.NET RichTextBox undo

Here's just some code I decided to mess around with:

        string buffer = String.Empty;
        string buffer2 = String.Empty;
        public Form3()
        {
            InitializeComponent();
            this.richTextBox1.KeyDown += new KeyEventHandler(richTextBox1_KeyDown);
            this.richTextBox1.TextChanged += new EventHandler(richTextBox1_TextChanged);
        }

        void richTextBox1_TextChanged(object sender, EventArgs e)
        {
            buffer2 = buffer;
            buffer = richTextBox1.Text;
        }

        void richTextBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Control && e.KeyCode == Keys.Z)
            {
                this.richTextBox1.Text = buffer2;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            richTextBox1.Text = "Changed";
        }

It's basically me writing my own Undo feature. All I'm doing is storing the old value in one buffer variable, and the new value in a second buffer variable. Every time the text changes, these values get update. Then, if the user hits "CTRL-Z" it replaces the text with the old value. Hack? A little. But, it works for the most part.