C# 平到下个月，因为这对我来说似乎更有用

我还加入了一个LINQPad可运行的示例程序

void Main()
{
    DayOfWeek dow = DayOfWeek.Friday;
    int y = 2014;
    int m = 2;

    String.Format("First {0}: {1}", new object[] { dow, DateHelper.FirstDayOfWeekInMonth(y, m, dow) }).Dump();

    "".Dump();

    String.Format("Last {0}: {1}", new object[] { dow, DateHelper.LastDayOfWeekInMonth(y, m, dow) }).Dump();

    "".Dump();

    for(int i = 1; i <= 6; i++)
        String.Format("{0} #{1}: {2}", new object[] { dow, i, DateHelper.XthDayOfWeekInMonth(y, m, dow, i) }).Dump();
}


public class DateHelper
{
    public static DateTime FirstDayOfWeekInMonth(int year, int month, DayOfWeek day)
    {
        DateTime res = new DateTime(year, month, 1);
        int offset = -(res.DayOfWeek - day);

        if (offset < 0)
            offset += 7;

        res = res.AddDays(offset);

        return res;
    }

    public static DateTime LastDayOfWeekInMonth(int year, int month, DayOfWeek day)
    {
        DateTime dt = new DateTime(year, month, 1).AddMonths(1);        
        DateTime res = FirstDayOfWeekInMonth(dt.Year, dt.Month, day);

        res = res.AddDays(-7);

        return res;
    }


    public static DateTime XthDayOfWeekInMonth(int year, int month, DayOfWeek day, int x)
    {
        DateTime res = DateTime.MinValue;

        if (x > 0)
        {
            res = FirstDayOfWeekInMonth(year, month, day);

            if (x > 1)
                res = res.AddDays((x - 1) * 7);

            res = res.Year == year && res.Month == month ? res : DateTime.MinValue;
        }

        return res;
    }
}

这是我的算法：

查找距即将到来的星期五的天数

初始化计数器并将其设置为1。从[1]返回的日期减去七天，然后将从返回日期算起的月份与从（1）返回的日期进行比较。

如果月份不相等，请从[2]返回计数器

如果月份相等，则递归到[2]，并将1添加到[2]中创建的计数器

---

柜台将为您提供该日期当月的第n个星期五（或即将到来的星期五）。

我将此日期时间传递给您，作为我所查看的月初的日期时间

    private DateTime thirdSunday(DateTime timeFrom)
    {
        List<DateTime> days = new List<DateTime>();
        DateTime testDate = timeFrom;

        while (testDate < timeFrom.AddMonths(1))
        {
            if (testDate.DayOfWeek == DayOfWeek.Friday)
            {
                days.Add(testDate);
            }
            testDate = testDate.AddDays(1);
        }

        return days[2];
    }

private DateTime thirdSunday（DateTime timeFrom）
{
列表天数=新列表（）；
DateTime testDate=timeFrom；
而（测试日期

以下方法效果很好，未提供事件验证。您可以找到给定日期月份的任何第n天，从开始或最后。如果要查找上一个事件，请提供负发生值

 public static DateTime GetDayOfMonth(DateTime dateValue, DayOfWeek dayOfWeek, int occurance)
    {
        List<DateTime> dayOfWeekRanges = new List<DateTime>();

        //move to the first of th month
        DateTime startOfMonth = new DateTime(dateValue.Year, dateValue.Month, 1);

        //move startOfMonth to the dayOfWeek requested
        while (startOfMonth.DayOfWeek != dayOfWeek)
            startOfMonth = startOfMonth.AddDays(1);

        do
        {
            dayOfWeekRanges.Add(startOfMonth);
            startOfMonth = startOfMonth.AddDays(7);
        } while (startOfMonth.Month == dateValue.Month);

        bool fromLast = occurance < 0;
        if (fromLast)
            occurance = occurance * -1;

        if (fromLast)
            return dayOfWeekRanges[dayOfWeekRanges.Count - occurance];
        else
            return dayOfWeekRanges[occurance - 1];
    }

public static DateTime GetDayOfMonth（DateTime dateValue、DayOfWeek-DayOfWeek、int-occurrence）
{
List dayOfWeekRanges=新列表（）；
//移到第个月的第一天
DateTime startOfMonth=新的日期时间（dateValue.Year，dateValue.Month，1）；
//将startOfMonth移动到请求的星期几
while（startOfMonth.DayOfWeek！=DayOfWeek）
startOfMonth=startOfMonth.AddDays（1）；
做
{
dayOfWeekRanges.Add（startOfMonth）；
startOfMonth=startOfMonth.AddDays（7）；
}while（startOfMonth.Month==dateValue.Month）；
bool fromLast=发生率<0；
if（fromLast）
发生率=发生率*-1；
if（fromLast）
返回Weekranges日[dayOfWeekRanges.Count-发生]；
其他的
周末回归日[发生-1]；
}

