public T Copy() {
            MemoryStream mem = new MemoryStream();
            DataContractJsonSerializer json = new DataContractJsonSerializer(typeof(T));
            json.WriteObject(mem, item);
            mem.Seek(0, SeekOrigin.Begin);//這里沒有報錯，因為當前位置在最后邊
            var r = (T)json.ReadObject(mem);
            mem.Close();
            return r;
        }