خطا در مقدار بازگشتی

با درود و عرض ادب
دوستان من یه وب سرویس احتیاج دارم که مقدار بازگشتیش به صورت Json باید باشه

using System.Collections.Generic;using System.Configuration;

using System.Data;

using System.Data.SqlClient;

using System.ServiceModel.Web;

namespace HRJSOFT_Sana_WCF_plus

{

    public class Service1 : IService1

    {

        public List<IpVisitors> GetAllIPS()

        {

            List<IpVisitors> IPList = new List<IpVisitors>();

            using (SqlConnection conn = new SqlConnection())

            {

                conn.ConnectionString = ConfigurationManager.ConnectionStrings["sql"].ConnectionString;

                using (SqlCommand cmmd = new SqlCommand("dbo.Sana_www_Sp_xxx", conn))

                {

                    cmmd.CommandType = CommandType.StoredProcedure;

                    cmmd.Parameters.AddWithValue("@_Layer", 1);

                    using (SqlDataReader sdr = cmmd.ExecuteReader())

                    {

                        while (sdr.Read())

                        {

                            IPList.Add(new IpVisitors

                            {

                                Ip = sdr["cdate"].ToString(),

                                Number = sdr["list"].ToString()

                            });

                        }

                    }

                }

            }

            return IPList;

        }

        public class IpVisitors

        {

            public string Ip { get; set; }

            public string Number { get; set; }

        }

    }

}

using System;

using System.Collections.Generic;

using System.Linq;

using System.Runtime.Serialization;

using System.ServiceModel;

using System.ServiceModel.Web;

using System.Text;

using static HRJSOFT_Sana_WCF_plus.Service1;





namespace HRJSOFT_Sana_WCF_plus

{

    // NOTE: You can use the "Rename" command on the "Refactor" menu to change the interface name "IService1" in both code and config file together.

    [ServiceContract]

    public interface IService1

    {





        [OperationContract]

        [WebInvoke(Method = "GET", RequestFormat = WebMessageFormat.Json,

            ResponseFormat = WebMessageFormat.Json, UriTemplate = "/iplist/")]

        List<IpVisitors> GetAllIPS();





        // TODO: Add your service operations here

    }

}

اما تو مقدار بازگشتی خطای The server encountered an error processing the request. See server logs for more details. رو صادر میکنه