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。

一年有52周零1天，如果是一圈年(52 x 7 = 364)则为2天。2012-12-31是第53周，一周只有2天，因为2012年是一圈年。

下面是il_guru答案的扩展版本和可空版本。

扩展:

public static int GetIso8601WeekOfYear(this DateTime time)
{
    var day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

可以为空:

public static int? GetIso8601WeekOfYear(this DateTime? time)
{
    return time?.GetIso8601WeekOfYear();
}

Usages:

new DateTime(2019, 03, 15).GetIso8601WeekOfYear(); //returns 11
((DateTime?) new DateTime(2019, 03, 15)).GetIso8601WeekOfYear(); //returns 11
((DateTime?) null).GetIso8601WeekOfYear(); //returns null

如果您需要日历周和年的组合(例如。“1/22”表示2022年日历周1)请注意，年份部分并不总是等于输入日期的年份。这是因为如果一年中至少有4天属于去年，那么每年的第一天属于前一年的最后一个日历周。因此，如果一周中至少有4天属于下一年，那么一年的最后几天属于下一年的第一个日历周。

下面是一个基于@Jogge在回答中提供的扩展

public static class DateTimeExtensions
{
    public static int GetIso8601WeekOfYear(this DateTime time)
    {
        var day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
        if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
        {
            time = time.AddDays(3);
        }

        return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek,
            DayOfWeek.Monday);
    }

    public static int? GetIso8601WeekOfYear(this DateTime? time)
    {
        return time?.GetIso8601WeekOfYear();
    }

    public static string GetIso8601WeekAndYearString(this DateTime time)
    {
        var week = time.GetIso8601WeekOfYear();

        var year = time.Month == 1 && week >= 52
            ? time.Year - 1
            : time.Month == 12 && week == 1
                ? time.Year + 1
                : time.Year;

        return $"{week}/{new DateTime(year, 1, 1):yy}";
    }

    public static string GetIso8601WeekAndYearString(this DateTime? time)
    {
        return time?.GetIso8601WeekAndYearString();
    }
}

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);
    }
}