asp.net验证码的实现

1.private void Page_Load(object sender, System.EventArgs e)
        {    
            //建立位图对象
            Bitmap newBitmap=new Bitmap(36,16,System.Drawing.Imaging.PixelFormat.Format32bppArgb);
            //根据上面创建的位图对象创建绘图层
            Graphics g=Graphics.FromImage(newBitmap);
            //以指定的颜色填宠矩形区
            g.FillRectangle(new SolidBrush(Color.White),new Rectangle(0,0,36,16));
            //创建字体对象
            Font textFont=new Font("Times new Roman",10);
            //创建RectangleF结构指定一个区域
            RectangleF rectangle=new RectangleF(0,0,36,16);
            //创建随机数对象
            Random rd=new Random();
            //取得随机数
            int valationNo=1000+rd.Next(8999);
            //使用指定的颜色填充上面的RectangleF结构指定的矩形区域
            g.FillRectangle(new SolidBrush(Color.BurlyWood),rectangle);
            //在上面填充的矩形区域中填充上面生成的随机数
            g.DrawString(valationNo.ToString(),textFont,new SolidBrush(Color.Blue),rectangle);
            //把创建的位图保存到指定的路径
            newBitmap.Save(Server.MapPath("img")+"\\RandomImg.gif",System.Drawing.Imaging.ImageFormat.Gif);
        }

    首先创建Bitmap和Graphics对象,然后通过Font和RectangleF对象把Random对象生成的随机数绘制到位图上，最后把创建的位图保存到指定的目录下.
    注:首先运行程序,把img目录下生成的RandomImg.gif图片包括在工程中,然后设置页面Image的ImageUrl属性=RandomImg.gif.

2.

 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();
        try
        {
            int flag;
            flag = Int16.Parse(Request.QueryString["flag"]);
            random = new Random(flag);
        }
        catch
        {
        }

        for (int i = 0; i < 5; i++)
        {
            number = random.Next();

            if (number % 2 == 0)
                code = (char)('O' + (char)(number % 10));
            else
                code = (char)('A' + (char)(number % 26));

            checkCode += code.ToString();
        }

        Session.Add("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", 11, (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, 1.1f, true);
            g.DrawString(checkCode, font, brush, 2, 2);

            //画图片的前景噪音点
            for (int i = 0; i < 150; 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/Jpeg";
            Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            image.Dispose();
        }
    }
}