blocks|key|223793|text|一年有52周零1天，如果是圈年(52+x+7=+364)。2012-12-31将是第53周，这一周只有2天，因为2012年是一个圈年。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|223794|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

A year has 52 weeks and 1 day or 2 in case of a lap year (52 x 7 = 364). 2012-12-31 would be week 53, a week that would only have 2 days because 2012 is a lap year.

blocks|key|226010|text|问题是:你如何定义一周是在2012年还是2013年？我猜你的猜测是，既然一周中有6天是在2013年，那么这一周应该被标记为2013年的第一周。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|226011|我不确定这是不是正确的方法。那一周开始于2012年(12月31日星期一)，所以它应该被标记为2012年的最后一周，因此它应该是2012年的53天。2013年的第一周应该从7号星期一开始。|226012|现在，您可以使用星期信息处理边缘周(一年的第一周和最后一周)的特定情况。这完全取决于你的逻辑。|226013|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

The question is: How do you define if a week is in 2012 or in 2013?
Your supposition, I guess, is that since 6 days of the week are in 2013, this week should be marked as the first week of 2013. 

Not sure if this is the right way to go. 
That week started on 2012 (On monday 31th Dec), so it should be marked as the last week of 2012, therefore it should be the 53rd of 2012. The first week of 2013 should start on monday the 7th.

Now, you can handle the particular case of edge weeks (first and last week of the year) using the day of week information. It all depends on your logic.

blocks|key|2875895|text|一年中可能有超过52周。每一年有52个完整的星期%2B1或%2B2+(闰年)天。他们补足了53周的时间。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2875896|2875897|52周*7天=+364天。|unordered-list-item|2875898|2875899|因此，每一年，你至少有一个额外的一天。两个表示闰年。这些额外的日子是否被算作他们自己的单独的星期？|2875900|有多少周取决于你一周的开始日期。让我们考虑一下2012年的情况。|2875901|2875902|US+(周日->周六)：52周%2B一周短2天，2012-12-30和2012-12-31。这将导致总共53周。今年的最后两天(周日%2B周一)组成了他们自己的短短一周。|2875903|2875904|检查当前“区域性”的设置，查看它用作每周的第一天的内容。|2875905|如你所见，结果是53是很正常的。|2875906|2875907|欧洲(星期一和星期日)：1月2dn+(2012-1-2)是第一个星期一，所以这是第一周的第一天。询问1月1日的周编号，你会得到52，因为它被认为是2011年上周的一部分。|2875908|2875909|甚至有可能会有54周。每28年发生一次，将1月1日和12月31日视为不同的星期。今年肯定也是闰年。|2875910|例如，2000年有54周。1月1日(星期六)为第一周日，12月31日(星期日)为第二周日。|2875911|var+d+=+new+DateTime(2012,+12,+31);
CultureInfo+cul+=+CultureInfo.CurrentCulture;

var+firstDayWeek+=+cul.Calendar.GetWeekOfYear(
++++d,
++++CalendarWeekRule.FirstDay,
++++DayOfWeek.Monday);

int+weekNum+=+cul.Calendar.GetWeekOfYear(
++++d,
++++CalendarWeekRule.FirstDay,
++++DayOfWeek.Monday);

int+year+=+weekNum+==+52+&&+d.Month+==+1+?+d.Year+-+1+:+d.Year;
Console.WriteLine("Year:+{0}+Week:+{1}",+year,+weekNum);|code-block|syntax|javascript|2875912|打印数量:年份:+2012周:+54|2875913|将上面示例中的CalendarWeekRule更改为FirstFullWeek或FirstFourDayWeek，您将返回53。让我们把起跑日保持在周一，因为我们是在和德国打交道。|2875914|因此，第53周从2012-12-31星期一开始，持续一天，然后停止。|2875915|53是正确答案。如果您想尝试一下，请将“文化”更改为“德国”。|2875916|CultureInfo+cul+=+CultureInfo.GetCultureInfo("de-DE");|2875917|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1H|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|1J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|1L|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|1M|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]|$1|L|3|M|5|E|7|1O|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1P|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1R|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1S|8|@]|9|@]|A|$]]|$1|T|3|U|5|E|7|1T|8|@]|9|@]|A|$]]|$1|V|3|-4|5|6|7|1U|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1V|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1W|8|@]|9|@]|A|$]]|$1|10|3|11|5|12|7|1X|8|@]|9|@]|A|$13|14]]|$1|15|3|16|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|17|3|18|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|19|3|1A|5|6|7|20|8|@]|9|@]|A|$]]|$1|1B|3|1C|5|6|7|21|8|@]|9|@]|A|$]]|$1|1D|3|1E|5|12|7|22|8|@]|9|@]|A|$13|14]]|$1|1F|3|-4|5|6|7|23|8|@]|9|@]|A|$]]]|1G|$]]

