public string Save(Book abook) { if (bookGateway.IsISBNExists(abook.Isbn)) { return("ISBN exists"); } if (abook.Isbn.Length != 13) { return("ISBN must thirteen 13 characters long."); } int rowAffected = bookGateway.Save(abook); if (rowAffected > 0) { return("Book saved succesfully"); } return("Book saved failed"); }