static void Main(string[] args) { var ym1 = new YearMonth(2019, 2); var ym2 = new YearMonth(2017, 12); var ym3 = new YearMonth(2016, 2); var ym4 = new YearMonth(2013, 11); var ym5 = new YearMonth(2019, 12); var ym = new YearMonth[] { ym1, ym2, ym3, ym4, ym5 }; for (int i = 0; i < ym.Length; i++) { Console.WriteLine(ym[i]); } string a = ym.Contains(Is21(ym)) ? Is21(ym).Year.ToString(): "21世紀のデータがありません"; Console.WriteLine(a); var years = ym.Select(s => s.AddOneMonth()).OrderBy(y => y.Year); foreach (var b in years) { Console.WriteLine(b); } }