There can be more than 52 weeks in a year. Each year has 52 full weeks + 1 or +2 (leap year) days extra. They make up for a 53th week.

<ul>
<li>52 weeks * 7days = 364 days.</li>
</ul>

So for each year you have at least one an extra day. Two for leap years. Are these extra days counted as separate weeks of their own?

How many weeks there are really depends on the starting day of your week. Let's consider this for 2012.

<ul>
<li>US (Sunday -> Saturday): 52 weeks + one short 2 day week for 2012-12-30 &amp; 2012-12-31. This results in a total of 53 weeks. Last two days of this year (Sunday + Monday) make up their own short week.</li>
</ul>

Check your current Culture's settings to see what it uses as the first day of the week.

As you see it's normal to get 53 as a result.

<ul>
<li>Europe (Monday -> Sunday): January 2dn (2012-1-2) is the first monday, so this is the first day of the first week. Ask the week number for the 1st of January and you'll get back 52 as it is considered part of 2011 last's week.</li>
</ul>

It's even possible to have a 54th week. Happens every 28 years when the 1st of January and the 31st of December are treated as separate weeks. It must be a leap year too. 

For example, the year 2000 had 54 weeks. January 1st (sat) was the first one week day, and 31st December (sun) was the second one week day.

<pre><code>var d = new DateTime(2012, 12, 31);
CultureInfo cul = CultureInfo.CurrentCulture;

var firstDayWeek = cul.Calendar.GetWeekOfYear(
 d,
 CalendarWeekRule.FirstDay,
 DayOfWeek.Monday);

int weekNum = cul.Calendar.GetWeekOfYear(
 d,
 CalendarWeekRule.FirstDay,
 DayOfWeek.Monday);

int year = weekNum == 52 &amp;&amp; d.Month == 1 ? d.Year - 1 : d.Year;
Console.WriteLine("Year: {0} Week: {1}", year, weekNum);
</code></pre>

Prints out: Year: 2012 Week: 54

Change CalendarWeekRule in the above example to FirstFullWeek or FirstFourDayWeek and you'll get back 53. Let's keep the start day on Monday since we are dealing with Germany.

So week 53 starts on monday 2012-12-31, lasts one day and then stops.

53 is the correct answer. Change the Culture to germany if want to to try it.

<pre><code>CultureInfo cul = CultureInfo.GetCultureInfo("de-DE");
</code></pre>

blocks|key|2875926|text|正如本MSDN+page中所指出的，ISO8601+week和.Net+week的编号略有不同。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2875927|你可以参考MSDN+Blog中的这篇文章来获得更好的解释："ISO+8601+Week+of+Year+format+in+Microsoft+.Net“|2875928|简而言之，.Net允许将几周时间分成几年，而国际标准化组织标准不允许。在这篇文章中，还有一个简单的函数可以获得一年中最后一周的正确ISO+8601周数字。|2875929|更新以下方法实际上为2012-12-31返回1，这在ISO8601(例如德国)中是正确的。|style|BOLD|CODE|2875930|//+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);
}+|code-block|syntax|javascript|2875931|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/dotnet/api/system.globalization.calendarweekrule|1|https://docs.microsoft.com/archive/blogs/shawnste/iso-8601-week-of-year-format-in-microsoft-net^0|3|9|0|0|U|1A|1|0|0|0|2|A|A|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@$A|12|B|13|1|14]]|C|$]]|$1|D|3|E|5|6|7|15|8|@]|9|@$A|16|B|17|1|18]]|C|$]]|$1|F|3|G|5|6|7|19|8|@]|9|@]|C|$]]|$1|H|3|I|5|6|7|1A|8|@$A|1B|B|1C|J|K]|$A|1D|B|1E|J|L]]|9|@]|C|$]]|$1|M|3|N|5|O|7|1F|8|@]|9|@]|C|$P|Q]]|$1|R|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]]|S|$T|$5|U|V|W|C|$X|Y]]|Z|$5|U|V|W|C|$X|10]]]]

As noted in this <a href="https://docs.microsoft.com/dotnet/api/system.globalization.calendarweekrule" rel="noreferrer">MSDN page</a> there is a slight difference between ISO8601 week and .Net week numbering.

