现在很多网页登陆的时候都使用了随机图片的方式，是一种简单、有效的防止黑客恶意攻击的手段。今天看了一些网上的资料，明白其生成原理：从样本中，获取随机字符串，随机字符串保存进session，并以位图的方式形成随机码图片。 实现：添加命名空间 using System.Drawing;using System.Drawing.Imaging;using System.Drawing.Drawing2D;生成页代码 using System;using System.Data;using System.Configuration;using System.Collections;using System.Web;using System.Web.Security;using System.Web.UI;using System.Web.UI.WebControls;using System.Web.UI.WebControls.WebParts;using System.Web.UI.HtmlControls;using System.Drawing;using System.Drawing.Imaging;using System.Drawing.Drawing2D; public partial class getRandImg : System.Web.UI.Page{    protected void Page_Load(object sender, EventArgs e)    {        //生成随机码图片        SetValidateCode();        //生成页面不保存到cache        Response.Cache.SetNoStore();    }     //设置验证码    private void SetValidateCode()    {        //新建位图        Bitmap newBitmap = new Bitmap(                                        71,                                         23,                                         PixelFormat.Format32bppArgb                                     );        //从位图获得绘图画面        Graphics g = Graphics.FromImage(newBitmap);        //随机数生成器        Random r = new Random();        //绘图画面清空        g.Clear(Color.White);        //绘图画面划线干扰        for (int i = 0; i < 50; i++)        {            int x1 = r.Next(newBitmap.Width);            int x2 = r.Next(newBitmap.Width);            int y1 = r.Next(newBitmap.Height);            int y2 = r.Next(newBitmap.Height);            g.DrawLine(new Pen(                                Color.FromArgb(r.Next())),                                 x1,                                 y1,                                 x2,                                 y2                              );        }        //绘图画面点数干扰        for (int i = 0; i < 100; i++)        {            int x = r.Next(newBitmap.Width);            int y = r.Next(newBitmap.Height);            newBitmap.SetPixel(                                x,                                 y,                                 Color.FromArgb(r.Next())                              );        }        //获得随机字符串（5位长度）        string value = GenerateRandom(5);        //随机字符串赋值给Session        Session["RandCode"] = value;        //定义图片显示字体样式        Font font = new Font(                               "Arial",                                14,                                FontStyle.Bold                            );        Random rr = new Random();        int yy = rr.Next(1, 4);        //定义随机字符串显示图片刷子        LinearGradientBrush brush = new LinearGradientBrush(                                                              new Rectangle(0, 0, 71, 23),                                                               Color.Red,                                                               Color.Blue,                                                               1.2f,                                                               true                                                           );        g.DrawString(value, font, brush, 2, yy);        g.DrawRectangle(new Pen(                                  Color.Silver),                                   0,                                   0,                                   70,                                   22                                );        System.IO.MemoryStream ms = new System.IO.MemoryStream();        newBitmap.Save(ms, ImageFormat.Gif);        //输出图片        Response.ClearContent();        Response.ContentType = "image/gif";        Response.BinaryWrite(ms.ToArray());    }     //常量集    private static char[] constant ={                                        '0','1','2','3','4','5','6','7','8','9',                                        'a','b','c','d','e','f','g','h','i','j',                                        'k','l','m','n','o','p','q','r','s','t',                                        'u','v','w','x','y','z'                                    };     //生成随机字符串    public static string GenerateRandom(int Length)    {        System.Text.StringBuilder newRandom = new System.Text.StringBuilder(36);        Random rd = new Random();        for (int i = 0; i < Length; i++)        {            newRandom.Append(constant[rd.Next(36)]);        }        return newRandom.ToString();    }} 使用随机图片的页面，IMAGE控件的写法如下： <asp:Image ID="Image1" ImageUrl="~/getRandImg.aspx" runat="server" /> 示例代码：http://www.cnblogs.com/Files/heekui/RandCode.rar

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