调试易

// 不要每次Paint事件,都重新初始化一个Bitmap
Bitmap bp = new Bitmap(drawPanel.Width, drawPanel.Height); 
        void drawPanel_Paint(object sender, PaintEventArgs e)
        {
            using(Graphics g = Graphics.FromImage(bp))
            {
           //画之前把前一次的内容清空
              g.Clear(Color.Empty);
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias; 
            Pen p = new Pen(Color.Black, 4);
            p.DashStyle = DashStyle.Custom;
            p.EndCap = LineCap.ArrowAnchor;
            foreach (Line line in lines)
            {
                g.DrawLine(p, line.pointStart, line.pointEnd);
            }
            
            }
            e.Graphics.DrawImage(bp, Point.Empty);
        }