BiomePainter.History.HistoryManager.OnChange C# (CSharp) Метод

        private void OnChange()
        {
            if (change != null)
            {
                //the top action on a stack may be the first of its kind and therefore
                //needs to be skipped over; the first action of a kind is only needed
                //when undoing the second action of that kind, via it's PreviousAction
                //property
                String redoDescription = null;
                if(redoStack.Count > 0)
                {
                    LinkedListNode<IAction> redo = redoStack.Last;
                    while (redo != null && redo.Value.PreviousAction == null)
                        redo = redo.Previous;
                    if (redo != null)
                        redoDescription = redo.Value.Description;
                }
                String undoDescription = null;
                if (undoStack.Count > 0)
                {
                    LinkedListNode<IAction> undo = undoStack.Last;
                    while (undo != null && undo.Value.PreviousAction == null)
                        undo = undo.Previous;
                    if (undo != null)
                        undoDescription = undo.Value.Description;
                }
                change(undoDescription, redoDescription);
            }
        }