网站验证码是一种很常用的技术。下面我介绍下技术上是如何实现的。

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）。

     
       
      看不清?换一张 
     
     
   
      
       验证码：
      
      
       
      
      
       
      
     

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：

public partial class ValidateImage : System.Web.UI.Page    {        protected void Page_Load(object sender, EventArgs e)        {            this.CreateCheckCodeImage(RndNum());        }        /// 
生成验证码        /// 
        /// 
             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;        }        /// 
向页面生成验证码Gif图片        /// 
        ///         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：

#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("");            }            else            {               // Response.Write("");            }        }  #endregion