using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Drawing;
using System.IO;

namespace SecurityCodePic
{
 public class DrawingSecurityCode
 {
  /// summary>
  /// 生成验证码，并返回
  /// /summary>
  /// returns>/returns>
  public string GetSecurityCode(int n)
  {
   string code = GenerateCheckCode(n);
   CreateCheckCodeImage(code);
   return code;
  }

  /// summary>
  /// 动态生成指定数目的随机数或字母
  /// /summary>
  /// param name="num">整数/param>
  /// returns>返回验证码字符串/returns>
  private string GenerateCheckCode(int num)
  {
   int number;//定义变量
   char code;
   string checkCode = String.Empty; //空字符串，只读
   Random random = new Random(); //定义随机变量实例
   for (int i=0; i  num;i++ )
   {
    //利用for循环生成指定数目的随机数或字母
    number = random.Next(); //返回一个小于指定的最大值的非负的随机数 next有三个构造函数 
    if (number % 2 == 0)
    {//产生一个一位数
     code = (char)('0' + (char)(number % 10));
    }
    else
    { //产生一个大写字母
     code = (char)('A'+(char)(number % 26));
    }
    checkCode += code.ToString();
   }
   return checkCode;
  }

  /// summary>
  /// 根据验证码字符串生成验证码图片
  /// /summary>
  /// param name="checkCode">验证码字符串/param>
  private void CreateCheckCodeImage(string checkCode)
  {

   if (checkCode == null || checkCode.Trim() == String.Empty) return;
   // 引用System.Drawing类库
   Bitmap myImage = new Bitmap(80, 30);//生成一个指定大小的位图
   Graphics graphics = Graphics.FromImage(myImage); //从一个位图生成一个画布
   try
   {
    graphics.Clear(Color.White); //清除整个绘画面并以指定的背景色填充,这里是把背景色设为白色
    Random random = new Random(); //实例化一个伪随机数生成器

    //画图片的前景噪音点,这里有100个
    for (int i = 0; i  100; i++)
    {
     int x = random.Next(myImage.Width);
     int y = random.Next(myImage.Height);
     myImage.SetPixel(x, y, Color.FromArgb(random.Next()));//指定坐标为x,y处的像素的颜色
    }

    //画图片的背景噪音线,这里为2条
    for (int i = 0; i  2; i++)
    {
     int x1 = random.Next(myImage.Width);
     int x2 = random.Next(myImage.Width);
     int y1 = random.Next(myImage.Height);
     int y2 = random.Next(myImage.Height);
     graphics.DrawLine(new Pen(Color.Black), x1, y1, x2, y2); //绘制一条坐标x1,y1到坐标x2,y2的指定颜色的线条，这里的线条为黑色
    }

    Font font = new Font("Arial", 15, FontStyle.Bold); //定义特定的文本格式,这里的字体为Arial，大小为15,字体加粗
    //根据矩形、起始颜色和结束颜色以及方向角度产生一个LinearGradientBrush实例---线性渐变
    System.Drawing.Drawing2D.LinearGradientBrush brush =
     new System.Drawing.Drawing2D.LinearGradientBrush(
      new Rectangle(0, 0, myImage.Width, myImage.Height),//在坐标0,0处实例化一个和myImage同样大小的矩形
      Color.Blue, Color.Red, 1.2f, true);
    //绘制文本字符串
    graphics.DrawString(checkCode, font, brush, 2, 2);

    //绘制有坐标对、宽度和高度指定的矩形---画图片的边框线
    graphics.DrawRectangle(new Pen(Color.Silver), 0, 0, myImage.Width - 1, myImage.Height - 1);
    //创建其支持存储器为内存的流
    MemoryStream ms = new MemoryStream();
    //将此图像以指定格式保存到指定的流中
    myImage.Save(ms, System.Drawing.Imaging.ImageFormat.Gif); //这里是以gif的格式保存到内存中
    HttpContext.Current.Response.ClearContent(); //清除缓冲区流中的所有内容输出
    HttpContext.Current.Response.ContentType = "image/Gif"; //获取或设置输出流的HTTP MIME类型
    HttpContext.Current.Response.BinaryWrite(ms.ToArray()); //将一个二进制字符串写入HTTP输出流
   }
   finally
   {
    //释放占用资源
    graphics.Dispose();
    myImage.Dispose();
   }
  }
 }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace SecurityCodePic
{
 /// summary>
 /// SecurityCode1 的摘要说明
 /// /summary>
 public class SecurityCode1 : IHttpHandler
 {

  public void ProcessRequest(HttpContext context)
  {
   DrawingSecurityCode sc = new DrawingSecurityCode();
   string SecurityCode = sc.GetSecurityCode(6);
  }

  public bool IsReusable
  {
   get
   {
    return false;
   }
  }
 }
}

%@ Page Language="C#" AutoEventWireup="true" CodeBehind="SecurityCode_Test.aspx.cs" Inherits="SecurityCodePic.SecurityCode_Test" %>

!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>验证码的实现/title>
 style type="text/css">
 #VCodeImg { cursor: pointer;}
 /style>
/head>
body>
 form id="form1" runat="server">
 div>
 img id="VCodeImg" src="SecurityCode.ashx" alt="验证码" onclick="javascript:RefreshCode();" />
 /div>
 /form>
 script type="text/javascript">
  function RefreshCode() {
   var random = Math.random();
   var img = document.getElementById("VCodeImg");
   img.src = "SecurityCode.ashx?" + random; //加上无意义的随机参数，浏览器才会认为是新地址，就会重新读取数据
  }
 /script>
/body>
/html>