row is deleted");
                }
                catch (InvalidOperationException e)
                {
                    throw new InvalidOperationException("There is no customer with this ID", e);
                }

            }
        }
        public static void UpdateCustomer(string customerID, string companyName, string contactName = null, string contactTitle = null, string address = null, string city = null, string region = null, string postalCode = null, string country = null, string phone = null, string fax = null)
        {
            using (NorthwindEntities DBcontext = new NorthwindEntities())
            {
                Customer customer = DBcontext.Customers.First(x => x.CustomerID == customerID);

                customer.CompanyName = companyName ?? customer.CompanyName;
                customer.ContactName = contactName ?? customer.ContactName;
                customer.ContactTitle = contactTitle ?? customer.ContactTitle;
                customer.Address = address ?? customer.Address;
                customer.City = city ?? customer.City;
                customer.Region = region ?? customer.Region;
                customer.PostalCode = postalCode ?? customer.PostalCode;
                customer.Country = country ?? customer.Country;
                customer.Phone = phone ?? customer.Phone;
                customer.Fax = fax ?? customer.Fax;

                DBcontext.SaveChanges();
                Console.WriteLine("Row is updated");
            }
        }