namespace error when using namespaces that don't match names

Question

Suppose I create an asp.net website project that loads data into griview from the class library, declaring the library to use loading data for griview

//file: ChitietSP.aspx

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Diagnostics;
// Debug using connectSQL;
// warning
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) { LoadData(); }
    }
    string sSQL;
    private void LoadData()
    {
        sSQL = "SELECT * FROM TABKHACHHANG "; sSQL += "ORDER BY MASOPHIEU;";
        GridView1.DataSource = ClsConnecSQL.FillDatatable(sSQL);
        //warning
    }
}

//file: clsConnecSQL.cs

using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
//using connectSQL;
namespace connectSQL
{
    public class ClsConnecSQL
    { public static DataTable FillDatatable(string sSQL)
        { //Do somthing...
        }
    }
}

I have declared the library namespace to be used is still error, asking you to help me

 

using connectSQL; // warning

 

GridView1.DataSource = ClsConnecSQL.FillDatatable (sSQL); // error not found connectSQL namespace