后台文件:

1using System;
2using System.Data;
3using System.Configuration;
4using System.Collections;
5using System.Drawing;
6using System.Web;
7using System.Web.Security;
8using System.Web.UI;
9using System.Web.UI.WebControls;
10using System.Web.UI.WebControls.WebParts;
11using System.Web.UI.HtmlControls;
12
13public partial class Default2 : System.Web.UI.Page
14{
15 protected void Page_Load(object sender, EventArgs e)
16 {
17 if(!this.IsPostBack)
18 {
19 this.GenImg(this.GenCode(4));
20 }
21
22 }
23 //产生随机字符串
24 private string GenCode(int num)
25 {
26 string[] source={"0","1","2","3","4","5","6","7","8","9",
27 "A","B","C","D","E","F","G","H","I","J","K","L","M","N",
28 "O","P","Q","R","S","T","U","V","W","X","Y","Z"};
29 string code="";
30 Random rd=new Random();
31 for(int i=0;i < num;i++)
32 {
33 code += source[rd.Next(0,source.Length)];
34 }
35 return code;
36 }
37
38 //生成图片
39 private void GenImg(string code)
40 {
41 Bitmap myPalette = new Bitmap(60, 20);//定义一个画板
42
43 Graphics gh = Graphics.FromImage(myPalette);//在画板上定义绘图的实例
44
45 Rectangle rc = new Rectangle(0, 0, 60, 20);//定义一个矩形
46
47 gh.FillRectangle(new SolidBrush(Color.Blue), rc);//填充矩形
48 gh.DrawString(code, new Font("宋体", 16), new SolidBrush(Color.White), rc);//在矩形内画出字符串
49
50 myPalette.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);//将图片显示出来
51
52 Session["ValidateCode"] = code;//将字符串保存到Session中，以便需要时进行验证
53
54 gh.Dispose();
55 myPalette.Dispose();
56 }
57
58}

　　HTML文件:

　　加入一个HTML控件Image即可

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>

来源：网络