后台自动添加的服务器控件以及其相应的事件，该事件无法进入、求解决
        protected void AddDropList(int pid, int i)
        {
            DataTable dt = gbll.GetList("ParentID=" + pid).Tables[0];
            if (dt.Rows.Count > 0)
            {
                DropDownList drp = new DropDownList();
                drp.ID = "drpGroup" + i;
                drp.AutoPostBack = true;
                drp.SelectedIndexChanged += new EventHandler(drp_SelectedIndexChanged);

                drp.DataSource = dt;
                drp.DataTextField = "GrpName";
                drp.DataValueField = "ID";

                drp.DataBind();
                tbProduct.Rows[0].Cells[1].Controls.Add(drp);

                AddDropList(Convert.ToInt32(dt.Rows[0]["ID"]), i + 1);
            }

        }

        public void drp_SelectedIndexChanged(object sender, EventArgs e)
        {
            int id = Convert.ToInt32(drpGroup1.SelectedValue);
            int classnum = GetClassNum(id, 2);
            DropDownList drp = Page.FindControl("drpGroup" + classnum) as DropDownList;
            AddDropList(id, classnum);
        }