export gridview to excel

hi
iam using following code to export gridview to excel..but the generated excel file does not contain rows and columns and normal excel file ...
can any on e suggest a solutio for that
here is my code


StringWriter sw = new StringWriter();
            HtmlTextWriter htw = new HtmlTextWriter(sw);
            String savefile = heading;
            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
            HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=" + savefile + ".xls");
    &nb ...