You can refer to this article in MSDN Blog for a better explanation: "<a href="https://docs.microsoft.com/archive/blogs/shawnste/iso-8601-week-of-year-format-in-microsoft-net" rel="noreferrer">ISO 8601 Week of Year format in Microsoft .Net</a>" 

Simply put, .Net allow weeks to be split across years while the ISO standard does not.
In the article there is also a simple function to get the correct ISO 8601 week number for the last week of the year.

Update The following method actually returns 1 for <code>2012-12-31</code> which is correct in ISO 8601 (e.g. Germany).

<pre><code>// 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 &gt;= DayOfWeek.Monday &amp;&amp; day &lt;= DayOfWeek.Wednesday)
 {
 time = time.AddDays(3);
 }

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

blocks|key|223841|text|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;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|223842|它对美国和俄罗斯的文化都很有效。ISO+8601也将是正确的，因为俄罗斯周从周一开始。|unstyled|223843|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>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 &amp;&amp; lastDayOfYear.DayOfWeek != lastDayOfWeek 
 ? 1 
 : weekNumber;
</code></pre>

It works well both for US and Russian cultures. ISO 8601 also will be correct, `cause Russian week starts at Monday.

blocks|key|223994|text|由于似乎没有.Net文化产生正确的ISO-8601周数字，我宁愿完全绕过内置的周确定，并手动进行计算，而不是试图纠正部分正确的结果。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|223995|我最终得到的是以下扩展方法：|223996|///+<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+%2B+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+%2B+6)+%25+7;
}|code-block|syntax|javascript|223997|首先，((int)date.DayOfWeek+%2B+6)+%25+7)确定工作日数字、0=monday、6=sunday。|offset|length|style|CODE|223998|date.AddDays(-((int)date.DayOfWeek+%2B+6)+%25+7)根据请求的周数确定星期一的日期。|223999|三天后是目标周四，这决定了一周是哪一年。|224000|如果将一年中的(从零开始的)天数除以7(向下舍入)，就会得到一年中的(从零开始的)周数。|224001|在c#中，整数计算结果隐式向下舍入。|224002|entityMap^0|0|0|0|3|U|0|0|18|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@$K|12|L|13|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|14|8|@$K|15|L|16|M|N]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|17|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|18|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|19|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|X|$]]

Since there doesn't seem to be a .Net-culture that yields the correct ISO-8601 week number, I'd rather bypass the built-in week determination altogether, and do the calculation manually, instead of attempting to correct a partially correct result. 

What I ended up with is the following extension method:

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

/// &lt;summary&gt;
/// 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.
/// &lt;/summary&gt;
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());
}

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



First of all, <code>((int)date.DayOfWeek + 6) % 7)</code> determines the weekday number, 0=monday, 6=sunday.

<code>date.AddDays(-((int)date.DayOfWeek + 6) % 7)</code> determines the date of the monday preceiding the requested week number.

Three days later is the target thursday, which determines what year the week is in.

If you divide the (zero based) day-number within the year by seven (round down), you get the (zero based) week number in the year. 

In c#, integer calculation results are round down implicitly.

blocks|key|3089968|text|从上面的代码从il_guru到Powershell端口的C#|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3089969|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
}|code-block|syntax|javascript|3089970|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

C# to Powershell port from code above from il_guru:

