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。

在PowerShell 7.x.y中: 如果您需要匹配的WeekYear，您将需要以下两个代码行。

[System.Globalization.ISOWeek]::GetWeekOfYear((get-date))
[System.Globalization.ISOWeek]::GetYear((get-date))

var cultureInfo = CultureInfo.CurrentCulture;
var calendar = cultureInfo.Calendar;

var calendarWeekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
var firstDayOfWeek = cultureInfo.DateTimeFormat.FirstDayOfWeek;
var lastDayOfWeek = cultureInfo.LCID == 1033 //En-us
                    ? DayOfWeek.Saturday
                    : DayOfWeek.Sunday;

var lastDayOfYear = new DateTime(date.Year, 12, 31);

var weekNumber = calendar.GetWeekOfYear(date, calendarWeekRule, firstDayOfWeek);

 //Check if this is the last week in the year and it doesn`t occupy the whole week
return weekNumber == 53 && lastDayOfYear.DayOfWeek != lastDayOfWeek 
       ? 1  
       : weekNumber;

它对美国文化和俄罗斯文化都适用。ISO 8601也是正确的，因为俄罗斯周从周一开始。

是这样的:

public int GetWeekNumber()
{
    CultureInfo ciCurr = CultureInfo.CurrentCulture;
    int weekNum = ciCurr.Calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    return weekNum;
}

最重要的是CalendarWeekRule参数。

在这里看到的: https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=IT-IT&k=k (System.Globalization.CalendarWeekRule)、k (TargetFrameworkMoniker -.NETFramework

如果您想要全球化的周数，另一个答案是: 我也把答案贴在这里了

从原来的答案:

基于这个答案:@bunny4

但并不是每个人都在美国，或者可能需要支持多种文化。 使用此解决方案来支持文化定义的周规则和第一天规则。例:丹麦有“一周的头四天”规则，“星期一”是一周的第一天。

//for now, take the the current executing thread's Culture
var cultureInfo = Thread.CurrentThread.CurrentCulture;

//let's pick a date
DateTime dt = new DateTime(2020, 12, 21);

DayOfWeek firstDay = cultureInfo.DateTimeFormat.FirstDayOfWeek;
CalendarWeekRule weekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
Calendar cal = cultureInfo.Calendar;
int week = cal.GetWeekOfYear(dt, weekRule, firstDay);

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();
    }
}