C#判断每月的天数及闰年包括2月

/// <summary>
        /// 判断闰，平年,获取每月的天数  包括2月
        /// </summary>
        /// <param name="year"></param>
        /// <param name="month"></param>
        /// <returns></returns>
        public static string GetMonthLastDay(int year, int month)
        {
            string day = DateTime.Now.Day.ToString();
            if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)
            {
                day = "31";
            }
            else if (month == 2)
            {
                if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
                {
                    day = "29";
                }
                else
                {
                    day = "28";
                }
            }
            else
            {
                day = "30";
            }
            return day;
        }