c#实现输出本月的月历

格式要求：

SU MO TU WE TH FR SA
         01 02 03 04
05 06 07 08 09 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30

代码：

    class Interview1
    {
        static void Main()
        {
            PrintCalender(2011, 10);
        }
        public static void PrintCalender(int year, int month)
        {
            formatDate fd = new formatDate(year, month);
            string calender =
           @"SU MO TU WE TH FR Sa
            {0} {0} {0} {0} {0} {0} {0} {0}
            {0} {0} {0} {0} {0} {0} {0} {0}
            {0} {0} {0} {0} {0} {0} {0} {0}
            {0} {0} {0} {0} {0} {0} {0} {0}
            {0} {0} {0} {0} {0} {0} {0} {0}";

            calender = string.Format(calender, fd).TrimEnd();
            Console.WriteLine(calender);
        }
    }
    class formatDate : IFormattable
    {
        int num;
        int max;
        public formatDate(int year, int month)
        {
            DateTime dt = new DateTime(year, month, 1);
            num = (int)dt.DayOfWeek * -1;
            max = DateTime.DaysInMonth(year, month);
        }
        public string ToString(string format,IFormatProvider formatProvider)
        {
            return num++ < 0 || num > max ? "  " : num.ToString("00");
        }
    }