ASP.NET随机码生成示例

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,
     &nb