Memento

public class Memento
{
    private readonly string _state;

    public Memento(string state)
    {
        _state = state;
    }

    public string GetState()
    {
        return _state;
    }
}

public class Caretaker
{
    private readonly List<Memento> _mementos = [];

    public void AddMemento(Memento memento)
    {
        _mementos.Add(memento);
    }

    public Memento GetMemento(int index)
    {
        return _mementos[index];
    }
}

public class TextState
{
    private readonly string _text;

    internal TextState(string text)
    {
        _text = text;
    }

    internal string GetText()
    {
        return _text;
    }
}

public class TextHistory
{
    private readonly Stack<TextState> _undoStack = [];
    private readonly Stack<TextState> _redoStack = [];

    public void Backup(TextState textState)
    {
        _undoStack.Push(textState);
        _redoStack.Clear();
    }

    public void Undo(TextBox textBox)
    {
        if (_undoStack.Count < 1)
        {
            return;
        }

        _redoStack.Push(_undoStack.Pop());
        textBox.Restore(_undoStack.Peek());
    }

    public void Redo(TextBox textBox)
    {
        if (_redoStack.Count < 1)
        {
            return;
        }

        var redoState = _redoStack.Pop();

        _undoStack.Push(redoState);
        textBox.Restore(redoState);
    }
}