C# 画第二个矩形时，第一个矩形会消失，怎么办。
源代码如下

C# code

using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;namespace WindowsFormsApplication2{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        private Point sp = new Point(0, 0);        private Point ep = new Point(0, 0);        private bool d1 = false;        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)        {            if (!d1)             {                sp=new Point(e.X,e.Y);                ep=new Point(e.X,e.Y);            }            d1 = !d1;        }        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)        {            if (!d1)            {                return;            }            ep = new Point(e.X, e.Y);            this.pictureBox1.Refresh();         }        private void pictureBox1_Paint(object sender, PaintEventArgs e)        {            ColorDialog cd = new ColorDialog();            if (cd.ShowDialog() == DialogResult.OK)            {                MessageBox.Show(cd.Color.Name);            }            Pen pen1 = new Pen(cd, 3);            if (sp.Equals(ep)) return;            int a = ep.Y - sp.Y;            int b = ep.X - sp.X;            e.Graphics.DrawRectangle(pen1, sp.X,sp.Y,b,a);        }    }}

[解决办法]
看看这个效果，注册 PictureBox 的 MouseDown MouseUp Paint 3个事件，对应下面的方法，鼠标先在 PictureBox 上按下，按住移动一段距离，再放开，选择个颜色，画出一个矩形

C# code

using System;using System.Collections.Generic;using System.Drawing;using System.Windows.Forms;namespace CommonPlatform{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();        }        private Point sp = Point.Empty;        private List<object[]> rects = new List<object[]>();        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)        {            this.sp = e.Location;        }        private void pictureBox1_Paint(object sender, PaintEventArgs e)        {            foreach (var item in this.rects)            {                Rectangle r = (Rectangle)item[0];                Color c = (Color)item[1];                e.Graphics.DrawRectangle(new Pen(c), r);            }        }        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)        {            if (sp.IsEmpty && sp == e.Location)            {                return;            }            int x = e.X > sp.X ? sp.X : e.X;            int y = e.Y > sp.Y ? sp.Y : e.Y;            int width = Math.Abs(sp.X - e.X);            int height = Math.Abs(sp.Y - e.Y);            ColorDialog dialog = new ColorDialog();            dialog.ShowDialog(this);            this.rects.Add(new object[] { new Rectangle(x, y, width, height), dialog.Color });            this.pictureBox1.Invalidate();            this.sp = Point.Empty;        }    }}
[解决办法]
不要跟踪OnPaint事件，直接在mousemove事件中做几件事：
1.invalidate鼠标上次经过的点与sp形成的矩形；
2.绘制由当前鼠标坐标与sp形成的新矩形