This code of export to excel saves the whole page with extension .aspx

How to correct this .xls or .xlsx

  private void ExportToExcel(string strFileName, GridView dg)
    {
        Response.Clear();
        Response.Buffer = true;
        Response.ContentType = "application/vnd.ms-excel";
        Response.Charset = "";
        this.EnableViewState = false;
        System.IO.StringWriter oStringWriter = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter oHtmlTextWriter = new System.Web.UI.HtmlTextWriter(oStringWriter);
        GridView1.RenderControl(oHtmlTextWriter);
        Response.Write(oStringWriter.ToString());
        Response.End();
    }