C# 验证码生成

后台：

　　　　 //生成验证码
　　    public void CreateImage()
        {
　　　　　　　//获取4位验证码，并转成小写。
            string code = getRandomNumber().ToLower();
            //验证码赋值Cookie
            HttpCookie myCookie = new HttpCookie(Constant.LOGIN_VALIDATE_CODE);
            myCookie.Value = MD5Utils.getMD5(code);
            myCookie.Expires = DateTime.Now.AddMinutes();
            Response.Cookies.Add(myCookie);//字体名
            string strFontName = "宋体";
            //字体大小
            int intFontSize = ;
            //图像宽
            int intWidth = ;
            //图像长
            int intHeight = ;
            //背景颜色
            Color bgColor = ColorTranslator.FromHtml("#EFB3FF");
            //前景颜色
            Color foreColor = ColorTranslator.FromHtml("#FF0000");
            //产生字体
            Font forFont = new Font(strFontName, intFontSize, FontStyle.Bold);
            //生成图片
            Bitmap newBitmap = new Bitmap(intWidth, intHeight, PixelFormat.Format32bppArgb);
            Graphics g = Graphics.FromImage(newBitmap);
            //定义一个四方形框与字片一样大小
            Rectangle newRect = new Rectangle(, , intWidth, intHeight);
            //涂上背景色
            g.FillRectangle(new SolidBrush(bgColor), newRect);
            //写字
            g.DrawString(code, forFont, new SolidBrush(foreColor), , );
            MemoryStream mStream = new MemoryStream();
            //存入MemoryStream
            newBitmap.Save(mStream, ImageFormat.Gif);
            g.Dispose();
            newBitmap.Dispose();
            //发送
            Response.ClearContent();
            Response.ContentType = "image/GIF";
            Response.BinaryWrite(mStream.ToArray());
            HttpContext.ApplicationInstance.CompleteRequest();
        }
　　　　 //生成4位验证码
　　　　 public static string getRandomNumber(int Length)
        {
            System.Text.StringBuilder newRandom = new System.Text.StringBuilder();
            Random rd = new Random();
            for (int i = ; i < Length; i++)
            {
                //得到数字（根据情况二选一）
                newRandom.Append(constant[rd.Next()]);
                //得到字符和数字
                newRandom.Append(constant[rd.Next()]);
            }
            return newRandom.ToString();
        }
　　　　 //验证码库
　　　　 private static char[] constant =
        {
            '','','','','','','','','','',
            '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',
            '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'
        };                                

前台(点击图片触发更改验证码的方法)：

<img border="0" class="text4" align="absmiddle" style="cursor: pointer;" onclick="changeImages();" src="" name="createImage" title="点击换一张图片" id="Img3" />

JavaScript：

function changeImages() {
        var d = new Date();
        var obj = document.getElementById('Img');
        obj.src = "/Common/CreateImage?tmep_seq=" + d.getTime();
}
changeImages();