在需要验证的页面,加上<IMG alt="" src="CheckCode.aspx">

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 common
{
 /// <summary>
 /// CheckCode 的摘要说明。
 /// </summary>
 public class ValidateCode : System.Web.UI.Page
 {
  private void Page_Load(object sender, System.EventArgs e)
  {
   this.CreateCheckCodeImage(GenerateCheckCode());
  }

#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

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

System.Random random = new Random();

for(int i=0; i<4; i++) // 控制长度
   {
    number = random.Next();

if(number % 2 == 0)
     code = (char)('0' + (char)(number % 10));
    else
     code = (char)('0' + (char)(number % 10)); // 使用数字
     //code = (char)('A' + (char)(number % 26)); // 使用字母

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.Silver), 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.Black, Color.Black, 1.2f, true); // 两个Color.Black为字母混合颜色
    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();
   }
  }
 }
}