create json wcf web service

Question

Hello guys. i wanted to create a json web service. so i tried to use this code:

        [OperationContract]
        [WebInvoke(Method = "GET", ResponseFormat = WebMessageFormat.Json)]
        DisCalResult[] DiscuntCalculaterJson(string fn, string ln);


        public DisCalResult[] DiscuntCalculaterJson(string fn, string ln)
        {
            DisCalResult[] DCResult = null;
            Calculater GetDis = new Calculater();
            IPClass IP = new IPClass();
            string iP = IP.GetIPAddress();

            DCResult = GetDis.DisCal(fn, ln, iP);
            return DCResult;
        }

but it doesn't return the respond in json format. i foun another way i just use this one:

        [OperationContract]
        string DiscuntCalculaterJson(string fn, string ln);

        public string DiscuntCalculaterJson(string fn, string ln)
        {
            DisCalResult[] DCResult = null;
            Calculater GetDis = new Calculater();
            IPClass IP = new IPClass();
            string iP = IP.GetIPAddress();

            DCResult = GetDis.DisCal(fn, ln, iP);
            return new JavaScriptSerializer().Serialize(DCResult);
        }

i have no idea what i have done but the second code worked.

whoud you please explain it to me and tell me what is the best way to create json web service?

Thank you.