addCmd

    public sealed class AddVertexCmd : ICommand
    {
        private Vertex _vertex;
        private List<Vertex> _deleted;
        private Polygon _polygon;

        public AddVertexCmd(Vertex vertex, Polygon polygon)
        {
            _vertex = vertex;
            _polygon = polygon;
            _deleted = new List<Vertex>();
            if (_polygon.Count >= 3)
            {
                Polygon clone = _polygon;
                clone.Add(vertex);
                clone.MakeConvex();
                _deleted = _polygon.Except(clone).ToList();
            }
        }

        public void Execute()
        {
            _polygon.Add(_vertex);
            if (_polygon.Count >= 3)
                _polygon.MakeConvex();
        }

        public void Undo()
        {
            _polygon.Remove(_vertex);
            if (_polygon.Count >= 3)
            {
                _polygon.AddRange(_deleted);
                _polygon.MakeConvex();
            }
        }
    }