c#读取并异步写入文件

  StreamReader objReader = new StreamReader("E://workspace//zzz//read.txt", Encoding.GetEncoding("utf-8"));
            string sLine = "";
            ArrayList LineList = new ArrayList();
            while (sLine != null)
            {
                sLine = objReader.ReadLine();
                if (sLine != null && !sLine.Equals(""))
                    LineList.Add(sLine);
            }
            objReader.Close();

            string str = "";
            foreach (var item in LineList)
            {
                str += item + Environment.NewLine;
            }
            string filePath = "E://workspace//zzz//" + DateTime.Now.Millisecond + ".txt";
            FileStream fs = new FileStream(filePath, FileMode.Create);
            StreamWriter sw = new StreamWriter(fs, Encoding.GetEncoding("utf-8"));
            //开始写入
            sw.Write(str);
            //清空缓冲区
            sw.Flush();
            //关闭流
            sw.Close();
            fs.Close();

 /// <summary>
        /// 异步将xml内容写入文件
        /// </summary>
        /// <param name="str"></param>
        /// <param name="encoding"></param>
        public static void XmlToFile(string str, string fileName, string encoding)
        {
            string filePath = "E://workspace_VS//zzz//" + fileName + DateTime.Now + ".xml";
            byte[] bytes = Encoding.GetEncoding(encoding).GetBytes(str.ToString());
            FileStream writer = new FileStream(filePath, FileMode.Create, FileAccess.Write);
            writer.BeginWrite(bytes, 0, str.Length, new AsyncCallback(endWrite), writer);
        }
        /// <summary>
        /// 结束异步写入
        /// </summary>
        /// <param name="asr"></param>
        private static void endWrite(IAsyncResult asr)
        {
            using (Stream str = (Stream)asr.AsyncState)
            {
                str.EndWrite(asr);
            }
        }