/// <summary>
        /// 生成驗證碼圖片
        /// </summary>
        /// <param name="code"></param>
        /// <returns></returns>
        public byte[] GenerateImg(string code)
        {
            Bitmap image = new Bitmap(code.Length * 25, 25);
            Graphics g = Graphics.FromImage(image);
            try
            {
                //清空圖片背景色
                g.Clear(Color.White);

                //增加背景干擾線
                Random random = new Random();
                for (int i = 0; i < 30; i++)
                {
                    int x1 = random.Next(image.Width);
                    int x2 = random.Next(image.Width);
                    int y1 = random.Next(image.Height);
                    int y2 = random.Next(image.Height);
                    //顏色可自定義
                    g.DrawLine(new Pen(Color.FromArgb(186, 212, 231)), x1, y1, x2, y2);
                }

                //定義驗證碼字體
                Font font = new Font("黑體", 12, (FontStyle.Bold | FontStyle.Italic));
                //定義驗證碼的刷子，這里采用漸變的方式，顏色可自定義
                LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.FromArgb(67, 93, 230), Color.FromArgb(70, 128, 228), 1.5f, true);

                //增加干擾點
                for (int i = 0; i < 100; i++)
                {
                    int x = random.Next(image.Width);
                    int y = random.Next(image.Height);
                    //顏色可自定義
                    image.SetPixel(x, y, Color.FromArgb(random.Next()));
                }

                //將驗證碼寫入圖片
                g.DrawString(code, font, brush, random.Next(5, 50), random.Next(5, 10));


                //圖片邊框
                g.DrawRectangle(new Pen(Color.FromArgb(93, 142, 228)), 0, 0, image.Width - 1, image.Height - 1);

                //保存圖片數據
                MemoryStream stream = new MemoryStream();
                image.Save(stream, ImageFormat.Jpeg);
                return stream.ToArray();
            }
            finally
            {
                g.Dispose();
                image.Dispose();
            }

        }

 private void button1_Click(object sender, EventArgs e)
        {
            GenerateCode();
        }

       private void GenerateCode()
        {
            string[] symbols = new string[]{
                "+","-","*"
            };
            Random random = new Random();
            string code = random.Next(0, 99) + symbols[random.Next(0,symbols.Length)] + random.Next(0, 99);

            //這里拿到結果用來驗證
            DataTable dt = new DataTable();
            string result = Convert.ToString(dt.Compute(code, null));
            label1.Text = result;

            byte[] bytes = GenerateImg(code);
            MemoryStream ms = new MemoryStream(bytes);
            Image image = System.Drawing.Image.FromStream(ms);
            pictureBox1.Image = image;
        }