C# อยากสอบถามเรื่อง Webservice ครับผมทำ Dropdownlist แบบใช้ Webserice

 <tr>
        <td style="width: 80px">
            Country:
        </td>
        <td>
            <asp:DropDownList ID="ddlCountries" runat="server" Width="150">
            </asp:DropDownList>
            <cc1:CascadingDropDown ID="cdlCountries" TargetControlID="ddlCountries" PromptText="Select Country"
                PromptValue="" ServicePath="ServiceCS.asmx" ServiceMethod="GetCountries" runat="server"
                Category="CountryId" LoadingText="Loading..." />
        </td>
    </tr>
    <tr>
        <td>
            State:
        </td>
        <td>
            <asp:DropDownList ID="ddlStates" runat="server" Width="150">
            </asp:DropDownList>
            <cc1:CascadingDropDown ID="cdlStates" TargetControlID="ddlStates" PromptText="Select State"
                PromptValue="" ServicePath="ServiceCS.asmx" ServiceMethod="GetStates" runat="server"
                Category="StateId" ParentControlID="ddlCountries" LoadingText="Loading..." />
        </td>
    </tr>
    <tr>
        <td>
            City:
        </td>
        <td>
            <asp:DropDownList ID="ddlCities" runat="server" Width="150">
            </asp:DropDownList>
            <cc1:CascadingDropDown ID="cdlCities" TargetControlID="ddlCities" PromptText="Select City"
                PromptValue="" ServicePath="ServiceCS.asmx" ServiceMethod="GetCities" runat="server"
                Category="CityId" ParentControlID="ddlStates" LoadingText="Loading..." />
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" />
        </td>
    </tr>

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class ServiceCS : System.Web.Services.WebService
{
    [WebMethod]
    public CascadingDropDownNameValue[] GetCountries(string knownCategoryValues)
    {
        string query = "SELECT CountryName, CountryId FROM Countries";
        List<CascadingDropDownNameValue> countries = GetData(query);
        return countries.ToArray();
    }
 
    [WebMethod]
    public CascadingDropDownNameValue[] GetStates(string knownCategoryValues)
    {
        string country = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues)["CountryId"];
        string query = string.Format("SELECT StateName, StateId FROM States WHERE CountryId = {0}", country);
        List<CascadingDropDownNameValue> states = GetData(query);
        return states.ToArray();
    }
 
    [WebMethod]
    public CascadingDropDownNameValue[] GetCities(string knownCategoryValues)
    {
        string state = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues)["StateId"];
        string query = string.Format("SELECT CityName, CityId FROM Cities WHERE StateId = {0}", state);
        List<CascadingDropDownNameValue> cities = GetData(query);
        return cities.ToArray();
    }
 
    private List<CascadingDropDownNameValue> GetData(string query)
    {
       string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        SqlCommand cmd = new SqlCommand(query);
        List<CascadingDropDownNameValue> values = new List<CascadingDropDownNameValue>();
        using (SqlConnection con = new SqlConnection(conString))
        {
            con.Open();
            cmd.Connection = con;
            using (SqlDataReader reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    values.Add(new CascadingDropDownNameValue
                    {
                        name = reader[0].ToString(),
                        value = reader[1].ToString()
                    });
                }
                reader.Close();
                con.Close();
                return values;
            }
        }
    }
}