C # and Winforms TextBox control: how do I get the text changed?

I have an event handler for the TextBox.TextChanged event on my form. To support cancellation, I would like to identify exactly what has changed in the TextBox so that I can undo the change if the user asks for it. (I know the inline textbox supports undo, but I would like to have one undo for the entire application)

Is there a sane way to do this? If not, is there a better way to support such a cancellation function?

EDIT: Seems like something like the following: Are there any better ideas? (It seems to me that I really would like .NET to have something like an STL algorithm std::mismatch

...

    class TextModification
    {
        private string _OldValue;
        public string OldValue
        {
            get
            {
                return _OldValue;
            }
        }
        private string _NewValue;
        public string NewValue
        {
            get
            {
                return _NewValue;
            }
        }
        private int _Position;
        public int Position
        {
            get
            {
                return _Position;
            }
        }
        public TextModification(string oldValue, string newValue, int position)
        {
            _OldValue = oldValue;
            _NewValue = newValue;
            _Position = position;
        }
        public void RevertTextbox(System.Windows.Forms.TextBox tb)
        {
            tb.Text = tb.Text.Substring(0, Position) + OldValue + tb.Text.Substring(Position + NewValue.Length);
        }
    }

    private Stack<TextModification> changes = new Stack<TextModification>();
    private string OldTBText = "";
    private bool undoing = false;

    private void Undoit()
    {
        if (changes.Count == 0)
            return;
        undoing = true;
        changes.Pop().RevertTextbox(tbFilter);
        OldTBText = tbFilter.Text;
        undoing = false;
    }

    private void UpdateUndoStatus(TextBox caller)
    {
        int changeStartLocation = 0;
        int changeEndTBLocation = caller.Text.Length;
        int changeEndOldLocation = OldTBText.Length;
        while (changeStartLocation < Math.Min(changeEndOldLocation, changeEndTBLocation) &&
            caller.Text[changeStartLocation] == OldTBText[changeStartLocation])
            changeStartLocation++;
        while (changeEndTBLocation > 1 && changeEndOldLocation > 1 &&
            caller.Text[changeEndTBLocation-1] == OldTBText[changeEndOldLocation-1])
        {
            changeEndTBLocation--;
            changeEndOldLocation--;
        }
        changes.Push(new TextModification(
            OldTBText.Substring(changeStartLocation, changeEndOldLocation - changeStartLocation),
            caller.Text.Substring(changeStartLocation, changeEndTBLocation - changeStartLocation),
            changeStartLocation));
        OldTBText = caller.Text;
    }

    private void tbFilter_TextChanged(object sender, EventArgs e)
    {
        if (!undoing)
            UpdateUndoStatus((TextBox)sender);
    }