这是我的两分钱。。。 没有不必要的循环或测试的优化解决方案：

public static DateTime ThirdFridayOfMonth(DateTime dateTime)
{
    int day = dateTime.Day;
    return dateTime.AddDays(21 - day - ((int)dateTime.DayOfWeek + 37 - day) % 7);
}

---

我编写了@justcoding121代码的扩展版本，可以从每月的最后一天获得。我不知道这个算法是对的，但到目前为止它仍然有效

public static int? GetNthDayOfWeekInMonth(int year, int month, DayOfWeek dow, int weekNumOfMonth)
{
    if (weekNumOfMonth < -5 || weekNumOfMonth == 0 || weekNumOfMonth > 5)
        throw new ArgumentOutOfRangeException("weekNumOfMonth", $"must be between 1~5 or -1~-5. ({weekNumOfMonth})");

    int daysOfMonth = DateTime.DaysInMonth(year, month);

    if (weekNumOfMonth > 0)
    {
        var firstDay = new DateTime(year, month, 1);
        var firstDayOfTargetDOW = (int)dow - (int)firstDay.DayOfWeek;
        if (firstDayOfTargetDOW < 0)
            firstDayOfTargetDOW += 7;
        var resultedDay = (firstDayOfTargetDOW + 1) + (7 * (weekNumOfMonth - 1));

        if (resultedDay > daysOfMonth)
            return null;

        return resultedDay;
    }
    else
    {
        var lastDay = new DateTime(year, month, daysOfMonth);
        var firstDayOfTargetDOW = (int)lastDay.DayOfWeek - (int)dow;
        if (firstDayOfTargetDOW < 0)
            firstDayOfTargetDOW += 7;
        var resultedDay = firstDayOfTargetDOW + (7 * (Math.Abs(weekNumOfMonth) - 1));

        if (resultedDay > daysOfMonth)
            return null;

        return (daysOfMonth - resultedDay);
    }
}

---

我知道这个帖子很旧。我有这个解决方案，试图找到一个更干净的代码#伯伯叔叔

publicstaticboolshirdwednesday（DateTime-inputDate）
{
DateTime firstDayOfMonth=新的日期时间（inputDate.Year，inputDate.Month，1）；
DateTime firstDayOfNextMonth=firstDayOfMonth.AddMonths（1）；
int周三计数=0；
while（每月第一天<下个月第一天）
{
如果（每月的第一天。星期三的第二天==星期三的第二天）
星期三计数++；
如果（星期三计数=3）
{
如果（inputDate==每月的第一天）
返回true；
其他的
返回false；
}
firstDayOfMonth=每月第一天添加天数（1）；
}
返回false；
}

+1解释而不是给出代码，因为这是一个算法问题，而不是技术问题。这应该是答案，因为它很清楚，普遍适用，并且实际有效。我已经在它的基础上创建了一个C#类，并添加了它作为任何想要快速解决问题的人的答案。你有一个bug。它应该如下所示。否则我认为这是最干净的解决方案更改为每月第一天DateTime dayOfMonth=instance.AddDays（1-instance.day）；此测试：if（occurrence<0 | | occurrence>5）将允许出现值为#0，我认为这是一个错误。这是可行的，但TryGet方法是否应该引发异常？我认为您是根据Int32.TryParseMost等方法建模的，如果不是所有以Try开头的Microsoft.Net framework方法都不会对正常错误引发异常。相反，如果尝试以布尔值形式成功，它们将返回，并在out参数中返回有效结果（如不抛出异常）。现在考虑到我添加了额外的参数，验证参数并在这些参数无效时抛出异常仍然是最佳做法（我可能不应该检查实例，因为DateTime不能为null）。例如，可以在反射时同时抛出

