c#读取并异步写入文件，简单版，指定编码，保持原格式。

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);
}
}