<pre><code>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
}
</code></pre>

blocks|key|3089972|text|这就是方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3089973|public+int+GetWeekNumber()
{
++++CultureInfo+ciCurr+=+CultureInfo.CurrentCulture;
++++int+weekNum+=+ciCurr.Calendar.GetWeekOfYear(DateTime.Now,+CalendarWeekRule.FirstFourDayWeek,+DayOfWeek.Monday);
++++return+weekNum;
}|code-block|syntax|javascript|3089974|对于来说，最重要的是CalendarWeekRule参数。|offset|length|style|CODE|3089975|查看此处：https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=IT-IT&k=k(System.Globalization.CalendarWeekRule);k(TargetFrameworkMoniker-.NETFramework|3089976|entityMap|0|LINK|mutability|MUTABLE|url|https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=IT-IT&k=k(System.Globalization.CalendarWeekRule);k(TargetFrameworkMoniker-.NETFramework^0|0|0|A|G|0|5|47|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@$I|Z|J|10|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@$I|12|J|13|1|14]]|A|$]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

This is the way:

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

Most important for is the <code>CalendarWeekRule</code> parameter.

See here:
<a href="https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&amp;l=IT-IT&amp;k=k(System.Globalization.CalendarWeekRule);k(TargetFrameworkMoniker-.NETFramework" rel="noreferrer">https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&amp;l=IT-IT&amp;k=k(System.Globalization.CalendarWeekRule);k(TargetFrameworkMoniker-.NETFramework</a>

blocks|key|3089978|text|++DateTimeFormatInfo+dfi+=+DateTimeFormatInfo.CurrentInfo;
++DateTime+date1+=+new+DateTime(2011,+1,+1);
++Calendar+cal+=+dfi.Calendar;

++Console.WriteLine("{0:d}:+Week+{1}+({2})",+date1,+
++++++++++++++++++++cal.GetWeekOfYear(date1,+dfi.CalendarWeekRule,+
++++++++++++++++++++++++++++++++++++++dfi.FirstDayOfWeek),
++++++++++++++++++++cal.ToString().Substring(cal.ToString().LastIndexOf(".")+%2B+1));++++++|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3089979|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code> DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
 DateTime date1 = new DateTime(2011, 1, 1);
 Calendar cal = dfi.Calendar;

 Console.WriteLine("{0:d}: Week {1} ({2})", date1, 
 cal.GetWeekOfYear(date1, dfi.CalendarWeekRule, 
 dfi.FirstDayOfWeek),
 cal.ToString().Substring(cal.ToString().LastIndexOf(".") + 1)); 
</code></pre>

blocks|key|226112|text|使用c#和DateTime类确定周数的最简单方法是ISO8601样式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|226113|问这个问题:一年中有多少个星期四就是本周的星期四。答案等于想要的周数。|226114|var+dayOfWeek+=+(int)moment.DayOfWeek;
//+Make+monday+the+first+day+of+the+week
if+(--dayOfWeek+<+0)
++++dayOfWeek+=+6;
//+The+whole+nr+of+weeks+before+this+thursday+plus+one+is+the+week+number
var+weekNumber+=+(moment.AddDays(3+-+dayOfWeek).DayOfYear+-+1)+/+7+%2B+1;|code-block|syntax|javascript|226115|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The easiest way to determine the week number ISO 8601 style using c# and the DateTime class.

Ask this: the how-many-eth thursday of the year is the thursday of this week.
The answer equals the wanted week number.

<pre><code>var dayOfWeek = (int)moment.DayOfWeek;
// Make monday the first day of the week
if (--dayOfWeek &lt; 0)
 dayOfWeek = 6;
// The whole nr of weeks before this thursday plus one is the week number
var weekNumber = (moment.AddDays(3 - dayOfWeek).DayOfYear - 1) / 7 + 1;
</code></pre>

blocks|key|220734|text|好消息！刚刚合并了一个将System.Globalization.ISOWeek添加到.NET核心的pull+request，目前计划在3.0版本中使用。希望它能在不久的将来推广到其他.NET平台。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|220735|该类型具有以下签名，应涵盖大多数ISO+week需求：|220736|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);
++++}
}|code-block|syntax|javascript|220737|你可以找到源代码here。|220738|更新：这些API也已经被included+in+the+2.1+version+of+.NET+Standard。|BOLD|220739|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/dotnet/coreclr/pull/18456|1|https://en.wikipedia.org/wiki/ISO_week_date|2|https://github.com/dotnet/runtime/blob/b41f1c5f2fde25d752d857a54c3af24145060cdd/src/libraries/System.Private.CoreLib/src/System/Globalization/ISOWeek.cs|3|https://github.com/dotnet/standard/pull/963^0|C|S|1E|C|0|0|G|8|1|0|0|8|4|2|0|0|2|C|18|3|0^^$0|@$1|2|3|4|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|F|3|G|5|6|7|1B|8|@]|D|@$9|1C|A|1D|1|1E]]|E|$]]|$1|H|3|I|5|J|7|1F|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1G|8|@]|D|@$9|1H|A|1I|1|1J]]|E|$]]|$1|O|3|P|5|6|7|1K|8|@$9|1L|A|1M|B|Q]]|D|@$9|1N|A|1O|1|1P]]|E|$]]|$1|R|3|-4|5|6|7|1Q|8|@]|D|@]|E|$]]]|S|$T|$5|U|V|W|E|$X|Y]]|Z|$5|U|V|W|E|$X|10]]|11|$5|U|V|W|E|$X|12]]|13|$5|U|V|W|E|$X|14]]]]

