Export Gridview Within gridview to excel

public void ExportToExcel(DataTable dt)
    {
            con.Open();
            string sql = "select *from test";
            cmd = new SqlCommand(sql, con);
            dt = new DataTable();
            da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            GridView1.DataSource = dt;
            GridView1.DataBind();
            cmd.Dispose();


            string filename = "DownloadTest.xls";
            System.IO.StringWriter tw = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
            DataGrid dgGrid = new DataGrid();
            dgGrid.DataSource = dt;
            dgGrid.DataBind();

            dgGrid.RenderControl(hw);
            
            Response.ContentType = "application/vnd.ms-excel";
            Response.AppendHeader("Content-Disposition", "attachment; filename=" + filename + "");
            this.EnableViewState = false;
            Response.Write(tw.ToString());
            Response.End();


            con.Close();
    }