SiteHelper.cs

internal static string GetNoLastNumber(int? random = null)
        {
            var dictionary = new[] { "a", "b", "c", "d", "e", "f", "g", "h", "k", "m", "p", "r" };
            
            if (random.HasValue==false)
            {
                random = new Random().Next(0, 9);
            }
            return dictionary[random.Value];
        }
        
        
        /// <summary>
        /// 產出活動序號
        /// </summary>
        /// <param name="eventDate">The event date.</param>
        /// <returns>System.String.</returns>
        public static string GenerateNo(DateTime eventDate)
        {
            var day1 = (eventDate.Day / 10);
            var day2 = (eventDate.Day % 10);
            return
                $"{GetNoLastNumber(eventDate.Month - 1)}{GetNoLastNumber(day1)}{GetNoLastNumber(day2)}{GetNoLastNumber()}";
        }

Thanks a lot. Also,