ArgumentException

和

NotSupportedException

，将XthDayOfWeekInMonth更改为接受负数“x”表示“最后一个”、“倒数第二个”等是非常好的。但是我没有个人的使用案例，所以我没有这样做：o）LastDayOfWeekInMonth在12月失败，因为DateTime不能用第13个月来计算出您需要下一年的1月。该死的，我怎么从来没有注意到这一点？我会编辑一个补丁谢谢你的功能！效果很好，补充了马克的答案。谢谢。。。这真的很有帮助。：）欢迎来到SO。回答这个问题可能有助于解释您演示的代码背后的逻辑/基本原理。

public static DateTime GetThirdFriday(int year, int month)
{
   DateTime baseDay = new DateTime(year, month, 15);
   int thirdfriday = 15 + ((12 - (int)baseDay.DayOfWeek) % 7);
   return new DateTime(year, month, thirdfriday);
}

  private readonly static int[] thirdfridays =
      new int[] { 20, 19, 18, 17, 16, 15, 21 };

  public static int GetThirdFriday(int year, int month)
  {
     DateTime baseDay = new DateTime(year, month, 15);
     return thirdfridays[(int)baseDay.DayOfWeek];
  }

    public DateTime GetThirdThursday(DateTime now)
    {
        DateTime ThirdThursday;
        now = DateTime.Now;
        string wkday;
        DateTime firstday = new DateTime(now.Year, now.Month, 1);
        ThirdThursday = firstday.AddDays(15);

        // ThirdThursday = now.AddDays((now.Day - 1) * -1).AddDays(14);
        wkday = ThirdThursday.DayOfWeek.ToString();

        while (wkday.CompareTo("Thursday") < 0)

        {
            ThirdThursday.AddDays(1);
        }
        return ThirdThursday;
    }

public static bool TryGetDayOfMonth(this DateTime instance, 
                                 DayOfWeek dayOfWeek, 
                                 int occurance, 
                                 out DateTime dateOfMonth)
{
    if (instance == null)
    {
        throw new ArgumentNullException("instance");
    }

    if (occurance <= 0 || occurance > 5)
    {
        throw new ArgumentOutOfRangeException("occurance", "Occurance must be greater than zero and less than 6.");
    }

    bool result;
    dateOfMonth = new DateTime();

    // Change to first day of the month
    DateTime dayOfMonth = instance.AddDays(1 - instance.Day);

    // Find first dayOfWeek of this month;
    if (dayOfMonth.DayOfWeek > dayOfWeek)
    {
        dayOfMonth = dayOfMonth.AddDays(7 - (int)dayOfMonth.DayOfWeek + (int)dayOfWeek);
    }
    else
    {
        dayOfMonth = dayOfMonth.AddDays((int)dayOfWeek - (int)dayOfMonth.DayOfWeek);
    }

    // add 7 days per occurance
    dayOfMonth = dayOfMonth.AddDays(7 * (occurance - 1));

    // make sure this occurance is within the original month
    result = dayOfMonth.Month == instance.Month;


    if (result)
    {
        dateOfMonth = dayOfMonth;
    }

    return result;
}

DateTime myDate = new DateTime(2013, 1, 1)
DateTime dateOfMonth;

myDate.TryGetDayOfMonth(DayOfWeek.Sunday, 1, out dateOfMonth) 
// returns: true; dateOfMonth = Sunday, 1/6/2013

myDate.TryGetDayOfMonth(DayOfWeek.Sunday, 4, out dateOfMonth) 
// returns: true; dateOfMonth = Sunday, 1/27/2013

myDate.TryGetDayOfMonth(DayOfWeek.Sunday, 5, out dateOfMonth) 
// returns: false; 

myDate.TryGetDayOfMonth(DayOfWeek.Wednesday, 1, out dateOfMonth) 
// returns: true; dateOfMonth = Wednesday, 1/2/2013

myDate.TryGetDayOfMonth(DayOfWeek.Wednesday, 4, out dateOfMonth) 
// returns: true; dateOfMonth = Wednesday, 1/23/2013

myDate.TryGetDayOfMonth(DayOfWeek.Wednesday, 5, out dateOfMonth) 
// returns: true; dateOfMonth = Wednesday, 1/30/2013 

// etc

int thirdFriday = DayFinder.FindDay(2013, 12, DayOfWeek.Friday, 3);

  public class DayFinder
  {

    //For example to find the day for 2nd Friday, February, 2016
    //=>call FindDay(2016, 2, DayOfWeek.Friday, 2)
    public static int FindDay(int year, int month, DayOfWeek Day, int occurance)
    {

        if (occurance <= 0 || occurance > 5)
            throw new Exception("Occurance is invalid");

