public string ReadFile() 
  {
     try
      {
         StreamReader sr=new StreamReader(fileName,Encoding.Default);
         string result=sr.ReadToEnd();
         sr.Close();
         return result;
       }
       catch(Exception e){MessageBox.Show(e.Message);}
       return null; 
  }

   public void WriteFile(string str)
  {
     try
     {
        StreamWriter sw=new StreamWriter(fileName,false,Encoding.Default);
        sw.Write(str);
        sw.Close();
      }
      catch(Exception e){MessageBox.Show(e.Message,"保存文件出错！");} 
  }
}