Asp.Net 动态生成验证码

我们在设计用户登录模块时，经常会用到验证码，可以有效地防止黑客软件的恶意破解，现公开我常用的验证码的源代码。
 
使用方法： 
         1、在Web项目中添加一个类，如“CreateImage.cs”，然后将我公布的源代码Copy进去； 
         2、再新建一个Web窗体，如“Image.aspx”，在Page_Load中加入代码 “CreateImage.DrawImage ();”当然别忘了加上对类的引用哦！！ 
         3、在页面的合适位置上（你想放验证码的位置）上加上如下javascript代码就OK 了。 
<script language="JavaScript">
<!--
var numkey = Math.random();
numkey = Math.round(numkey*10000);
document.write("<img src=/"Image.aspx?k="+ numkey +"/" width=/"52/" height=/"23/" hspace=/"4/"");
//-->
</script><script language="JavaScript">
<!--
var numkey = Math.random();
numkey = Math.round(numkey*10000);
document.write("<img src=/"Image.aspx?k="+ numkey +"/" width=/"52/" height=/"23/" hspace=/"4/"");
//-->
</script>
源代码如下： 
/// <summary>
/// 验证码模块
/// </summary>
public class CreateImage
{
    public static void DrawImage()
    {
        CreateImage img = new CreateImage();
        HttpContext.Current.Session["CheckCode"] = img.RndNum(4);
        img.CreateImages(HttpContext.Current.Session["CheckCode"].ToString());
    }

    /// <summary>
    /// 生成验证图片
    /// </summary>
    /// <param name="checkCode">验证字符</param>
    private void CreateImages(string checkCode)
    {
        int iwidth = (int) (checkCode.Length * 13);
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
        Graphics g = Graphics.FromImage(image);
        g.Clear(Color.White);
        //定义颜色
        Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
        //定义字体 
        string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
        Random rand = new Random();
        //随机输出噪点
        for (int i = 0; i < 50; i++)
        {
            int x = rand.Next(image.Width);
            int y = rand.Next(image.Height);
            g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
        }

        //输出不同字体和颜色的验证码字符
        for (int i = 0; i < checkCode.Length; i++)
        {
            int cindex = rand.Next(7);
            int findex = rand.Next(5);

            Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(c[cindex]);
            int ii = 4;
            if ((i + 1) % 2 == 0)
            {
                ii = 2;
            }
            g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * 12), ii);
        }
        //画一个边框
        g.DrawRectangle(new Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1);

        //输出到浏览器
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
        HttpContext.Current.Response.ClearContent();
        //Response.ClearContent();
        HttpContext.Current.Response.ContentType = "image/Jpeg";
        HttpContext.Current.Response.BinaryWrite(ms.ToArray());
        g.Dispose();
        image.Dispose();
    }

    /// <summary>
    /// 生成随机的字母
    /// </summary>
    /// <param name="VcodeNum">生成字母的个数</param>
    /// <returns>string</returns>
    private string RndNum(int VcodeNum)
    {
        string Vchar = "0,1,2,3,4,5,6,7,8,9";
        string[] VcArray = Vchar.Split(',');
        string VNum = ""; //由于字符串很短，就不用StringBuilder了
        int temp = -1; //记录上次随机数值，尽量避免生产几个一样的随机数

        //采用一个简单的算法以保证生成随机数的不同
        Random rand = new Random();
        for (int i = 1; i < VcodeNum + 1; i++)
        {
            if (temp != -1)
            {
                rand = new Random(i * temp * unchecked((int) DateTime.Now.Ticks));
            }
            int t = rand.Next(VcArray.Length);
            if (temp != -1 && temp == t)
            {
                return RndNum(VcodeNum);
            }
            temp = t;
            VNum += VcArray[t];
        }
        return VNum;
    }
}