        DateTime firstDayOfMonth = new DateTime(year, month, 1);
        //Substract first day of the month with the required day of the week 
        var daysneeded = (int)Day - (int)firstDayOfMonth.DayOfWeek;
        //if it is less than zero we need to get the next week day (add 7 days)
        if (daysneeded < 0) daysneeded = daysneeded + 7;
        //DayOfWeek is zero index based; multiply by the Occurance to get the day
        var resultedDay = (daysneeded + 1) + (7 * (occurance - 1));

        if (resultedDay > (firstDayOfMonth.AddMonths(1) - firstDayOfMonth).Days)
            throw new Exception(String.Format("No {0} occurance(s) of {1} in the required month", occurance, Day.ToString()));

        return resultedDay;
    }
}

public DateTime DateOfWeekOfMonth(int year, int month, DayOfWeek dayOfWeek, byte weekNumber)
{
    DateTime tempDate = new DateTime(year, month, 1);
    tempDate = tempDate.AddDays(-(tempDate.DayOfWeek - dayOfWeek));

    return
        tempDate.Day > (byte)DayOfWeek.Saturday
            ? tempDate.AddDays(7 * weekNumber)
            : tempDate.AddDays(7 * (weekNumber - 1));
}

public DateTime DateOfWeekOfMonth(DateTime sender, DayOfWeek dayOfWeek, byte weekNumber)
{
    return DateOfWeekOfMonth(sender.Year, sender.Month, dayOfWeek, weekNumber);
}

DateTime thirdFridayOfMonth = DateOfWeekOfMonth(DateTime.Now, DayOfWeek.Friday, 3);

void Main()
{
    DayOfWeek dow = DayOfWeek.Friday;
    int y = 2014;
    int m = 2;

    String.Format("First {0}: {1}", new object[] { dow, DateHelper.FirstDayOfWeekInMonth(y, m, dow) }).Dump();

    "".Dump();

    String.Format("Last {0}: {1}", new object[] { dow, DateHelper.LastDayOfWeekInMonth(y, m, dow) }).Dump();

    "".Dump();

    for(int i = 1; i <= 6; i++)
        String.Format("{0} #{1}: {2}", new object[] { dow, i, DateHelper.XthDayOfWeekInMonth(y, m, dow, i) }).Dump();
}


public class DateHelper
{
    public static DateTime FirstDayOfWeekInMonth(int year, int month, DayOfWeek day)
    {
        DateTime res = new DateTime(year, month, 1);
        int offset = -(res.DayOfWeek - day);

        if (offset < 0)
            offset += 7;

        res = res.AddDays(offset);

        return res;
    }

    public static DateTime LastDayOfWeekInMonth(int year, int month, DayOfWeek day)
    {
        DateTime dt = new DateTime(year, month, 1).AddMonths(1);        
        DateTime res = FirstDayOfWeekInMonth(dt.Year, dt.Month, day);

        res = res.AddDays(-7);

        return res;
    }


    public static DateTime XthDayOfWeekInMonth(int year, int month, DayOfWeek day, int x)
    {
        DateTime res = DateTime.MinValue;

        if (x > 0)
        {
            res = FirstDayOfWeekInMonth(year, month, day);

            if (x > 1)
                res = res.AddDays((x - 1) * 7);

            res = res.Year == year && res.Month == month ? res : DateTime.MinValue;
        }

        return res;
    }
}

First Friday: 07/02/2014 00:00:00

Last Friday: 28/02/2014 00:00:00

Friday #1: 07/02/2014 00:00:00
Friday #2: 14/02/2014 00:00:00
Friday #3: 21/02/2014 00:00:00
Friday #4: 28/02/2014 00:00:00
Friday #5: 01/01/0001 00:00:00
Friday #6: 01/01/0001 00:00:00

    private DateTime thirdSunday(DateTime timeFrom)
    {
        List<DateTime> days = new List<DateTime>();
        DateTime testDate = timeFrom;

        while (testDate < timeFrom.AddMonths(1))
        {
            if (testDate.DayOfWeek == DayOfWeek.Friday)
            {
                days.Add(testDate);
            }
            testDate = testDate.AddDays(1);
        }

        return days[2];
    }

 public static DateTime GetDayOfMonth(DateTime dateValue, DayOfWeek dayOfWeek, int occurance)
    {
        List<DateTime> dayOfWeekRanges = new List<DateTime>();

        //move to the first of th month
        DateTime startOfMonth = new DateTime(dateValue.Year, dateValue.Month, 1);

