如何根据同月的日期查找一个月中某一天的第5天或结束日期

我一直试图找到一个月中第一周的第5周日期，例如第5周的星期一，第5周的星期二，星期三……等等，根据同月的日期。 此日期可能属于同月的任何一周。 我试过了

DateTime MonthEventDate=05/01/2016; //Date format in dd/MM/yyyy format DayOfWeek EventDay="Sunday"; //For Example i want to find 5th Sunday in January Month, but days too will change everytime based on user selection string SelectedWeek="5"; //Here i'm getting the week in which i need to find the given date ie, 5th Monday or Tuesday & so on if (SelectedWeek == "5") { //Here i tried to add number of days to my initial day to find 5th day date, but every time its returning next month value MonthEventDate = MonthEventDate.AddDays((EventDay < MonthEventDate.DayOfWeek ? 31 : 28) + EventDay - MonthEventDate.DayOfWeek); } 

我知道逻辑是错误的，但是我希望得到一周中第5天的日期，如果那天不存在，则返回0.寻找一些指导注意：这里的月份会根据用户输入而改变，所以如何返回日期第五天，如果它存在于给定的月份

Answer 1

这应该给你第5天（如果有的话）……

  DateTime dayInMonth = DateTime.Now; DayOfWeek dayToFind = DayOfWeek.Friday; var fifth= Enumerable.Range(1, DateTime.DaysInMonth(dayInMonth.Year, dayInMonth.Month)) .Select(day => new DateTime(dayInMonth.Year, dayInMonth.Month, day)) .Where(day => day.DayOfWeek == dayToFind) .Skip(4) .FirstOrDefault(); 

Answer 2

扩展@SonerGönül的答案。 在输入中，您输入所需的星期几，所需的月份和所需的年份。 在输出时，您将获得第五周中同一天的日期，或者为null

  public DateTime? FifthDay(DayOfWeek dayOfWeek, byte monthNum, int year) { if (monthNum > 12 || monthNum < 1) { throw new Exception("Month value should be between 1 and 12"); } var searchDate = new DateTime(year, monthNum, 1); var weekDay = searchDate.DayOfWeek; if (weekDay == dayOfWeek) { searchDate = searchDate.AddDays(28); } for (DateTime d = searchDate; d < d.AddDays(7); d = d.AddDays(1)) { if (d.DayOfWeek == dayOfWeek) { searchDate = searchDate.AddDays(28); break; } } if (searchDate.Month == monthNum) return searchDate; return null; } 

Answer 3

您可以使用这样的简单数学（不包括validation）

 static DateTime? FindDate(int year, int month, DayOfWeek dayOfWeek, int weekOfMonth = 5) { var baseDate = new DateTime(year, month, 1); int firstDateOffset = ((int)dayOfWeek - (int)baseDate.DayOfWeek + 7) % 7; var date = baseDate.AddDays(firstDateOffset + 7 * (weekOfMonth - 1)); return date.Month == month ? date : (DateTime?)null; } 

我认为代码是自我解释的。 可能需要解释的唯一技巧是线

  int firstDateOffset = ((int)dayOfWeek - (int)baseDate.DayOfWeek + 7) % 7; 

当我说星期五开始月份并且你要求星期一时，处理这种情况，并且相当于

 int firstDateOffset = (int)dayOfWeek - (int)baseDate.DayOfWeek; if (firstDateOffset < 0) firstDateOffset += 7; 

在您的示例中使用

 var monthEventDate = FindDate(2016, 1, DayOfWeek.Sunday, 5); 

Answer 4

您可以参考此演示代码

  int requiredDay = 5; //Day number ie 0-6(Sunday to SaturDay) DateTime day = new DateTime(2016, 1, 1); //Month,year,Date if (DateTime.DaysInMonth(day.Year, day.Month) > 28) { //Get the first day name of the month int firstMonthDay = (int)day.DayOfWeek; int offset=0; //Number of days from the first day for the required day if (firstMonthDay <= requiredDay) offset = requiredDay - firstMonthDay; else offset = 7 - firstMonthDay + requiredDay; // DateTime firstoccurence = day.AddDays((double)offset); DateTime fifthOccurence = firstoccurence.AddDays(28); if (fifthOccurence.Month == firstoccurence.Month) MessageBox.Show(fifthOccurence.ToString()); else MessageBox.Show("No 5th Occurence for this day"); }