如何从代码后面显示GridView中的网格线

在aspx代码中我设置了GridLines="None"

导出网格视图到excel文件时，如何从代码后面显示GridLines

protected void btnexcel_Click(object sender, EventArgs e)
{
        Response.Clear();
        Response.Buffer = true;
        Response.AddHeader("content-disposition",
         "attachment;filename=UsersReport.xls");
        Response.Charset = "";
        Response.ContentType = "application/vnd.ms-excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        gridusers.DataBind();
        gridusers.RenderControl(hw);
        //style to format numbers to string
        string style = @"<style> .textmode { mso-number-format:'@; } </style>";
        Response.Write(style);
        Response.Output.Write(sw.ToString());
        Response.Flush();
        Response.End();
}

grdAdslCompanyAdvisers.GridLines = GridLines.Both;

grdAdslCompanyAdvisers.GridLines = GridLines.None;