网页的验证码

1.首先可以写一个产生随机验证码的aspx文件，如下产生四位数字：
    private void Page_Load(object sender, System.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 < 4; i++)
        {
            number = random.Next();

            code = (char)('0' + (char)(number % 10));
            checkCode += code.ToString();
        }

Response.Cookies.Add(new HttpCookie("CheckCode", checkCode));
//为了今后的验证

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 < 15; 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 < 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.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();
        }
    }
这个文件名设为CheckCode，它放在CheckCode.aspx的CheckCode.aspx.cs下就可。
2.然后在需要放验证图片的地方加上如下代码：
<img src="common/CheckCode.aspx" alt=""/>就能产生验证码了
3.当需要取得验证图片值的时候可参考编写如下代码
if (String.Compare(Request.Cookies["CheckCode"].Value, validateTb.Text, true) != 0)
        {
            //validateTb为文本框
            validateTb.Focus();
            ErrorMsg.Text = "验证码输入错误!";
            return;
        }