public class Memento
{
    private string _msg;
    public Memento(string msg)
    {
      _msg = msg;
    }
    public string GetText()
    {
      return _msg;
    }
}

public class Notepad
{
    private string _msg;
    public Memento SetMsg(string msg)
    {
      Memento m = new Memento(msg);
      _msg = msg;
      return m;
    }
     
    public string GetMsg()
    {
      return _msg;
    }
    public void Undo(Memento previousState)
    {
      if (previousState != null)
      {
        _msg = previousState.GetText();
      }
      else
      {
        _msg = "";
      }
      
    }
}

class Program
{
    static void Main(string[] args)
    {
      IList<Memento> undos = new List<Memento>();
      Notepad notepad = new Notepad();
      Memento undo;
      undo = notepad.SetMsg("Hello");
      undos.Add(undo);
      undo = notepad.SetMsg("World");
      undos.Add(undo);
      Console.WriteLine("进入Nodepad查看信息");
      Console.WriteLine(notepad.GetMsg());
      Console.WriteLine("1、接着执行一个回退操作");
      notepad.Undo(undos[0]);
      Console.WriteLine("回退操作以后查看信息");
      Console.WriteLine(notepad.GetMsg());
      Console.WriteLine("2、再执行一个回退操作");
      notepad.Undo(null);
      Console.WriteLine("回退操作以后再查看信息");
      Console.WriteLine(notepad.GetMsg());
      Console.ReadKey();
    }
}