在picturebox中画连续线 C#

急求代码！！！

using System;
using System.Collections.Generic;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace CSharpTemp
{
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();

            Controls.Add(new MyControl() { Dock = DockStyle.Fill });
        }

    }

    class Line
    {
        protected List points = new List();

        public void Add(Point p)
        {
            points.Add(p);
        }

        public void Draw(Graphics g, Pen pen)
        {
            if (points.Count >= 2)
                g.DrawLines(pen, points.ToArray());
        }
    }

    class MyControl : Control 
    {
        protected List lines = new List();

        public MyControl()
        {
            DoubleBuffered = true;
        }

        protected override void OnMouseDown(MouseEventArgs e)
        {
            Line line = new Line();
            line.Add(e.Location);
            lines.Add(line);
        }

        protected override void OnMouseMove(MouseEventArgs e)
        {
            if (e.Button == System.Windows.Forms.MouseButtons.Left)
            {
                lines.Last().Add(e.Location);
                Invalidate();
            }
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            foreach (Line l in lines)
                l.Draw(e.Graphics, Pens.Black);
        }
    }
}

清除image： graphics g = graphics.fromimage(picturebox1.image); g.clear(color.white); g.dispose(); picturebox1.refresh(); 清除背景： graphics g = graphics.fromimage(picturebox1.backgroundimage); g.clear(color.white); g.dispose(); picturebox1.refresh(); 去掉image，显示出背景 picturebox1.image = null; picturebox1.refresh();