月末最後の営業日の曜日 (paizaランク B 相当)

問題文
YouTube解説動画

        static void Main(string[] args)
        {
            int X = int.Parse(Console.ReadLine().Trim(' '));
            int all = 0;
            int[] lastBusinessDays = new int[7]; //日、月、火、水、木、金、土
           
            DateTime date = DateTime.Now;
            DateTime endDate = date.AddYears(400 * 7);
            while(date < endDate)
            {
                DateTime lastDate = new DateTime(date.Year, date.Month, DateTime.DaysInMonth(date.Year, date.Month));
                if(lastDate.DayOfWeek == DayOfWeek.Saturday)
                {
                    lastDate = lastDate.AddDays(-1);
                }else if(lastDate.DayOfWeek == DayOfWeek.Sunday)
                {
                    lastDate = lastDate.AddDays(-2);
                }
                DayOfWeek dow = lastDate.DayOfWeek;
                switch (dow)
                {
                    case DayOfWeek.Monday:
                        lastBusinessDays[1]++;
                        break;
                    case DayOfWeek.Tuesday:
                        lastBusinessDays[2]++;
                        break;
                    case DayOfWeek.Wednesday:
                        lastBusinessDays[3]++;
                        break;
                    case DayOfWeek.Thursday:
                        lastBusinessDays[4]++;
                        break;
                    case DayOfWeek.Friday:
                        lastBusinessDays[5]++;
                        break;
                }
                date = date.AddMonths(1);
                all++;
            }
            double result = (int)(((double)lastBusinessDays[X] / all) * 1000000 ) / 1000000.0;
            Console.WriteLine(result);
        }

メモ

月末最終日は年月の日数をDateTime.DaysInMonth(year,month)メソッドで求め、DateTimeインスタンスをつくる。

int lastDay = DateTime.DaysInMonth(year,mont)
DateTime day = new DateTime(year,month,lastDay)