如何在Panel上画出直线？

public class LineObj
        {
            private Point m_start;
            private Point m_end;
            public LineObj(Point start,Point end)
            {
                this.m_start = start;
                this.m_end = end;            }
            public void draw(Graphics g, Pen pen)
            {
                g.DrawLine(pen,m_start,m_end);
            }
        
        }        private Point m_startPoint = Point.Empty;
        List<LineObj> lineList = new List<LineObj>();
        private void drawLine(Graphics graphics, Point startPoint, Point endPoint)
        {
            BufferedGraphicsContext context = BufferedGraphicsManager.Current;
            BufferedGraphics bg = context.Allocate(graphics,this.ClientRectangle);
            bg.Graphics.Clear(this.BackColor);
            foreach(LineObj line in this.lineList)
            {
                line.draw(bg.Graphics,SystemPens.ControlText);
            }
            bg.Graphics.DrawLine(SystemPens.ControlText,startPoint,endPoint);
            bg.Render();
            bg.Dispose();
            bg = null;
             
        }        private void onPaint(object sender, PaintEventArgs e)
        {
            foreach(LineObj line in this.lineList)
            {
                line.draw(e.Graphics,SystemPens.ControlText);
            }
        }        private void panel1_MouseDown(object sender, MouseEventArgs e)
        {
           // base.OnMouseDown(e);
            this.m_startPoint = new Point(e.X,e.Y);
        }        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
          //base.OnMouseMove(e);
            if(e.Button==MouseButtons.Left)
            {
                this.drawLine(this.CreateGraphics(),m_startPoint,new Point(e.X,e.Y));
            }
        }        private void panel1_MouseUp(object sender, MouseEventArgs e)
        {
            //base.OnMouseUp(e);
            LineObj line = new LineObj(this.m_startPoint,e.Location);
            this.lineList.Add(line);
这个它不会显示出画在panel上的直线，只会有画在窗体上的直线，该如何修改呢？