1.添加一个.ashx文件

<%@ WebHandler Language="C#" class="CheckCode" %>

using System;
using System.Web;
using System.Drawing;

public class CheckCode : IHttpHandler {

    public void ProcessRequest(HttpContext context)
    {
        string g = GenerateCheckCode();
        CreateCheckCodeImage(g);

    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

    private string GenerateCheckCode()
    {
        int number;
        char code;
        string checkCode = String.Empty;

        System.Random random = new Random();
 
        for (int i = 0; i < 5; i++)
        {
            number = random.Next();

            if (number % 2 == 0)
                //生成'0'-'9'字符
                code = (char)('0' + (char)(number % 10));
            else
                //生成'A'-'Z'字符
                code = (char)('A' + (char)(number % 26));

            checkCode += code.ToString();
        }

        HttpContext.Current.Response.Cookies.Add(new HttpCookie("checkcode", checkCode));

        return checkCode;
        //两个字符相加等于=asicc码加
    }

    private void CreateCheckCodeImage(string checkCode)
    {
        if (checkCode == null || checkCode.Trim() == String.Empty)
            return;
        //(int)Math.Ceiling((checkCode.Length * 12.5)),22
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(80, 30);
        Graphics g = Graphics.FromImage(image);

        try
        {
            //生成随机生成器
            Random random = new Random();

            //清空图片背景色
            g.Clear(Color.White);

            //画图片的背景噪音线
            for (int i = 0; i < 5; i++)
            {
                int x1 = random.Next(image.Width+10);
                int x2 = random.Next(image.Width+10);
                int y1 = random.Next(image.Height+10);
                int y2 = random.Next(image.Height+10);

                Pen p = new Pen(Color.Gray, 4);
                g.DrawLine(p, x1, y1, x2, y2);
            }

            Font font = new System.Drawing.Font("Arial", 15, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, System.Drawing.Color.DarkRed, 1.2f, true);
            g.DrawString(checkCode, font, brush, 5, 5);

            //画图片的前景噪音点
            for (int i = 0; i < 200; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);

                image.SetPixel(x, y, System.Drawing.Color.FromArgb(random.Next()));
            }

            //画图片的边框线
            g.DrawRectangle(new System.Drawing.Pen(System.Drawing.Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);

            HttpContext.Current.Response.ClearContent();
            HttpContext.Current.Response.ContentType = "image/jpg";
            HttpContext.Current.Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            image.Dispose();
        }
    }
   

}

2.在用到验证码的界面添加一个img控件，并且src属性指向.ashx文件

<img alt="看不清，换一张" src="CheckCode.ashx" onclick="this.src='CheckCode.ashx?abc='+Math.random()" />

3.提交后先判断验证码是否正确

 //检查验证码是否正确
        if(this.txt_checkcode.Text.ToLower ()!=this.Request.Cookies["checkcode"].Value.ToLower () )
        {
            //写脚本提示
           if( this.ClientScript.IsStartupScriptRegistered ("checkcode")==false)
           {
               this.ClientScript.RegisterStartupScript(this.GetType (),"checkcode","<script>alert('验证码错误!');</script>");
           }
            return;
        }