Good news! A <a href="https://github.com/dotnet/coreclr/pull/18456" rel="noreferrer">pull request</a> adding <code>System.Globalization.ISOWeek</code> to .NET Core was just merged and is currently slated for the 3.0 release. Hopefully it will propagate to the other .NET platforms in a not-too-distant future.
The type has the following signature, which should cover most <a href="https://en.wikipedia.org/wiki/ISO_week_date" rel="noreferrer">ISO week</a> needs:
<pre class="lang-cs prettyprint-override"><code>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);
 }
}
</code></pre>
You can find the source code <a href="https://github.com/dotnet/runtime/blob/b41f1c5f2fde25d752d857a54c3af24145060cdd/src/libraries/System.Private.CoreLib/src/System/Globalization/ISOWeek.cs" rel="noreferrer">here</a>.
UPDATE: These APIs have also been <a href="https://github.com/dotnet/standard/pull/963" rel="noreferrer">included in the 2.1 version of .NET Standard</a>.

blocks|key|224031|text|这是il_guru的answer的扩展版本和可空版本。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|224032|扩展名：|224033|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);
}|code-block|syntax|javascript|224034|可以为空：|224035|public+static+int?+GetIso8601WeekOfYear(this+DateTime?+time)
{
++++return+time?.GetIso8601WeekOfYear();
}|224036|用法：|224037|new+DateTime(2019,+03,+15).GetIso8601WeekOfYear();+//returns+11
((DateTime?)+new+DateTime(2019,+03,+15)).GetIso8601WeekOfYear();+//returns+11
((DateTime?)+null).GetIso8601WeekOfYear();+//returns+null|224038|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/11155102/7108481|1^0|2|7|0|A|6|1|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@$A|12|B|13|1|14]|$A|15|B|16|1|17]]|C|$]]|$1|D|3|E|5|6|7|18|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|19|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|C|$]]|$1|M|3|N|5|H|7|1B|8|@]|9|@]|C|$I|J]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|C|$]]|$1|Q|3|R|5|H|7|1D|8|@]|9|@]|C|$I|J]]|$1|S|3|-4|5|6|7|1E|8|@]|9|@]|C|$]]]|T|$U|$5|V|W|X|C|$Y|Z]]|10|$5|V|W|X|C|$Y|Z]]]]

Here is an extension version and nullable version of <a href="https://stackoverflow.com/a/11155102/7108481">il_guru</a>'s <a href="https://stackoverflow.com/a/11155102/7108481">answer</a>.
<h2>Extension:</h2>
<pre><code>public static int GetIso8601WeekOfYear(this DateTime time)
{
 var day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
 if (day &gt;= DayOfWeek.Monday &amp;&amp; day &lt;= DayOfWeek.Wednesday)
 {
 time = time.AddDays(3);
 }

 return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
</code></pre>
<h2>Nullable:</h2>
<pre><code>public static int? GetIso8601WeekOfYear(this DateTime? time)
{
 return time?.GetIso8601WeekOfYear();
}
</code></pre>
<h3>Usages:</h3>
<pre><code>new DateTime(2019, 03, 15).GetIso8601WeekOfYear(); //returns 11
((DateTime?) new DateTime(2019, 03, 15)).GetIso8601WeekOfYear(); //returns 11
((DateTime?) null).GetIso8601WeekOfYear(); //returns null
</code></pre>

blocks|key|226165|text|在.NET+3.0和更高版本中，您可以使用ISOWeek.GetWeekOfDate-Method。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|226166|请注意，year+%2B+week数字格式的年份可能与DateTime的年份不同，因为跨年边界的周可能不同。|226167|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/api/system.globalization.isoweek.getweekofyear^0|L|L|L|S|0|0|P|8|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

In .NET 3.0 and later you can use the <a href="https://docs.microsoft.com/en-us/dotnet/api/system.globalization.isoweek.getweekofyear" rel="noreferrer"><code>ISOWeek.GetWeekOfDate</code>-Method</a>.

