可以看看，应该没什么问题

using System;

namespace MyDate
{
    class Program
    {
        static void Main(string[] args)
        {
            MyDate oneDate = new MyDate();
            MyDate twoDate = new MyDate(3, 22, 2021);
            twoDate.Day = 21;
            twoDate.DisplayDate();
        }
    }
    
     class MyDate
    {
        private int day, month, year;

        public int Day
        {
            get { return day; }
            set
            {
                if (Check(value, month, year)) day = value;
                Console.WriteLine("时间设置为：{0}", this);//this的用法？
            }
        }

        public int Month
        {
            get { return month; }
            set
            {
                if (Check(day, value, year)) month = value;
                Console.WriteLine("时间设置为：{0}", this);
            }
        }

        public int Year
        {
            get { return year; }
            set
            {
                if (Check(day, month, value)) year = value;
                Console.WriteLine("时间设置为：{0}", this);
            }
        }

        static private bool Check(int day,int month,int year)
        {
            if (Vaildity(day, month, year)) return true;
            Console.WriteLine("日期{0}/{1}/{2}不合法", day, month, year);
            return false;
        }

        static int[] day_12 = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
        static private bool Vaildity(int day, int month, int year)//检查日期的合法性
        {
            if (day < 0 || day > 31 || month < 0 || month > 12 || year < 0) return false;
            if (day > day_12[month - 1]) return false;
            if(month==2&&day==29)
            {
                if (year % 4 == 0) return true;
                else return false;
            }return true;
        }

        public MyDate() : this(1, 1, 1990) { }

        public MyDate(int a, int b, int c)
        {
            if(Vaildity(a,b,c))
            {
                day = a;month = b;year = c;
            }
            else
            {
                Console.WriteLine("设置的日期不合法");
                a = 1; b = 1; c = 1990;
                Console.WriteLine("默认日期为：{0}", this);
            }
        }
        public override string ToString() { return string.Format("{0}/{1}/{2}", day, month, year); }
        public void DisplayDate() { Console.WriteLine("\n" + this + "\n"); }
    }
  
}