读取

      //直接读取出字符串
   string text = System.IO.File.ReadAllText(@"D:\web\test.txt");
   Console.WriteLine(text);

        //按行读取为字符串数组
    string[] lines = System.IO.File.ReadAllLines(@"D:\web\test.txt");
    foreach (string line in lines) 
    {
          Console.WriteLine(line);
    }
    //从头到尾以流的方式读出文本文件
        //该方法会一行一行读出文本
        using (System.IO.StreamReader sr = new System.IO.StreamReader(@"D:\web\test.txt"))
        {
            string str;
            while ((str = sr.ReadLine()) != null)
            {
                Console.WriteLine(str);
            }
        }
        Console.Read();

写入

        //该方法写入字符数组换行显示
        string[] lines = { "一", "二", "三","四" };
        System.IO.File.WriteAllLines(@"D:\web\test.txt", lines, Encoding.UTF8);

       
        string strTest = "我是一个字符串";
        System.IO.File.WriteAllText(@"D:\web\test1.txt", strTest, Encoding.UTF8);

        
        
        //StreamWriter第二个参数为false覆盖现有文件，为true则把文本追加到文件末尾   默认为false
        using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"D:\web\test.txt",true))
        {
            foreach (string line in lines)
            {
                if (!line.Contains("second"))
                {
                    file.Write(line);//直接追加文件末尾，不换行
                    file.WriteLine(line);// 直接追加文件末尾，换行   
                }
            }
        }