一个无法描述的错误

if (Session["user_name"] == null)
            {
                for (int i = 0; i < DataList1.Items.Count; i++)
                {
                    (Panel)DataList1.Items[i].FindControl("PanelControl").Visible = false;//错在这里
                }

            }

望各位高手指点一二,如果写错了,请不吝赐教,该如何的写!多谢多谢