C#如何获取一个月中的第几天

Question

如何获取特定年份特定工作日的所有日期，例如：

Monday : 07.01.2015,14.01.2015,...,21.02.2015, ,28.12.2015
Tuesday : 08.01.2015, 15.01.2015,..., 22.02.2015, 29.12.2015

等...

谢谢

Answer 1

public static IEnumerable<int> DaysInMonth(int year, int month, DayOfWeek dow)
{ 
    DateTime monthStart = new DateTime(year, month, 1);
    return Enumerable.Range(0, DateTime.DaysInMonth(year, month))
        .Select(day => monthStart.AddDays(day))
        .Where(date => date.DayOfWeek == dow)
        .Select(date => date.Day);
}

你的例子：

var wednesdaysInSeptember2015 = DaysInMonth(2015, 9, DayOfWeek.Wednesday);
Console.Write(String.Join(",", wednesdaysInSeptember2015)); // 2,9,16,23,30

值得一提的是，这是一个性能优化版本：

public static IEnumerable<int> DaysInMonthMicroOptimized(int year, int month, DayOfWeek dayOfWeek)
{
    DateTime monthStart = new DateTime(year, month, 1);
    int distanceToDayOfWeek = (dayOfWeek < monthStart.DayOfWeek ? 7 : 0) + dayOfWeek - monthStart.DayOfWeek;
    DateTime dayOfWeekInMonth = monthStart.AddDays(distanceToDayOfWeek);
    yield return dayOfWeekInMonth.Day;
    while((dayOfWeekInMonth = dayOfWeekInMonth.AddDays(7)).Month == month)
        yield return dayOfWeekInMonth.Day;
}

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