Note that the year in the year + week number format might differ from the year of the <code>DateTime</code> because of weeks that cross the year boundary.

blocks|key|226172|text|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。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|226173|++++///+<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);
++++}|code-block|syntax|javascript|226174|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Based on il_guru's answer, I created this version for my own needs that also returns the year component.

<pre><code> /// &lt;summary&gt;
 /// This presumes that weeks start with Monday.
 /// Week 1 is the 1st week of the year with a Thursday in it.
 /// &lt;/summary&gt;
 /// &lt;param name="time"&gt;The date to calculate the weeknumber for.&lt;/param&gt;
 /// &lt;returns&gt;The year and weeknumber&lt;/returns&gt;
 /// &lt;remarks&gt;
 /// Based on Stack Overflow Answer: https://stackoverflow.com/a/11155102
 /// &lt;/remarks&gt;
 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 &gt;= DayOfWeek.Monday &amp;&amp; day &lt;= 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 &gt;= 52 &amp; time.Month == 1 ? time.Year - 1 : time.Year), week);
 }
</code></pre>

blocks|key|220770|text|public+int+GetWeekNumber()
{
+++CultureInfo+ciCurr+=+CultureInfo.CurrentCulture;
+++int+weekNum+=+ciCurr.Calendar.GetWeekOfYear(DateTime.Now,+
+++CalendarWeekRule.FirstFourDayWeek,+DayOfWeek.Monday);
+++return+weekNum;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|220771|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

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

blocks|key|224063|text|这两种方法会有所帮助，假设我们的一周从周一开始。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|224064|///+<summary>
++++///+Returns+the+weekId
++++///+</summary>
++++///+<param+name="DateTimeReference"></param>
++++///+<returns>Returns+the+current+week+id</returns>
++++public+static+DateTime+GetDateFromWeek(int+WeekReference)
++++{
++++++++//365+leap
++++++++int+DaysOffset+=+0;
++++++++if+(WeekReference+>+1)
++++++++{
++++++++++++DaysOffset+=+7;
++++++++++++WeekReference+=+WeekReference+-+1;
++++++++}
++++++++DateTime+DT+=+new+DateTime(DateTime.Now.Year,+1,+1);
++++++++int+CurrentYear+=+DT.Year;
++++++++DateTime+SelectedDateTime+=+DateTime.MinValue;

++++++++while+(CurrentYear+==+DT.Year)
++++++++{
++++++++++++int+TheWeek+=+WeekReportData.GetWeekId(DT);
++++++++++++if+(TheWeek+==+WeekReference)
++++++++++++{
++++++++++++++++SelectedDateTime+=+DT;
++++++++++++++++break;
++++++++++++}
++++++++++++DT+=+DT.AddDays(1.0D);
++++++++}

++++++++if+(SelectedDateTime+==+DateTime.MinValue)
++++++++{
++++++++++++throw+new+Exception("Please+check+week");
++++++++}

++++++++return+SelectedDateTime.AddDays(DaysOffset);
++++}
///+<summary>
++++///+Returns+the+weekId
++++///+</summary>
++++///+<param+name="DateTimeReference"></param>
++++///+<returns>Returns+the+current+week+id</returns>
++++public+static+int+GetWeekId(DateTime+DateTimeReference)
++++{
++++++++CultureInfo+ciCurr+=+CultureInfo.InvariantCulture;
++++++++int+weekNum+=+ciCurr.Calendar.GetWeekOfYear(DateTimeReference,
++++++++CalendarWeekRule.FirstFullWeek,+DayOfWeek.Monday);
++++++++return+weekNum;
++++}|code-block|syntax|javascript|224065|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

