using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace AuthCode
{
    [ToolboxData("〈{0}:AuthCode runat=server>〈/{0}:AuthCode>")]
    public class AuthCode : WebControl
    {
        /// 〈summary>
        /// 获得验证码的值
        /// 〈/summary>
        /// 〈returns>验证码〈/returns>
        public string GetValue()
        {
            return HttpContext.Current.Session["value"].ToString();
        }
        [Bindable(true)]
        [Category("Appearance")]
        [Description("验证码字符长度")]
        [DefaultValue("ss")]
        [Localizable(true)]
        //长度
        internal static int mySize;

        public int MySize
        {
            get { return AuthCode.mySize; }
            set
            {
                AuthCode.mySize = value;
              
            }
        }
     

        public AuthCode()
            : base(HtmlTextWriterTag.Img)//重写父类的构造（输出流的HTML标记）
        { }
        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            base.AddAttributesToRender(writer);//将要输出的的HTML标签的属性和样式添加到指定的 HtmlTextWriter中
            writer.AddStyleAttribute(HtmlTextWriterStyle.Cursor, "pointer");//添加样式

            /**-
             * 图片的onclick事件 "this.src='VerifyImg.jd?id='+Math.random()"
             * 每次单击一次就有一个新的图片请求路径（VerifyImg.jd?id='+Math.random()）参数只是
             * 告诉浏览器这是一个新的请求然后经过 IHttpHander处理生成新的图片 id 没有任何实际意思（创造一个新的请求）
             * -**/
            writer.AddAttribute("onclick", "this.src='img.jd?id='+Math.random()");//添加js VerifyImg.jd

            writer.AddAttribute(HtmlTextWriterAttribute.Src, "img.jd");
            writer.AddAttribute("alt", "点击刷新");
        }

    }
}
 

      3.2--》新建处理类（必须实现 IHttpHandler,IRequiresSessionState 两个接口）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI.WebControls;
using System.Web.UI;
using System.Web.SessionState;
using System.Drawing;
using System.IO;

namespace AuthCode
{
   public class AuthCodeHttpHander:IHttpHandler,IRequiresSessionState
    {
        /// 〈summary>
        /// 返回验证码字符
        /// 〈/summary>
        /// 〈param name="codeCount">验证码长度〈/param>
        /// 〈returns>〈/returns>
        private string GetRandomNumberString(int codeCount)
        {
            string strChoice = "2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z";
            string[] strResult = strChoice.Split(new Char[] { ',' });
            string strReturn = "";
            Random rnd = new Random();
            for (int i = 0; i 〈 codeCount; i++)
            {
                int j = rnd.Next(strResult.Length);//随机数不能大于数组的长度
                strReturn = strReturn + strResult[j].ToString();
            }
            return strReturn;
        }

        private Color GetColor()
        {
            return Color.Black;
        }
        private Bitmap CreateImage(string str_AuthCode)
        {
            /* -----------------------------绘制图片的样式 ------------------------------------*/

            int width =str_AuthCode.Length* 21;
            int height = 30;
            Random rad = new Random();
            Bitmap bmp = new Bitmap(width, height);
            Graphics grp = Graphics.FromImage(bmp);// 在图片上绘制图形
            grp.Clear(Color.YellowGreen);//填充bmp的背景色
            grp.DrawRectangle(new Pen(Color.Red, 1), 0, 0, width - 1, height - 1);//绘制边框
            int num = width * height;
            for (int i = 0; i 〈 num; i++)//在图片的指定坐标上画上有颜色的圆点
            {
                int x = rad.Next(width);
                int y = rad.Next(height);
                int r = rad.Next(255);
                int g = rad.Next(255);
                int b = rad.Next(255);
                Color c = Color.FromArgb(r, g, b);
                bmp.SetPixel(x, y, c);//在图片的指定坐标上画上有颜色的圆点
            }

            /*-------------------------- 在图片绘制字符串------------------------------------ */

            Font f = new Font("宋体", 20, FontStyle.Bold);//定义字体
            Brush br = new SolidBrush(Color.Black);//定义画笔的颜色 及字体的颜色
            for (int i = 0; i 〈 str_AuthCode.Length; i++)
            {
                string s = str_AuthCode.Substring(i, 1);//单个单个的将字画到图片上
                Point p = new Point(i * 20 + rad.Next(3), rad.Next(3) + 1);//字体出现的位置（坐标）
                grp.DrawString(s, f, br, p);//绘制字符串
            }
            grp.Dispose();
            return bmp;//返回

        }

        /// 〈summary>
        /// 是否可以处理远程的HTTP请求
        /// 〈/summary>
        public bool IsReusable
        {
            get { return true; }
        }

        /// 〈summary>
        /// 将验证码图片发送给WEB浏览器
        /// 〈/summary>
        /// 〈param name="context">〈/param>
        public void ProcessRequest(HttpContext context)
        {
            int size = AuthCode.mySize; //Int32.Parse((String)context.Session["Size"]);
            MemoryStream ms = new MemoryStream(); //  创建内存流(初始长度为0 自动扩充)
            string NumStr = GetRandomNumberString(size);// 获得验证码字符
            context.Session.Add("value", NumStr);//将验证码字符保存到session里面
            Bitmap theBitmap = CreateImage(NumStr);// 获得验证码图片
            theBitmap.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);//将位图写入内存流
            context.Response.ClearContent();  //清除缓冲区里的所有内容输出
            context.Response.ContentType = "image/jpeg"; //需要输出图象信息 要修改HTTP头
            context.Response.BinaryWrite(ms.ToArray()); //将内存流写入HTTP输出流
            theBitmap.Dispose(); //释放资源
            ms.Close();//释放资源
            ms.Dispose();//释放资源
            context.Response.End();
        }

  
    }
}

〈%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default"  %>

〈%@ Register assembly="AuthCode" namespace="AuthCode" tagprefix="cc1" %>

〈!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>无标题页〈/title>
〈/head>
〈body>
    〈form id="form1" runat="server">
    〈div>
        〈cc1:AuthCode ID="AuthCode1" runat="server" MySize="5" />
    〈/div>
    〈/form>
〈/body>
〈/html>

设置验证码字符长度

6--》在webconfig文件添加节点

〈system.web>
〈httpHandlers>
      〈add verb="*" path="*.jd" type="AuthCode.AuthCodeHttpHander" />
〈/httpHandlers>
〈/system.web>