【问题标题】：How to get date from week number and week day C#如何从周数和工作日 C# 获取日期
【发布时间】：2016-06-04 06:22:46
【问题描述】：

如何从 C# 中的周数和周数获取日期。使用日历周规则。例如，我有第 43 周和第 3 周。现在我想要日期。

提前致谢。

【问题讨论】：

你能给出更具体的例子吗？
你的第一个问题是关于 SQLServer，人们给你三个答案，现在你完全改变了它，C# 也需要它。我建议您将问题回滚到第一个状态并创建有关 C# 的新问题。
我不能提出新问题，所以我改变了这个问题。也因为没有答案是有效的。
好吧，那么祝你好运:)

【解决方案1】：

static DateTime GetDateFromWeekNumberAndDayOfWeek(int weekNumber, int dayOfWeek)
        {
            DateTime jan1 = new DateTime(2012, 1, 1);
            int daysOffset = DayOfWeek.Tuesday - jan1.DayOfWeek;

            DateTime firstMonday = jan1.AddDays(daysOffset);

            var cal = CultureInfo.CurrentCulture.Calendar;
            int firstWeek = cal.GetWeekOfYear(jan1, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

            var weekNum = weekNumber;
            if (firstWeek <= 1)
            {
                weekNum -= 1;
            }

            var result = firstMonday.AddDays(weekNum * 7 + dayOfWeek - 1);
            return result;
        }

http://forums.asp.net/t/1850105.aspx?Get+date+from+weeknumber

【讨论】：

今天使用此代码时记得更改年份（也许将其添加为参数）:)

【解决方案2】：

可以这样计算：

declare @weeks table (week1 int, week2 int)
declare @today date = getdate()

select @@DATEFIRST

insert into @weeks(week1, week2)
values (12, 15)

;with computed_calendar as
(
  select
    @today today,
    datepart(week, @today) as current_week,
    datepart(weekday, @today) as current_weekday,

    w.week1,
    dateadd(week, w.week1-datepart(week, @today), @today) as week1_some_date,
    dateadd(day, 1-datepart(weekday, @today), dateadd(week, w.week1-datepart(week, @today), @today)) as week1_start,

    w.week2,
    dateadd(week, w.week2-datepart(week, @today), @today) as week2_some_date,
    dateadd(day, -datepart(weekday, @today), dateadd(week, w.week2+1-datepart(week, @today), @today)) as week2_end
  from @weeks w
)
select
  cc.today, cc.current_week, cc.current_weekday,

  cc.week1_start,
  datepart(week, cc.week1_start) week_1, 
  datename(weekday, cc.week1_start) week1_weekday,

  cc.week2_end,
  datepart(week, cc.week2_end) week_2,
  datename(weekday, cc.week2_end) week2_weekday
from computed_calendar cc

在最后选择周和工作日计算以确保一切正常。您可以尝试将 DATEFIRST 移动到星期一或星期日，并检查脚本是否返回正确的结果。

您也可以提取当前日期的年数并将周数添加到 1 月 1 日。我的方法演示了如何将周差添加到当前日期并从当前日期删除“额外”工作日以获得所需的工作日。

您要测试的时间段在week1_start 和week2_end 之间。

另一种测试周期的方法

您知道周数，并且您确切地知道范围界限总是从第 1 周的开始和第 2 周的结束，所以

只需检查您要测试的日期的周数是否在给定的周数内

select
... 
where datepart(week, my_date) between week1 and week2

注意，如果年份不同，它可能会失败。虽然最初的周数并不能说明它们属于哪一年。

【讨论】：

【解决方案3】：

以下查询将为您提供一年中所有周的星期一和星期日

DECLARE @y int = 2016 
-- Find if it is leap year or not
DECLARE @leap int = IIF(DATEPART(dd,(EOMONTH(CONCAT(@y,'0201')))) = 29,1,0)
DECLARE @totalDays int = IIF(@leap = 1, 366, 365)

;WITH tally AS (
    -- select 365 or 366 (for leap year) days 
    SELECT top (@totalDays) name
      , ROW_NUMBER() OVER (ORDER BY object_id) Rn
    from sys.all_objects
)
, cc2 as (
    SELECT cast(DATEADD(d, Rn - 1, '2016-01-01') AS date) dayNo
    FROM tally
)
, cc3 as (
    SELECT dayNo, DATEPART(ww, dayNo) weekNo
    FROM cc2
)
, cc4 as (
    SELECT weekNo, min(dayNo) FirstDay
    FROM cc3
    GROUP BY weekNo
)
SELECT weekNo
  , cast(DATEADD(wk, DATEDIFF(wk, 0, FirstDay), 0) as date) Monday
  , cast(DATEADD(wk, DATEDIFF(wk, 0, FirstDay), 6) as date) Sunday
FROM cc4

结果将类似于以下示例：

weekNo  Monday      Sunday
1       2015-12-28  2016-01-03
2       2016-01-04  2016-01-10
3       2016-01-11  2016-01-17
4       2016-01-18  2016-01-24
5       2016-01-25  2016-01-31
6       2016-02-01  2016-02-07
7       2016-02-08  2016-02-14
....
12      2016-03-14  2016-03-20
13      2016-03-21  2016-03-27
14      2016-03-28  2016-04-03
15      2016-04-04  2016-04-10
16      2016-04-11  2016-04-17
....

【讨论】：