FileStream fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite);
//讀
StreamReader sr=new StreamReader(fs);
string line=sr.readLine();

//寫(xiě)
StreamWriter s = new StreamWriter(fs);
s.WriteLine("xxx");

   /// <summary>
   /// 在文件流當(dāng)前位置開(kāi)始讀取改行的內(nèi)容 返回改行的長(zhǎng)度
   /// </summary>  
   private long readRow(FileStream fs20O)
   {
            long len = 0;

            byte[] b = new byte[1];

            bool exit = false;

            while (true)
            {
                if (fs20O.Read(b, 0, 1) == -1) break;

                len++;
                string str = Encoding.UTF8.GetString(b);
                Console.Write(str);
                if ("\n".Equals(str))
                {
                    return len;
                }
            }

            return len;
     }

 fs.Seek(-len, SeekOrigin.Current);

long len=0;
while((len=readRow(fs)!=-1)
{
    //......
    //修改內(nèi)容
    //文件指針移動(dòng)至行首
    if(true)
    {
        fs.Seek(-len, SeekOrigin.Current);
        byte[] readWriteByte = Encoding.UTF8.GetBytes("Test");
        //會(huì)直接把原來(lái)內(nèi)容替換
        fs.Write(readWriteByte, 0, readWriteByte.Length);
        //***繼續(xù)往下讀取 直到行尾 把指針移到下一行行首
        readRow(fs);

        //修改完成直接跳出循環(huán)，保存文件
        break;
    }
}