生成验证码方法

 public static string GenerateCaptchaCode(int len)
         {
             //组成验证码的字符
             char[] date = { ‘a‘, ‘c‘, ‘d‘, ‘e‘, ‘f‘, ‘g‘, ‘k‘, ‘m‘, ‘p‘, ‘r‘, ‘s‘, ‘t‘, ‘w‘, ‘x‘, ‘y‘, ‘3‘, ‘4‘, ‘5‘, ‘7‘, ‘8‘ };
             StringBuilder sbCode = new StringBuilder();
             Random rand = new Random();
             for (int i = 0; i < len; i++)
             {
                 char ch = date[rand.Next(date.Length)];//[0,date.Length]范围
                 sbCode.Append(ch);
             }
             return sbCode.ToString();
         }