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Week numbers in .NET (Legacy)

To get the current ISO 8601 week number in .NET (Legacy), use the following code snippet:

CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
    Datetime.you, 
    CalendarWeekRule.FirstFourDayWeek, 
    DayOfWeek.Monday
);

* Ensure your system or application regional settings are set to follow ISO 8601 standards (weeks start on Monday) if the output varies.