在c#中画三个圆分别代表红绿灯,然后间隔一定时间三个灯色轮流换,(换句话说模拟信号灯，该怎么编写代码

在c#中画三个圆分别代表红绿灯,然后间隔一定时间三个灯色轮流换,(换句话说模拟信号灯，该怎么编写代码

Graphics g1 = this.CreateGraphics();
            var Rec1 = new Rectangle(new Point(30, 30), new Size(150, 150));

            int time = 0, before = 0, flag = 0;
            System.Timers.Timer t = new System.Timers.Timer(1000);
            t.Elapsed += (s, e1) =>
            {
                int left = time - before;
                switch (flag)
                {
                    case 0:
                        if (left == 15)//red turn green,10s
                        {
                            flag = 2;
                            before = time;
                        }
                        g1.FillEllipse(Brushes.Red, Rec1);
                        break;
                    case 1:
                        if (left == 3)//yellow turn red,3s
                        {
                            flag = 0;
                            before = time;
                        }
                        g1.FillEllipse(Brushes.Yellow, Rec1);
                        break;
                    case 2:
                        if (left == 10)//green turn yellow,10s
                        {
                            flag = 1;
                            before = time;
                        }
                        g1.FillEllipse(Brushes.Green, Rec1);
                        break;
                    default:
                        break;
                }
                time++;
                button1.BeginInvoke(new Action(() =>
                {
                    button1.Text = time.ToString();
                }));
            };
            t.Start();