验证码：<input name="txtCode0" type="text" id="txtCode0" class="login-text" size="" maxlength="">

<img src="CodeHandler.ashx" id="imgCode" alt="验证码" title="点击刷新验证码" align="middle"

                                    style="height: 24px; width: 70px;" onclick="refreshRandCode()">

//刷新验证码

        function refreshRandCode() {

            $('#imgCode').hide().attr('src',

            'CodeHandler.ashx?' + Math.floor(Math.random() * 100)).fadeIn();

        }

Session["checkCode"]　　

<%@ WebHandler Language="C#" Class="CodeHandler" %>

using System;

using System.Web;

using System.Drawing;

public class CodeHandler : IHttpHandler, System.Web.SessionState.IRequiresSessionState{

    public string charSet = "0,1,2,3,4,5,6,8,9,A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z";

    public void ProcessRequest (HttpContext context) {

        string validateCode = CreateRandomCode();

        context.Session["checkCode"] = validateCode;

        CreateImage(validateCode, context);

    }

    public bool IsReusable {

        get {

            return false;

        }

    }

    /// <summary>

    /// 生成验证码

    /// <param name="n">位数</param>

    /// <returns>验证码字符串</returns>

    private string CreateRandomCode(int n)

    {

        string[] CharArray = charSet.Split(',');

        string randomCode = "";

        int temp = -;

        Random rand = new Random();

        for (int i = ; i < n; i++)

        {

            if (temp != -)

            {

                rand = new Random(i * temp * ((int)DateTime.Now.Ticks));

            } int t = rand.Next(CharArray.Length - );

            if (temp == t)

            {

                return CreateRandomCode(n);

            } temp = t;

            randomCode += CharArray[t];

        }

        return randomCode;

    }

    private void CreateImage(string checkCode, HttpContext context)

    {

        int iwidth = (int)(checkCode.Length * +);

        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, );

        Graphics g = Graphics.FromImage(image);

        Font f = new System.Drawing.Font("Arial", , (System.Drawing.FontStyle.Italic | System.Drawing.FontStyle.Bold));        // 前景色

        Brush b = new System.Drawing.SolidBrush(Color.Black);        // 背景色

        g.Clear(Color.White);        // 填充文字

        g.DrawString(checkCode, f, b, , );        // 随机线条

        Pen linePen = new Pen(Color.Gray, );

        Random rand = new Random();

        for (int i = ; i < ; i++)

        {

            int x1 = rand.Next(image.Width);

            int y1 = rand.Next(image.Height);

            int x2 = rand.Next(image.Width);

            int y2 = rand.Next(image.Height);

            g.DrawLine(linePen, x1, y1, x2, y2);

        }        // 随机点

        for (int i = ; i < ; i++)

        {

            int x = rand.Next(image.Width);

            int y = rand.Next(image.Height);

            image.SetPixel(x, y, Color.Gray);

        }        // 边框

        g.DrawRectangle(new Pen(Color.Gray), , , image.Width - , image.Height - );        // 输出图片

        System.IO.MemoryStream ms = new System.IO.MemoryStream();

        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);

        context.Response.ClearContent();

        context.Response.ContentType = "image/Jpeg";

        context.Response.BinaryWrite(ms.ToArray());

        g.Dispose();

        image.Dispose();

    }    

}