鉴于DateTime和DayOfWeek应该返回该月的最后一个DayOfWeek的日期。
E.g。 2009年3月1日和周日将于2009年3月29日回归
答案 0 :(得分：12)
找不到方便的单行，但这个有效：
static DateTime LastDayOfWeekInMonth(DateTime day, DayOfWeek dow)
{
DateTime lastDay = new DateTime(day.Year, day.Month, 1).AddMonths(1).AddDays(-1);
DayOfWeek lastDow = lastDay.DayOfWeek;
int diff = dow - lastDow;
if (diff > 0) diff -= 7;
System.Diagnostics.Debug.Assert(diff <= 0);
return lastDay.AddDays(diff);
}
答案 1 :(得分：11)
http://datetimeextensions.codeplex.com/有一个Last（dayOfWeek）扩展方法，可以为您执行此操作
测试包括： - ）
答案 2 :(得分：6)
获取下个月的第一天，然后找到该月的第一个工作日。回到七天，你就是在正确月份的最后一周的那个工作日：
DateTime date = new DateTime(2009, 3, 12);
DayOfWeek day = DayOfWeek.Sunday;
DateTime nextMonth = new DateTime(date.Year, date.Month, 1).AddMonths(1);
while (nextMonth.DayOfWeek != day) {
nextMonth = nextMonth.AddDays(1);
}
DateTime lastInMonth = nextMonth.AddDays(-7);
（您可以使用一些算法来替换循环，该算法根据DayOfWeek值的数值计算要添加的天数，但这更直接。）
编辑：
您当然也可以在当月的最后一天获得，然后向后循环，直到找到正确的工作日。
答案 3 :(得分：4)
static DateTime LastDateOfWeekForMonth(DayOfWeek weekday, int month, int year)
{
DateTime d = new DateTime(year, month, 1).AddMonths(1);
while (!(d.DayOfWeek == weekday && d.Month == month))
{
d = d.AddDays(-1);
}
return d;
}
答案 4 :(得分：1)
试试这个：从最后一天开始算起，直到你找到你想要的那天
static DateTime LastOccurenceOfDay(DateTime dt, DayOfWeek dow)
{
DateTime looperDate = new DateTime(dt.Year, dt.Month, 1)
.AddMonths(1).AddDays(-1);
while (looperDate.DayOfWeek!=dow)
looperDate =looperDate.AddDays(-1);
return looperDate;
编辑： 恢复了Dangph在评论中引用的旧版本。
static DateTime LastOccurenceOfDay(DateTime dt, DayOfWeek dow)
{
//set start of loop
DateTime looperDate = new DateTime(dt.Year, dt.Month, 1);
//initialze return value
DateTime returnDate = dt;
//loop until the month is over
while (looperDate.Month == dt.Month)
{
//if the current DayOfWeek is the date you're looking for
if (looperDate.DayOfWeek == dow)
//remember it.
returnDate = looperDate;
//increase day
looperDate=looperDate.AddDays(1);
}
return returnDate;
}
答案 5 :(得分：1)
public DateTime GetLastDayOfMonth(int year, int month, DayOfWeek dayOfWeek)
{
var daysInMonth = DateTime.DaysInMonth(year, month);
var lastDay = new DateTime(year, month, daysInMonth);
while (lastDay.DayOfWeek != dayOfWeek)
{
lastDay = lastDay.AddDays(-1);
}
return lastDay;
}
答案 6 :(得分：1)
以下是LastDayOfMonth（）和LastDayOfMonth（DayOfWeek）以及单元测试，其灵感来自@Paul Fryer在How to find the 3rd Friday in a month with C#?中的“Last Friday ”实现：
/**
/// <summary>An extension method that returns the last day of the month.</summary>
/// <param name="d">A date within the month to calculate.</param>
/// <returns>The last day of the current month.</returns>
**/
public static DateTime LastDayOfMonth(this DateTime d)
{
DateTime nextMonth = new DateTime(d.Year, d.Month, 1).AddMonths(1);
return nextMonth.AddDays(-1);
}
/**
/// <summary>An extension method that returns the last <see cref="DayOfWeek"> of the month.</summary>
/// <param name="d">A date within the month to calculate.</param>
/// <returns>The last day of the current month.</returns>
**/
public static DateTime LastDayOfMonth(this DateTime d, DayOfWeek dayOfWeek)
{
DateTime lastDayOfMonth = d.LastDayOfMonth();
int vector = (((int)lastDayOfMonth.DayOfWeek - (int)dayOfWeek + DaysInWeek) % DaysInWeek);
return lastDayOfMonth.AddDays(-vector);
}
#region LastDayOfMonth Tests
[TestCase("1/1/2011", "1/31/2011")]
[TestCase("2/1/2009", "2/28/2009")] //Leap Year
[TestCase("2/1/2008", "2/29/2008")] //Leap Year
[TestCase("3/10/2011", "3/31/2011")]
[TestCase("4/20/2011", "4/30/2011")]
[TestCase("5/15/2011", "5/31/2011")]
[TestCase("6/30/2011", "6/30/2011")]
[TestCase("12/1/2011", "12/31/2011")]
public void LastDayOfMonthReturnsCorrectDay(string startingDate, string expectedDate)
{
//Arrange
DateTime testDate = DateTime.Parse(startingDate);
DateTime expected = DateTime.Parse(expectedDate);
//Act
DateTime actual = testDate.LastDayOfMonth();
//Assert
Assert.That(actual, Is.EqualTo(expected));
}
[TestCase("1/1/2011", DayOfWeek.Monday, "1/31/2011")]
[TestCase("2/1/2009", DayOfWeek.Saturday, "2/28/2009")]
[TestCase("2/1/2009", DayOfWeek.Sunday, "2/22/2009")]
[TestCase("2/1/2008", DayOfWeek.Friday, "2/29/2008")] //Leap Year
[TestCase("2/1/2008", DayOfWeek.Thursday, "2/28/2008")] //Leap Year
[TestCase("3/10/2011", DayOfWeek.Wednesday, "3/30/2011")]
[TestCase("4/20/2011", DayOfWeek.Friday, "4/29/2011")]
[TestCase("4/20/2011", DayOfWeek.Saturday, "4/30/2011")]
[TestCase("5/15/2011", DayOfWeek.Tuesday, "5/31/2011")]
[TestCase("5/15/2011", DayOfWeek.Saturday, "5/28/2011")]
[TestCase("9/30/2011", DayOfWeek.Sunday, "9/25/2011")]
[TestCase("12/1/2011", DayOfWeek.Monday, "12/26/2011")]
public void LastDayOfMonthReturnsCorrectDayOfWeek(string startingDate, DayOfWeek dayOfWeek, string expectedDate)
{
//Arrange
DateTime testDate = DateTime.Parse(startingDate);
DateTime expected = DateTime.Parse(expectedDate);
//Act
DateTime actual = testDate.LastDayOfMonth(dayOfWeek);
//Assert
Assert.That(actual, Is.EqualTo(expected));
}
#endregion
答案 7 :(得分：0)
static DateTime GetDayOfWeekInMonthFromWeekIndex(this DateTime date, DayOfWeek dow, int weekIndex)
{
if (weekIndex > 4)
return LastDayOfWeekInMonth(date, dow);
DateTime newDate = new DateTime(date.Year, date.Month, 1);
DayOfWeek newDow = newDate.DayOfWeek;
int diff = dow - newDow;
diff += ((weekIndex - 1) * 7);
return newDate.AddDays(diff);
}
答案 8 :(得分：0)
值得一提的是
int LastDayOfMonth = DateTime.DaysInMonth（now.year，now.month）;