好消息!将System.Globalization.ISOWeek添加到. net Core的pull请求刚刚被合并，目前计划在3.0版本中发布。希望它能在不久的将来传播到其他。net平台。

该类型具有以下签名，应涵盖大多数ISO周需求:

namespace System.Globalization
{
    public static class ISOWeek
    {
        public static int GetWeekOfYear(DateTime date);
        public static int GetWeeksInYear(int year);
        public static int GetYear(DateTime date);
        public static DateTime GetYearEnd(int year);
        public static DateTime GetYearStart(int year);
        public static DateTime ToDateTime(int year, int week, DayOfWeek dayOfWeek);
    }
}

你可以在这里找到源代码。

更新:这些api也包含在。net标准的2.1版本中。

由于似乎没有. net文化可以产生正确的ISO-8601周数，因此我宁愿完全绕过内置的周数确定，并手动执行计算，而不是试图更正部分正确的结果。

我最终得到了以下扩展方法:

/// <summary>
/// Converts a date to a week number.
/// ISO 8601 week 1 is the week that contains the first Thursday that year.
/// </summary>
public static int ToIso8601Weeknumber(this DateTime date)
{
    var thursday = date.AddDays(3 - date.DayOfWeek.DayOffset());
    return (thursday.DayOfYear - 1) / 7 + 1;
}

/// <summary>
/// Converts a week number to a date.
/// Note: Week 1 of a year may start in the previous year.
/// ISO 8601 week 1 is the week that contains the first Thursday that year, so
/// if December 28 is a Monday, December 31 is a Thursday,
/// and week 1 starts January 4.
/// If December 28 is a later day in the week, week 1 starts earlier.
/// If December 28 is a Sunday, it is in the same week as Thursday January 1.
/// </summary>
public static DateTime FromIso8601Weeknumber(int weekNumber, int? year = null, DayOfWeek day = DayOfWeek.Monday)
{
    var dec28 = new DateTime((year ?? DateTime.Today.Year) - 1, 12, 28);
    var monday = dec28.AddDays(7 * weekNumber - dec28.DayOfWeek.DayOffset());
    return monday.AddDays(day.DayOffset());
}

/// <summary>
/// Iso8601 weeks start on Monday. This returns 0 for Monday.
/// </summary>
private static int DayOffset(this DayOfWeek weekDay)
{
    return ((int)weekDay + 6) % 7;
}

首先，(int)date。DayOfWeek + 6) % 7)决定工作日的数字，0=周一，6=周日。

date.AddDays (- ((int)日期。DayOfWeek + 6) % 7)确定所请求的周数之前的星期一。

三天后是目标星期四，这决定了一周是哪一年。

如果将一年中(以零为基数)的天数除以7(四舍五入)，就会得到一年中(以零为基数)的周数。

在c#中，整数计算结果隐式向下舍入。

根据il_guru的回答，我根据自己的需要创建了这个版本，它也返回year组件。

    /// <summary>
    /// This presumes that weeks start with Monday.
    /// Week 1 is the 1st week of the year with a Thursday in it.
    /// </summary>
    /// <param name="time">The date to calculate the weeknumber for.</param>
    /// <returns>The year and weeknumber</returns>
    /// <remarks>
    /// Based on Stack Overflow Answer: https://stackoverflow.com/a/11155102
    /// </remarks>
    public static (short year, byte week) GetIso8601WeekOfYear(DateTime time)
    {
        // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll
        // be the same week# as whatever Thursday, Friday or Saturday are,
        // and we always get those right
        DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
        if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
        {
            time = time.AddDays(3);
        }
        // Return the week of our adjusted day
        var week = (byte)CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
        return ((short)(week >= 52 & time.Month == 1 ? time.Year - 1 : time.Year), week);
    }

正如在这个MSDN页面中所指出的，ISO8601周和。net周编号之间有轻微的差异。

你可以参考MSDN博客中的这篇文章来获得更好的解释:“微软。net中的ISO 8601年度周格式”

简单地说，. net允许将周划分为年，而ISO标准不允许。 在本文中，还提供了一个简单的函数，用于获取每年最后一周的正确ISO 8601周数。

更新下面的方法实际上为2012-12-31返回1，这在ISO 8601中是正确的(例如德国)。

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
} 

如果没有。net 5.0，扩展DateTime类以包含周数。

public static class Extension {
    public static int Week(this DateTime date) {
        var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
        return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    }
}

从上面il_guru的代码到Powershell端口:

function GetWeekOfYear([datetime] $inputDate)
{
   $day = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetDayOfWeek($inputDate)
   if (($day -ge [System.DayOfWeek]::Monday) -and ($day -le [System.DayOfWeek]::Wednesday))
   {
      $inputDate = $inputDate.AddDays(3)
   }

   # Return the week of our adjusted day
   $weekofYear = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetWeekOfYear($inputDate, [System.Globalization.CalendarWeekRule]::FirstFourDayWeek, [System.DayOfWeek]::Monday)
   return $weekofYear
}