StreamReader sr = new StreamReader("c:\\a.txt");                
while(!sr.EndOfStream)  //读到文件流结尾退出                {                    
   string temp = sr.ReadLine(); // 一行一行的读                
}                
sr.Close(); 

StreamWriter sw = new StreamWriter("c:\\a.txt");                sw.WriteLine("your content"); // 没调用一次 写入一行                sw.Close();