These two methods will help, assumming our week starts on Monday

<pre><code>/// &lt;summary&gt;
 /// Returns the weekId
 /// &lt;/summary&gt;
 /// &lt;param name="DateTimeReference"&gt;&lt;/param&gt;
 /// &lt;returns&gt;Returns the current week id&lt;/returns&gt;
 public static DateTime GetDateFromWeek(int WeekReference)
 {
 //365 leap
 int DaysOffset = 0;
 if (WeekReference &gt; 1)
 {
 DaysOffset = 7;
 WeekReference = WeekReference - 1;
 }
 DateTime DT = new DateTime(DateTime.Now.Year, 1, 1);
 int CurrentYear = DT.Year;
 DateTime SelectedDateTime = DateTime.MinValue;

 while (CurrentYear == DT.Year)
 {
 int TheWeek = WeekReportData.GetWeekId(DT);
 if (TheWeek == WeekReference)
 {
 SelectedDateTime = DT;
 break;
 }
 DT = DT.AddDays(1.0D);
 }

 if (SelectedDateTime == DateTime.MinValue)
 {
 throw new Exception("Please check week");
 }

 return SelectedDateTime.AddDays(DaysOffset);
 }
/// &lt;summary&gt;
 /// Returns the weekId
 /// &lt;/summary&gt;
 /// &lt;param name="DateTimeReference"&gt;&lt;/param&gt;
 /// &lt;returns&gt;Returns the current week id&lt;/returns&gt;
 public static int GetWeekId(DateTime DateTimeReference)
 {
 CultureInfo ciCurr = CultureInfo.InvariantCulture;
 int weekNum = ciCurr.Calendar.GetWeekOfYear(DateTimeReference,
 CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday);
 return weekNum;
 }
</code></pre>

blocks|key|2876192|text|如果您没有.NET+5.0，可以扩展DateTime类以包含周数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2876193|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);
++++}
}|code-block|syntax|javascript|2876194|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you don't have .NET 5.0, extend the DateTime class to include week number.
<pre><code>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);
 }
}
</code></pre>

blocks|key|2876208|text|在PowerShell+7.x.y中:如果需要匹配的WeekYear，则需要以下两个代码行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2876209|[System.Globalization.ISOWeek]::GetWeekOfYear((get-date))
[System.Globalization.ISOWeek]::GetYear((get-date))|code-block|syntax|javascript|2876210|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

In PowerShell 7.x.y:
You will need both of the following codelines, if you have the need for the matching WeekYear.
<pre><code>[System.Globalization.ISOWeek]::GetWeekOfYear((get-date))
[System.Globalization.ISOWeek]::GetYear((get-date))
</code></pre>

I have Googled a lot and found a lot of solutions, but none of them give me the correct week number for the 2012-12-31. Even the example on MSDN (<a href="http://msdn.microsoft.com/en-us/library/system.globalization.calendar.getweekofyear.aspx">link</a>) fails.

2012-12-31 is Monday, therefore it should be Week 1, but every method I tried gives me 53. Here are some of the methods, that I have tried:

From the MDSN Library:

<pre><code>DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
Calendar cal = dfi.Calendar;

return cal.GetWeekOfYear(date, dfi.CalendarWeekRule, dfi.FirstDayOfWeek);
</code></pre>

Solution 2:

<pre><code>return new GregorianCalendar(GregorianCalendarTypes.Localized).GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
</code></pre>

Solution 3:

<pre><code>CultureInfo ciCurr = CultureInfo.CurrentCulture;
int weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
return weekNum;
</code></pre>

Update

The following method actually returns 1 when date is 2012-12-31. In other words, my problem was that my methods were not following the ISO-8601 standard.

<pre><code>// 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 &gt;= DayOfWeek.Monday &amp;&amp; day &lt;= DayOfWeek.Wednesday)
 {
 time = time.AddDays(3);
 }

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

Get the correct week number of a given date

我在谷歌上搜索了很多，找到了很多解决方案，但没有一个给我正确的2012-12-31周的数字。即使是MSDN ()上的例子也失败了。2012-12-31是星期一，所以应该是第一周，但我尝试的每种方法都会给我53。下面是我尝试过的一些方法：从MDSN库中：DateTimeFormatInfo dfi = DateTimeF...

问获取给定日期的正确周数
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