asp.net中验证码的实现

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
            {

//使用DateTime.Now.Millisecond作为生成随机数的参数，增加随机性
                randString  +=  RandCharString.Substring(random.Next( DateTime.Now.Millisecond) % 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;
            
        }
    }
}

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();
        }
    }
}

<% @ 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 ="width: 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; width: 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; width: 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 ="width: 83px" >
                 < asp:Button  ID ="btnLogin"  runat ="server"  Text ="登陆"  OnClick ="btnLogin_Click"   /></ td >< td  style ="width: 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 >