gridview导出excel没有网格，请高手指教，有图片，

protected void Button2_Click(object sender, EventArgs e)
{
//Response.Redirect("rsKaoQinSelectCrystal.aspx");
Export("application/vnd.xls", "学员信息表.xls");
}

private void Export(String FileType, String FileNmae)
{

HttpContext.Current.Response.Charset = "UTF-8";
HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.Default;
HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=Excel.xls");
HttpContext.Current.Response.ContentType = FileType;
GridView12.Page.EnableViewState = false;
System.IO.StringWriter tw = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
GridView12.RenderControl(hw);
HttpContext.Current.Response.Write(tw.ToString());
HttpContext.Current.Response.End();
//Response.ClearContent();
//Response.Charset = "GB2312";
//Response.ContentEncoding = System.Text.Encoding.UTF7;
//Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(FileNmae, System.Text.Encoding.UTF8).ToString());
//Response.ContentType = FileType;
//this.EnableViewState = false;
//StringWriter tw = new StringWriter();
//HtmlTextWriter hw = new HtmlTextWriter(tw);
//GridView12.RenderControl(hw);
//Response.Write(tw.ToString());
//Response.End();

}
public override void VerifyRenderingInServerForm(Control control)
{
}