public static Customer AddCustomer(this CustomerData customerData, string firstName, string lastName, string address, string city, string state, string zipCode) { int customerNumber = customerData.Any() ? customerData.Max(m => m.CustomerNumber) + 1 : CustomerData.CustomerNumberSeed; Customer customer = new Customer() { CustomerNumber = customerNumber, FirstName = firstName, LastName = lastName, Address = address, City = city, State = state, ZipCode = zipCode }; customerData.Add(customer); return(customer); }