大型网站中的验证码解决方案

        private string contentType = "image/pjpeg";
        private ImageFormat imageFormat = ImageFormat.Jpeg;
        public Bitmap Image
        {
            get;
            set;
        }
        public string ContentType
        {
            get { return contentType; }
            set { contentType = value; }
        }
        public ImageFormat ImageFormat
        {
            get { return imageFormat; }
            set { imageFormat = value; }
        }

    public interface IVerifyImage
    {
        VerifyImageInfo GenerateImage(string code, int width, int height, Color bgcolor, int textcolor);
    }

        private static Hashtable _instance = new Hashtable();
        private static object lockHelper = new object();
        //通过接口和反射结合配置文件信息来做整站多验证码技术
        public static IVerifyImage GetInstance(string assemlyName)
        {
            if (!_instance.Contains(assemlyName))
            {
                lock (lockHelper)
                {
                    if (!_instance.ContainsKey(assemlyName))
                    {
                        IVerifyImage p = null;
                        try
                        {
                            p = (IVerifyImage)Activator.CreateInstance(Type.GetType(string.Format("Plugin.VerifyImage.{0}.VerifyImage,Plugin.VerifyImage.{0}", assemlyName), false, true));
                        }
                        catch
                        {
                            p = new Plugin.VerifyImage.JpegImage.VerifyImage();
                        }
                        _instance.Add(assemlyName, p);
                    }
                }
            }
            return (IVerifyImage)_instance[assemlyName];
        }

        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);
            string verifyAssembly = "JpegImage";//此处信息可从配置文件获得
            string bgColor = "";//Request.QueryString["bgcolor"].Trim();
            int fontColor = 1; //StrToInt(Request.QueryString["fontcolor"].Trim(), 1);
            string[] bgcolorArray = bgColor.Split(',');
            Color bg = Color.White;
            if (bgcolorArray.Length == 1 && bgColor != string.Empty)
            {
                bg = ToColor(bgColor);
            }
            else if (bgcolorArray.Length == 3 && IsNumericArray(bgcolorArray))
            {
                bg = Color.FromArgb(StrToInt(bgcolorArray[0], 255), StrToInt(bgcolorArray[1], 255), StrToInt(bgcolorArray[2], 255));
            }
            VerifyImageInfo verifyimg = Plugin.VerifyImage.VerifyImageProvider.GetInstance(verifyAssembly).GenerateImage(CreateAuthStr(5, false), 120, 60, bg, fontColor);

            Bitmap image = verifyimg.Image;

            System.Web.HttpContext.Current.Response.ContentType = verifyimg.ContentType;

            //MemoryStream ms = new MemoryStream();

            image.Save(this.Response.OutputStream, verifyimg.ImageFormat);

        }

<script type="text/javascript">
   document.getElementById('vcodeimg').src='/tools/VerifyImagePage.aspx?id=4&time=' + Math.random();
</script>