1         验证码：<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><asp:Image ID="Image1" ImageUrl="YZM.aspx" runat="server" />
2         <asp:Button ID="Button1" runat="server" Text="验证" /><asp:Label ID="Label1" runat="server" Text=""></asp:Label>
3     

 1 //引用  using System.Drawing  命名空间
 2 
 3         // 创建画布
 4         Bitmap map = new Bitmap(100, 50);
 5         //指定在那个画布上绘制
 6         Graphics g = Graphics.FromImage(map);
 7         //随机数类
 8         Random r = new Random();
 9         //绘制背景色
10 
11         //准备好背景颜色放入集合中去
12         List<Color> clist = new List<Color>();
13         clist.Add(Color.Azure);
14         clist.Add(Color.LightGray);
15         clist.Add(Color.CornflowerBlue);
16         clist.Add(Color.SlateBlue);
17         clist.Add(Color.MediumBlue);
18         clist.Add(Color.SteelBlue);
19         clist.Add(Color.DarkGreen);
20         clist.Add(Color.GreenYellow);
21         clist.Add(Color.SlateBlue);
22         //绘制背景颜色
23         g.FillRectangle(new SolidBrush(clist[r.Next(0, clist.Count)]), 0, 0, 100, 50);
24          
25        //绘制干扰线
26         for(int i =0 ;i<10;i++)//循环绘制I条干扰线
27         {
28          //创建 Pen 对象 pen
29         Pen pen= new Pen(new SolidBrush(clist[r.Next(0, clist.Count)]),r.Next(0,10));
30         //绘制干扰线 
31         g.DrawLine(pen, r.Next(0, 100), r.Next(0, 50), r.Next(0, 100), r.Next(0, 50));
32         }
33         //验证码绘制内容
34         string ss = "ABCDEFHIJKLMNOPQRSTUVWXYZabxdefghijklmnopqrstuvwxyz1234567890";
35         string s = "";
36         for (int i = 0; i < 4; i++)//随机出四位验证码
37         {
38             s += ss.Substring((r.Next(0, ss.Length)), 1);
39         }
40         Session["yzm"] = s;
41         //绘制的字体
42         Font f = new Font("黑体", 30);
43         //绘制的画刷
44         Brush b = new SolidBrush(Color.Brown);
45         //开始绘制验证码
46         g.DrawString(s, f, b, 0, 0);
47 
48         //保存绘制的验证码
49         //Response.OutputStream输出流
50         //System.Drawing.Imaging.ImageFormat.Jpeg 保存的格式
51         map.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
52         Response.End();

//注册验证按钮点击事件
Button1.Click += Button1_Click;
   
 void Button1_Click(object sender, EventArgs e)
    {
        string t1 = TextBox1.Text;
        string t2 = Session["YZM"].ToString();

        if (t1.ToUpper() == t2.ToUpper())
        {
            Label1.Text = "验证成功！";
        }
        else
        {
            Label1.Text = "验证失败！";
        }
    } 

<script type="text/javascript">
    var a = 0;
    document.getElementById("Image1").onclick = function () {
        this.setAttribute("src", "yzm.aspx?id=" + a);
        a++;
    }
</script>