命令模式

    interface Command
    {
        void execute();
        void undo();
    }

    public class TextChangedCommand : Command
    {
        private TextBox ctrl;
        private String newStr;
        private String oldStr;

        public TextChangedCommand(TextBox ctrl, String newStr, String oldStr)
        {
            this.ctrl = ctrl;
            this.newStr = newStr;
            this.oldStr = oldStr;
        }

        public void execute()
        {
            this.ctrl.Text = newStr;
            this.ctrl.SelectionStart = this.ctrl.Text.Length;
        }

        public void undo()
        {
            this.ctrl.Text = oldStr;
            this.ctrl.SelectionStart = this.ctrl.Text.Length;
        }
    }

    public partial class Form1 : Form
    {
        Stack<Command> undoStack = new Stack<Command>();
        Stack<Command> redoStack = new Stack<Command>();

        string oldStr = null;
        bool flag = true;

        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            if (flag)
            {
                TextChangedCommand com = new TextChangedCommand((TextBox)textBox1, ((TextBox)textBox1).Text, oldStr);
                undoStack.Push(com);
                oldStr = ((TextBox)textBox1).Text;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (undoStack.Count == 0)
                return;
            flag = false;
            Command com = undoStack.Pop();
            com.undo();
            redoStack.Push(com);
            flag = true;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (redoStack.Count == 0)
                return;
            flag = false;
            Command com = redoStack.Pop();
            com.execute();
            undoStack.Push(com);
            flag = true;
        }
    }