asp.net 验证码功能

 public FileContentResult CreateImageCode()
        {
            string code = CreateVerifyCode();

            Session.SetCaptchaCode(code.ToUpper());//验证码存入Session中
            int fSize = FontSize;
            int fWidth = fSize + Padding;
            int imageWidth = (int)(code.Length * fWidth) + 4 + Padding * 2;
            int imageHeight = fSize * 2 + Padding;

            Bitmap image = new Bitmap(imageWidth, imageHeight);
            Graphics g = Graphics.FromImage(image);
            g.Clear(BackgroundColor);
            Random rand = new Random();

            //给背景添加随机生成的燥点
            if (this.Chaos)
            {
                Pen pen = new Pen(ChaosColor, 0);
                int c = Length * 10;

                for (int i = 0; i < c; i++)
                {
                    int x = rand.Next(image.Width);
                    int y = rand.Next(image.Height);

                    g.DrawRectangle(pen, x, y, 1, 1);
                }
            }

            int left = 0, top = 0, top1 = 1, top2 = 1;

            int n1 = (imageHeight - FontSize - Padding * 2);
            int n2 = n1 / 4;
            top1 = n2;
            top2 = n2 * 2;

            Font f;
            Brush b;
            int cindex, findex;

            //随机字体和颜色的验证码字符
            for (int i = 0; i < code.Length; i++)
            {
                cindex = rand.Next(Colors.Length - 1);
                findex = rand.Next(Fonts.Length - 1);

                f = new Font(Fonts[findex], fSize, FontStyle.Bold);
                b = new SolidBrush(Colors[cindex]);

                if (i % 2 == 1)
                {
                    top = top2;
                }
                else
                {
                    top = top1;
                }

                left = i * fWidth;

                g.DrawString(code.Substring(i, 1), f, b, left, top);
            }

            //画一个边框 边框颜色为Color.Gainsboro
            g.DrawRectangle(new Pen(Color.Gainsboro, 0), 0, 0, image.Width - 1, image.Height - 1);
            g.Dispose();

            //产生波形
            image = TwistImage(image, true, 0, 4);
            MemoryStream ms = new MemoryStream();
            //将图像保存到指定的流
            image.Save(ms, ImageFormat.Jpeg);
            return File(ms.GetBuffer(), "image/JPEG");
        }

  int length = 4;
        public int Length
        {
            get { return length; }
            set { length = value; }
        }

  int fontSize = 25;
        public int FontSize
        {
            get { return fontSize; }
            set { fontSize = value; }
        }

 int padding = 2;
        public int Padding
        {
            get { return padding; }
            set { padding = value; }
        }

 bool chaos = false;
        public bool Chaos
        {
            get { return chaos; }
            set { chaos = value; }
        }

 Color chaosColor = Color.LightGray;
        public Color ChaosColor
        {
            get { return chaosColor; }
            set { chaosColor = value; }
        }

 Color backgroundColor = Color.White;
        public Color BackgroundColor
        {
            get { return backgroundColor; }
            set { backgroundColor = value; }
        }

 Color[] colors = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
        public Color[] Colors
        {
            get { return colors; }
            set { colors = value; }
        }

 string[] fonts = { "Arial", "Georgia" };
        public string[] Fonts
        {
            get { return fonts; }
            set { fonts = value; }
        }

 string codeSerial = "1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,j,k,m,n,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,J,K,M,N,P,Q,R,S,T,U,V,W,X,Y,Z";
        public string CodeSerial
        {
            get { return codeSerial; }
            set { codeSerial = value; }
        }

  private const double PI = 3.1415926535897932384626433832795;
        private const double PI2 = 6.283185307179586476925286766559;

        /// <summary>
        /// 正弦曲线Wave扭曲图片（Edit By 51aspx.com）
        /// </summary>
        /// <param name="srcBmp">图片路径</param>
        /// <param name="bXDir">如果扭曲则选择为True</param>
        /// <param name="nMultValue">波形的幅度倍数，越大扭曲的程度越高，一般为3</param>
        /// <param name="dPhase">波形的起始相位，取值区间[0-2*PI)</param>
        /// <returns></returns>
        public Bitmap TwistImage(Bitmap srcBmp, bool bXDir, double dMultValue, double dPhase)
        {
            Bitmap destBmp = new Bitmap(srcBmp.Width, srcBmp.Height);

            // 将位图背景填充为白色
            Graphics graph = Graphics.FromImage(destBmp);
            graph.FillRectangle(new SolidBrush(Color.White), 0, 0, destBmp.Width, destBmp.Height);
            graph.Dispose();

            double dBaseAxisLen = bXDir ? (double)destBmp.Height : (double)destBmp.Width;

            for (int i = 0; i < destBmp.Width; i++)
            {
                for (int j = 0; j < destBmp.Height; j++)
                {
                    double dx = 0;
                    dx = bXDir ? (PI2 * (double)j) / dBaseAxisLen : (PI2 * (double)i) / dBaseAxisLen;
                    dx += dPhase;
                    double dy = Math.Sin(dx);

                    // 取得当前点的颜色
                    int nOldX = 0, nOldY = 0;
                    nOldX = bXDir ? i + (int)(dy * dMultValue) : i;
                    nOldY = bXDir ? j : j + (int)(dy * dMultValue);

                    Color color = srcBmp.GetPixel(i, j);
                    if (nOldX >= 0 && nOldX < destBmp.Width
                     && nOldY >= 0 && nOldY < destBmp.Height)
                    {
                        destBmp.SetPixel(nOldX, nOldY, color);
                    }
                }
            }

            return destBmp;
        }

 public string CreateVerifyCode(int codeLen = 4)
        {
            string[] arr = CodeSerial.Split(',');

            string code = string.Empty;

            int randValue = -1;

            Random rand = new Random(unchecked((int)DateTime.Now.Ticks));

            for (int i = 0; i < codeLen; i++)
            {
                randValue = rand.Next(0, arr.Length - 1);

                code += arr[randValue];
            }

            return code;
        }