c# - C#：找出每月的第三个星期一是哪个星期一？

Question

如果我正在编写一些贯穿一年日期（按天迭代）的 C# 代码，并且希望每个月的第三个星期一发生一些特别的事情，我该如何实现呢？

换句话说，找出当前星期一是一个月中哪个星期一的最佳方法是什么？

Answer 1

public bool IsThirdMondayOfMonth(DateTime dt)
{
  if(dt.DayOfWeek == DayOfWeek.Monday && dt.Day > 14 && dt.Day <= 21)
  {
    return true;
  }
  return false;
}

Answer 2

I don't think your "in other words" really restates the problem that you describe first, so I'll answer both.

Here's a fairly simple method that will determine the nth occurrence of a particular day of the week in a given month in a given year.

public static DateTime DayOccurrence(int year, int month, DayOfWeek day, 
    int occurrenceNumber)
{
    DateTime start = new DateTime(year, month, 1);
    DateTime first = start.AddDays((7 - ((int)start.DayOfWeek - (int)day)) % 7);

    return first.AddDays(7 * (occurrenceNumber - 1));
}

Determining which Monday (or any other day) of the month a date is is much easier; just take the ceiling of the day of the month / 7:

public static int DayOccurrence(DateTime date)
{
    return (int)Math.Ceiling(date.Day / 7.0);
}

Answer 3

Find the Monday that is between the 15th and the 21st, inclusive.

Answer 4

I don't know if there is a date manipulation library to do what you want, but you can write your own functions pretty easily:

using System;

class Program {
    static void Main(string[] args) {
        int year = 2010;
        int month = 05;
        DateTime thirdMonday = ThirdMonday(year, month);
    }

    private static DateTime ThirdMonday(int year, int month) {
        for (int day = 1; day <= DateTime.DaysInMonth(year, month); ++day) {
            DateTime dt = new DateTime(year, month, day);
            if (dt.DayOfWeek == DayOfWeek.Monday) {
                return dt.AddDays(14);
            }
        }
        // this should never happen
        throw new Exception();
    }
}