验证码是一张图片。我们需要在前台代码中写一段<img>，src指向一张页面（ValidateImage.aspx）。

代码如下:
<script language="javascript">

        function changeImg() {
            $("#imgCheckNo").attr("src", "ValidateImage.aspx?r=" + getRandom(999));
        }
        function getRandom(n) { return Math.floor(Math.random() * n + 1) }
</script>

<div>
<img id="imgCheckNo" src="ValidateImage.aspx" style="border-color: #000000; border-width: 1px;border-style: Solid">
 <span onclick="changeImg();">看不清?换一张</span>
 </div>
<div>
   <div class="labelCss">验证码：</div>
<div>
<asp:TextBox ID="tbxCheckNo" runat="server" CssClass="tbxCss"></asp:TextBox>
</div>
<div>
<asp:Button ID="btnSubmit" runat="server" Text="立即注册" OnClick="btnSubmit_Click" />
</div>

ValidateImage.aspx 用来生产验证码图片，并将验证码的码值保存到cookie中。
代码如下：

代码如下:
public partial class ValidateImage : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            this.CreateCheckCodeImage(RndNum());
        }

        /// <summary>生成验证码
        /// </summary>
        /// <returns></returns>
        private string RndNum()
        {
            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("yzmcode", checkCode));
            return checkCode;
        }

        /// <summary>向页面生成验证码Gif图片
        /// </summary>
        /// <param name="checkCode"></param>
        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.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();
            }
        }

    }

最后就是后台事件，判断当前验证码是否正确。
代码如下：

代码如下:
#region 页面控件事件
        protected void btnSubmit_Click(object sender, EventArgs e)
        {
            if (String.Compare(Request.Cookies["yzmcode"].Value, tbxCheckNo.Text, true) != 0)
            {
                Response.Write("<script>alert('验证码错误!')</script>");
            }
            else
            {
               // Response.Write("<script>alert('验证吗正确!!!')</script>");
            }
        }
  #endregion