using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace checkcode
{
 /// <summary>
 /// CheckCode 的摘要说明。
 /// </summary>
 public class CheckCode : System.Web.UI.Page
 {
  private void Page_Load(object sender, System.EventArgs e)
  {
   // 在此处放置用户代码以初始化页面
   CreateCheckCodeImage(GenerateCheckCode());
  }

  private string GenerateCheckCode()
  {
   int number;
   char code;
   string checkCode = String.Empty;
   System.Random random = new Random();

   for (int i = 0; i < 4; 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 < 2; 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.Black), x1, y1, x2, y2);
    }
    Font font = new System.Drawing.Font("Arial", 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 < 100; 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.Silver), 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();
    Response.ContentType = "image/Gif";
    Response.BinaryWrite(ms.ToArray());
   }
   finally
   {
    g.Dispose();
    image.Dispose();
   }
  }
  #region Web 窗体设计器生成的代码
  override protected void OnInit(EventArgs e)
  {
   //
   // CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
   //
   InitializeComponent();
   base.OnInit(e);
  }
  
  /// <summary>
  /// 设计器支持所需的方法 - 不要使用代码编辑器修改
  /// 此方法的内容。
  /// </summary>
  private void InitializeComponent()
  {   
   this.Load += new System.EventHandler(this.Page_Load);
  }
  #endregion
 }
}

<asp:Image id="Image1" style="Z-INDEX: 101; LEFT: 216px; POSITION: absolute; TOP: 0px" runat="server"
     Width="112px" src="CheckCode.aspx"></asp:Image>