将sql server中数据以Excel格式显示在浏览器的问题

如题，程序在dgExcel.RenderControl(hw)出错：类型“GridView”的控件“dgExcel”必须放在具有 runat=server 的窗体标记内。代码如下：
protected void Page_Load(object sender, EventArgs e)
    {
        String strsql;
        DataSet objDataset = new DataSet();
        SqlConnection con = new SqlConnection();
        con.ConnectionString = ConfigurationManager.ConnectionStrings["GoodsConnectionString"].ToString();
        strsql = "select * from products";
        SqlDataAdapter objAdapter = new SqlDataAdapter();
        con.Open();
        objAdapter.SelectCommand = new SqlCommand(strsql, con);
        objAdapter.Fill(objDataset);              //dgExcel是Gridview的id
        DataView oView = new DataView(objDataset.Tables[0]);
        dgExcel.DataSource = oView;
        dgExcel.DataBind();
        con.Close();
        con.Dispose();
        con = null;
        if (Request.QueryString["bExcel"] == "1")//另一页面穿的值
        {
            Response.ContentType = "application/vnd.ms-excel";
            Response.Charset = "";
            StringWriter tw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(tw);
            dgExcel.RenderControl(hw);//程序在此出错
            Response.Write(tw.ToString());
            Response.End();
            
        }
          检查<form id="form1" runat="server">在，无丢失