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.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
public partial class Tools_CheckCode : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        this.CreateCheckCodeImage(GenerateCheckCode());
    }
    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)
                code = (char)('0'+ (char)(number %10));
            else
                code = (char)('A'+ (char)(number %26));
            checkCode += code.ToString();
        }
        Response.Cookies.Add(new HttpCookie("CheckCode", checkCode));
        return checkCode;
    }
    private void CreateCheckCodeImage(string checkCode)
    {
        if (checkCode ==null|| checkCode.Trim() == String.Empty)
            return;
        System.Drawing.Bitmap image =new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length *12.5)), 22);
        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 <25; i++)
            {
                int x1 = random.Next(image.Width);
                int x2 = random.Next(image.Width);
                int y1 = random.Next(image.Height);
                int y2 = random.Next(image.Height);
                g.DrawLine(new Pen(Color.GreenYellow), x1, y1, x2, y2);
            }
            Font font =new System.Drawing.Font("Verdana", 12, (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, Color.DarkRed, 1.2f, true);
            g.DrawString(checkCode, font, brush, 2, 2);
            //画图片的前景噪音点
        for (int i =0; i <80; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);
                image.SetPixel(x, y, Color.FromArgb(random.Next()));
            }
            //画图片的边框线
            g.DrawRectangle(new Pen(Color.Red), 0, 0, image.Width -1, image.Height -1);
            System.IO.MemoryStream ms =new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            Response.ClearContent();
            hc.Response.ContentType ="image/gif";
            Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            image.Dispose();
        }
    }

}

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

using System;
using System.Web;
using System.Web.SessionState;
using System.Drawing;
using System.Drawing.Imaging;
using System.Text;

/**////
<summary>
/// ValidateImageHandler 生成网站验证码功能
///
</summary>
public class ValidateImageHandler : IHttpHandler, IRequiresSessionState
{
    int intLength =5;              //长度
string strIdentify ="Identify"; //随机字串存储键值，以便存储到Session中
public ValidateImageHandler()
    {       
    }
    /**////
<summary>
    ///  生成验证图片核心代码
    ///
</summary>
    ///
<param name="hc"></param>
public  void ProcessRequest(HttpContext hc)
    {
        //设置输出流图片格式
        hc.Response.ContentType ="image/gif";
       
        Bitmap b =new Bitmap(200, 60);
        Graphics g = Graphics.FromImage(b);
        g.FillRectangle(new SolidBrush(Color.YellowGreen), 0, 0, 200, 60);
        Font font =new Font(FontFamily.GenericSerif, 48, FontStyle.Bold, GraphicsUnit.Pixel);
        Random r =new Random();
        //合法随机显示字符列表
string strLetters ="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        StringBuilder s =new StringBuilder();
       
        //将随机生成的字符串绘制到图片上
for (int i =0; i < intLength; i++)
        {
            s.Append(strLetters.Substring(r.Next(0, strLetters.Length -1), 1));
            g.DrawString(s[s.Length -1].ToString(), font, new SolidBrush(Color.Blue), i *38, r.Next(0, 15));
        }
        //生成干扰线条
        Pen pen =new Pen(new SolidBrush(Color.Blue), 2);
        for (int i =0; i <10; i++)
        {
            g.DrawLine(pen, new Point(r.Next(0, 199), r.Next(0, 59)), new Point(r.Next(0, 199), r.Next(0, 59)));
        }
        b.Save(hc.Response.OutputStream, ImageFormat.Gif);
        hc.Session[strIdentify] = s.ToString(); //先保存在Session中，验证与用户输入是否一致
        hc.Response.End();
 
    }
   
    /**////
<summary>
    /// 表示此类实例是否可以被多个请求共用(重用可以提高性能)
    ///
</summary>
public
bool IsReusable
    {
        get
        {
            return true;
        }
    }
}