c# .net serializes List

/// <summary>
        /// 序列化List
        /// </summary>
        /// <param name="al"></param>
        /// <returns>string</returns>
        public string SerializeArrayList(ArrayList al)
        {
            Type[] extra = new Type[1];
            extra[0] = typeof(LocalNetInfo);
           // extra[0] = typeof(string);
            XmlSerializer xs = new XmlSerializer(typeof(ArrayList), extra);
            MemoryStream ms = new MemoryStream();
            XmlTextWriter tw = new XmlTextWriter(ms, Encoding.Default);
            xs.Serialize(tw, al);
            tw.Close();
            return Encoding.Default.GetString(ms.ToArray());
        }