        //move startOfMonth to the dayOfWeek requested
        while (startOfMonth.DayOfWeek != dayOfWeek)
            startOfMonth = startOfMonth.AddDays(1);

        do
        {
            dayOfWeekRanges.Add(startOfMonth);
            startOfMonth = startOfMonth.AddDays(7);
        } while (startOfMonth.Month == dateValue.Month);

        bool fromLast = occurance < 0;
        if (fromLast)
            occurance = occurance * -1;

        if (fromLast)
            return dayOfWeekRanges[dayOfWeekRanges.Count - occurance];
        else
            return dayOfWeekRanges[occurance - 1];
    }

public static DateTime ThirdFridayOfMonth(DateTime dateTime)
{
    int day = dateTime.Day;
    return dateTime.AddDays(21 - day - ((int)dateTime.DayOfWeek + 37 - day) % 7);
}

public static int? GetNthDayOfWeekInMonth(int year, int month, DayOfWeek dow, int weekNumOfMonth)
{
    if (weekNumOfMonth < -5 || weekNumOfMonth == 0 || weekNumOfMonth > 5)
        throw new ArgumentOutOfRangeException("weekNumOfMonth", $"must be between 1~5 or -1~-5. ({weekNumOfMonth})");

    int daysOfMonth = DateTime.DaysInMonth(year, month);

    if (weekNumOfMonth > 0)
    {
        var firstDay = new DateTime(year, month, 1);
        var firstDayOfTargetDOW = (int)dow - (int)firstDay.DayOfWeek;
        if (firstDayOfTargetDOW < 0)
            firstDayOfTargetDOW += 7;
        var resultedDay = (firstDayOfTargetDOW + 1) + (7 * (weekNumOfMonth - 1));

        if (resultedDay > daysOfMonth)
            return null;

        return resultedDay;
    }
    else
    {
        var lastDay = new DateTime(year, month, daysOfMonth);
        var firstDayOfTargetDOW = (int)lastDay.DayOfWeek - (int)dow;
        if (firstDayOfTargetDOW < 0)
            firstDayOfTargetDOW += 7;
        var resultedDay = firstDayOfTargetDOW + (7 * (Math.Abs(weekNumOfMonth) - 1));

        if (resultedDay > daysOfMonth)
            return null;

        return (daysOfMonth - resultedDay);
    }
}

Assert.AreEqual(02, DateTimeHelper.GetNthDayOfWeekInMonth(2019, 11, DayOfWeek.Saturday, 1));
Assert.AreEqual(30, DateTimeHelper.GetNthDayOfWeekInMonth(2019, 11, DayOfWeek.Saturday, -1));

    public static DateTime FindTheNthDay(
        int year, int month, DayOfWeek day, int occurrence)
    {
        var startDate = new DateTime(year, month, 1);

        while(startDate.DayOfWeek != day)
        {
            startDate = startDate.AddDays(1);
        }

        var nDays = 7 * (occurrence - 1);
        var result = startDate.AddDays(nDays);

        return result;
    }

> FindTheNthDay(2006, 11, DayOfWeek.Friday, 4)
[11/24/2006 12:00:00 AM]
> FindTheNthDay(2005, 11, DayOfWeek.Friday, 4)
[11/25/2005 12:00:00 AM]
> FindTheNthDay(2004, 11, DayOfWeek.Friday, 4)
[11/26/2004 12:00:00 AM]
> FindTheNthDay(2003, 11, DayOfWeek.Friday, 4)
[11/28/2003 12:00:00 AM]
> FindTheNthDay(1983, 11, DayOfWeek.Friday, 4)
[11/25/1983 12:00:00 AM]
> FindTheNthDay(1978, 11, DayOfWeek.Friday, 4)
[11/24/1978 12:00:00 AM]
> FindTheNthDay(1972, 11, DayOfWeek.Friday, 4)
[11/24/1972 12:00:00 AM]

    public static bool IsThirdWednesday(DateTime inputDate)
    {
        DateTime firstDayOfMonth = new DateTime(inputDate.Year, inputDate.Month, 1);
        DateTime firstDayOfNextMonth = firstDayOfMonth.AddMonths(1);

        int wednesdayCount = 0;
        while(firstDayOfMonth < firstDayOfNextMonth)
        {
            if (firstDayOfMonth.DayOfWeek == DayOfWeek.Wednesday)
                wednesdayCount++;

            if (wednesdayCount == 3)
            {
                if (inputDate == firstDayOfMonth)
                    return true;
                else
                    return false;
              
            }

            firstDayOfMonth = firstDayOfMonth.AddDays(1);
        }

        return false;
    }