how to export data in excel 2007 in asp.net c#?

DataTable dt = new DataTable();

            string str1;

            str1 = "SELECT Testt1.StudentCode, Student.StudentID,Student.Name, Testt1.FMark,'' as 'LMark','' as'Status',''as 'Promote' FROM Testt1 INNER JOIN School ON Testt1.SchoolCode = School.SchoolCode INNER JOIN Student ON Testt1.StudentCode = Student.StudentCode where Testt1.Promote ='Yes' and School.SchoolCode =" + DropDownList3.SelectedValue + "";

            dt = ObjConn.GetDataTable(str1);

            HttpResponse response = HttpContext.Current.Response;

            response.Clear();

            //response.Charset = "";

            response.ContentType = "application/vnd.ms-excel";

            response.AddHeader("Content-Disposition", "attachment;filename=Smarks.xls");

            //response.AddHeader("Content-Disposition", "C:\"" + fileName + "\"");

            StringWriter sw = new StringWriter();

            HtmlTextWriter htw = new HtmlTextWriter(sw);

            DataGrid dg = new DataGrid();

            dg.DataSource = dt;

            dg.DataBind();

            dg.RenderControl(htw);

            response.Write(sw.ToString());

            response.End();