asp.net中验证码的实现 (转自http://blog.csdn.net/zhoufoxcn/archive/2007/07/19/1698615.aspx)

主要用到一个类，一个aspx页面，还有一个使用图片验证码的aspx页面。

随机图片生成类：

using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;

namespace Sooyie.Common
{
    /// <summary>
    /// 产生随即图片
    /// </summary>
    public sealed class RandImage
    {
        private const string RandCharString = "0123456789";
        private int width;
        private int height;
        private int length;
        /// <summary>
        /// 默认构造函数，生成的图片宽度为48×24，随即字符串字符个数
        /// </summary>
        public RandImage():this(48,24,4)
        {
        }
        /// <summary>
        /// 指定生成图片的宽和高，默认生成图片的字符串长度为4个字符
        /// </summary>
        /// <param name="width"></param>
        /// <param name="height"></param>
        public RandImage(int width, int height):this(width,height,4)
        {
        }
        /// <summary>
        /// 指定生成图片的宽和高以及生成图片的字符串字符个数
        /// </summary>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <param name="length"></param>
        public RandImage(int width, int height, int length)
        {
            this.width = width;
            this.height = height;
            this.length = length;
        }
        /// <summary>
        /// 以默认的大小和默认的字符个数产生图片
        /// </summary>
        /// <returns></returns>
        public Image GetImage()
        {
            Bitmap image = new Bitmap(width, height);
            Graphics g = Graphics.FromImage(image);
            g.Clear(Color.White);
            string randString = "";
            Random random=new Random();
            do
            {
                randString += RandCharString.Substring(random.Next((int)DateTime.Now.Ticks)%RandCharString.Length, 1);
            }
            while (randString.Length < 4);
            float emSize=(float)width/randString.Length;
            Font font = new Font("Arial", emSize, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
            Pen pen = new Pen(Color.Silver);
            #region 画图片的背景噪音线
            int x1,y1,x2,y2;

            for (int i = 0; i < 25; i++)
            {
                x1 = random.Next(image.Width);
                y1 = random.Next(image.Height);
                x2 = random.Next(image.Width);
                y2 = random.Next(image.Height);
                g.DrawLine(pen, x1, y1, x2, y2);
            }
            #endregion

            #region 画图片的前景噪音点
            for (int i = 0; i < 100; i++)
            {
                x1 = random.Next(image.Width);
                y1 = random.Next(image.Height);
                image.SetPixel(x1, y1, Color.FromArgb(random.Next(Int32.MaxValue)));
            }
            #endregion

            g.DrawString(randString, font, Brushes.Red, 2, 2);
            g.Dispose();
            return image;

        }
    }
}

将图片转换成html资源的aspx页面，这个页面前台没有任何代码，主要后台(.cs)的功能。

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using Sooyie.Common;

public partial class CheckImage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            RandImage randImage=new RandImage();
            System.Drawing.Image image = randImage.GetImage();
            System.IO.MemoryStream memoryStream = new MemoryStream();
            image.Save(memoryStream, ImageFormat.Jpeg);
            Response.ClearContent();
            Response.ContentType = "image/gif";
            Response.BinaryWrite(memoryStream.ToArray());
            image.Dispose();
            Response.End();
        }
    }
}

下面是怎么使用的例子：

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

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>后台管理用户登录</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <table border="0" cellpadding="0" cellspacing="0" width="480">
        <tr>
            <td colspan="2" align="center">
                公文管理系统后台登陆</td>
        </tr>
        <tr>
            <td style=" 83px">
                用户名</td><td>
                <asp:TextBox ID="txtUserName" runat="server"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txtUserName"
                        ErrorMessage="用户名"></asp:RequiredFieldValidator></td>
        </tr>
        <tr>
            <td style="height: 19px;  83px;">
                密码</td><td>
                <asp:TextBox ID="txtPassword" runat="server" TextMode="Password"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="txtPassword"
                        ErrorMessage="密码"></asp:RequiredFieldValidator></td>
        </tr>
        <tr>
            <td style="height: 19px;  83px;">
                校验码</td><td>
                <asp:TextBox ID="txtCheckCode" runat="server"></asp:TextBox><img src="CheckImage.aspx" alt="校验码" />
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" ControlToValidate="txtCheckCode"
                        ErrorMessage="校验码必填"></asp:RequiredFieldValidator></td>
        </tr>
        <tr>
            <td style=" 83px">
                <asp:Button ID="btnLogin" runat="server" Text="登陆" OnClick="btnLogin_Click" /></td><td style=" 9px">
                    <input id="Reset1" type="reset" value="清除" /></td>
        </tr>
    </table>
    </div>
        <asp:Literal ID="lStatus" runat="server" Visible="False"></asp:Literal>
    </form>
</body>
</html>

请注意：<img src="CheckImage.aspx" alt="校验码" />这一句就使用了CheckImage.aspx.cs类中的功能，将图片通过HTTP输出。