/// <summary>
    /// 生成驗證碼圖片
    /// </summary>
    /// <returns></returns>
    public byte[] GetVerifyCode()
    {
      int codeW = 80;
      int codeH = 40;
      int fontSize = 18;
      string chkCode = string.Empty;
      //顏色列表，用于驗證碼、噪線、噪點 
      Color[] color = { Color.Black, Color.Red, Color.Blue, Color.Green, Color.Orange, Color.Brown, Color.Brown, Color.DarkBlue };
      //字體列表，用于驗證碼 
      string[] font = { "Times New Roman" };
      //驗證碼的字符集，去掉了一些容易混淆的字符 
      char[] character = { '2', '3', '4', '5', '6', '8', '9', 'a', 'b', 'd', 'e', 'f', 'h', 'k', 'm', 'n', 'r', 'x', 'y', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'R', 'S', 'T', 'W', 'X', 'Y' };
      Random rnd = new Random();
      //生成驗證碼字符串 
      for (int i = 0; i < 4; i++)
      {
        chkCode += character[rnd.Next(character.Length)];
      }

      //創建畫布
      Bitmap bmp = new Bitmap(codeW, codeH);
      Graphics g = Graphics.FromImage(bmp);
      g.Clear(Color.White);
      //畫噪線 
      for (int i = 0; i < 1; i++)
      {
        int x1 = rnd.Next(codeW);
        int y1 = rnd.Next(codeH);
        int x2 = rnd.Next(codeW);
        int y2 = rnd.Next(codeH);
        Color clr = color[rnd.Next(color.Length)];
        g.DrawLine(new Pen(clr), x1, y1, x2, y2);
      }
      //畫驗證碼字符串 
      for (int i = 0; i < chkCode.Length; i++)
      {
        string fnt = font[rnd.Next(font.Length)];
        Font ft = new Font(fnt, fontSize);
        Color clr = color[rnd.Next(color.Length)];
        g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * 18, (float)0);
      }
      //將驗證碼圖片寫入內存流，并將其以 "image/Png" 格式輸出 
      MemoryStream ms = new MemoryStream();
      try
      {
        bmp.Save(ms, ImageFormat.Png);
        return ms.ToArray();
      }
      catch (Exception)
      {
        return null;
      }
      finally
      {
        g.Dispose();
        bmp.Dispose();
      }
    }