method 500 error cascadingdropdown

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using AjaxControlToolkit;
using System.Collections.Specialized;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

/// 
/// Summary description for WebService1
/// 
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
 [System.Web.Script.Services.ScriptService()]
public class WebService1 : System.Web.Services.WebService {

    SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
    //Web method for bind country
    [WebMethod]
    public CascadingDropDownNameValue[] BindCountry(string knownCategoryValues, string category)
    {
        DataSet ds = new DataSet();
        conn.Open();
        SqlCommand cmd = new SqlCommand("select * from country", conn);
        SqlDataAdapter adp = new SqlDataAdapter(cmd);
        cmd.ExecuteNonQuery();
        adp.Fill(ds);
        conn.Close();
        List CountryDetails = new List();
        foreach (DataRow DR in ds.Tables[0].Rows)
        {
            string id = DR["id"].ToString();
            string Country = DR["Country"].ToString();
            CountryDetails.Add(new CascadingDropDownNameValue(Country, id));
        }
        return CountryDetails.ToArray();
    }
    //Web method for bind state
    [WebMethod]
    public CascadingDropDownNameValue[] BindState(string knownCategoryValues, string category)
    {
        DataSet ds = new DataSet();
        int id;
        StringDictionary CountryDetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        id = Convert.ToInt32(CountryDetails["Country"]);
        conn.Open();
        SqlCommand cmd = new SqlCommand("select * from State where id=@id", conn);
        cmd.Parameters.AddWithValue("@id", id);
        cmd.ExecuteNonQuery();
        SqlDataAdapter adp = new SqlDataAdapter(cmd);
        adp.Fill(ds);
        conn.Close();
        List StateDetails = new List();
        foreach (DataRow DR in ds.Tables[0].Rows)
        {
            string Sid = DR["Sid"].ToString();
            string StateName = DR["StateName"].ToString();
            StateDetails.Add(new CascadingDropDownNameValue(StateName, Sid));
        }
        return StateDetails.ToArray();
    }
    //Web method for bind city
    [WebMethod]
    public CascadingDropDownNameValue[] BindCity(string knownCategoryValues, string category)
    {
        DataSet ds = new DataSet();
        int Sid;
        StringDictionary statedetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        Sid = Convert.ToInt32(statedetails["State"]);
        conn.Open();
        SqlCommand cmd = new SqlCommand("Select * from Region where Sid=@Sid", conn);
        cmd.Parameters.AddWithValue("@Sid", Sid);
        cmd.ExecuteNonQuery();
        SqlDataAdapter adp = new SqlDataAdapter(cmd);
        adp.Fill(ds);
        conn.Close();
        List CityDetails = new List();
        foreach (DataRow DR in ds.Tables[0].Rows)
        {
            string Rid = DR["Rid"].ToString();
            string RegionName = DR[" RegionName"].ToString();
            CityDetails.Add(new CascadingDropDownNameValue(RegionName, Rid));
        }
        return CityDetails.ToArray();
    }

    
}