.NET จะ return Web Service ค่า Data set จากเว็บเซอร์วิสมันไม่ยอมReturn ครับ

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Configuration;
using System.Data.SqlClient;
using System.Data;
using System.ComponentModel;
using System.Web.Services.Protocols;
using System.Xml.Linq;
using System.Web.Configuration;


[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 Service : System.Web.Services.WebService
{
  
   
    public Service () {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }

    [WebMethod]
    public string HelloWorld() {
        return "Hello World";
    }
    [WebMethod]
    public string ShowNameAndLastName(string name, string lastname)
    {
       string name_str = name;
       string lastname_str = lastname;

       return string.Format("สวัสดีครับคุณ" + name_str + " " + lastname_str + "เป็นอย่างไรบ้าง");
            
   
    }
    [WebMethod]
    public DataSet ConnectDB()
    {




        SqlConnection con = new SqlConnection();
        con.ConnectionString = "Data Source= Lincon-PC ;DataBase=Test;Integrated Security=SSPI";
        string cmd = "SELECT customer.name, customer.surname, Departments.department FROM customer inner join Departments on customer.id_de = Departments.id_de";
        SqlDataAdapter da = new SqlDataAdapter(cmd, con);
        DataSet ds = new DataSet();
        da.Fill(ds, "ohm");
        return ds;
        
    }
   
    
}

 protected void Button1_Click(object sender, EventArgs e)
    {
        
       
        localhosts.Service call = new localhosts.Service();
        DataSet ds = call.ConnectDB();
        
        
        GridView1.DataSource =ds;
        GridView1.DataBind();
        GridView2.Visible = false;
        GridView1.Visible = true;
    }