public static string updateProductType(ProductType pt, string name, string description)
        {
            if (name.Length < 5)
            {
                return("Name must at least 5 characters!");
            }
            else if (description.Length <= 0)
            {
                return("Description must not be empty!");
            }
            else if (!name.Equals(pt.Name) && !ProductTypeHandler.validateProductTypeName(name))
            {
                return(String.Format("These product type name has already taken! " +
                                     "Use old name: '{0}' or other non-existent product type instead.", pt.Name));
            }

            try
            {
                ProductTypeHandler.updateProductType(pt.ID, name, description);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
                return("Database update on server failure, please try again!");
            }
            return("");
        }
        public static string insertProductType(string name, string description)
        {
            if (name.Length < 5)
            {
                return("Name must at least 5 characters!");
            }
            else if (description.Length <= 0)
            {
                return("Description must not be empty!");
            }
            else if (!ProductTypeHandler.validateProductTypeName(name))
            {
                return("These product type name has already taken!");
            }

            try
            {
                ProductTypeHandler.addProductType(name, description);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
                return("Database update on server failure, please try again!");
            }
            return("");
        }