protected void Page_Load(object sender, EventArgs e) { string term = Request.QueryString["term"]; Response.Clear(); Response.ContentType = "application/json; charset=utf-8"; using (var webClient = new WebClient()) { string rawData = webClient.DownloadString("https://www.anapioficeandfire.com/api/books"); BookList = JsonConvert.DeserializeObject <List <Book> >(rawData); foreach (var x in BookList) { ISBNs.Add(x.Isbn); } } //filtered ISBN HashSet HashSet <string> filteredISBN = new HashSet <string>(); //filtering the ISBN by input foreach (string ISBN in ISBNs) { if (ISBN.ToLower().Contains(term.ToLower())) { filteredISBN.Add(ISBN); } } string responseJSON = JsonConvert.SerializeObject(filteredISBN); Response.Write(responseJSON); Response.End(); }