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DateTime extension method to give Week number

By , 19 Jul 2011
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There certainly is a direct method in .NET to obtain the ISO week number.

It is even quite simple; it is, however, buggy - as ISO week numbering "always" has been at MSFT since VB 3.0 and Access Basic (of Access 2.0) and through all versions up to and including VBA of Office 2010. It is a big question why it hasn't been resolved in .NET. Fortunately, a tiny test for week 53 (study in-line comment please) will get it right.

/// <summary>
/// Gets the week number according to the ISO 8601 standard of this instance.
/// <para>This is the corrected value from Calendar.GetWeekOfYear(this, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday)</para>
/// </summary>
/// <param name="currentDate">The current date<
/// <returns>Returns the ISO 8601 week number of this instance.</returns>

public static int WeekIso8601(this DateTime currentDate)
    CultureInfo cultureInfo = CultureInfo.InvariantCulture;
    Calendar calendar = cultureInfo.Calendar;
    CalendarWeekRule calenderWeekRule = CalendarWeekRule.FirstFourDayWeek;
    DayOfWeek firstDayOfWeek = DayOfWeek.Monday;
    int weekNumber = 
        calenderWeekRule, firstDayOfWeek);
    if (weekNumber.Equals(_maxIso8601WeekNumber))
        // The week number appears to be 53. 
        // If so, the week number of the next week must be 1.
        // Obtain the week number of the next week and check if this is 1.
        int weekNumberNext = 
            calenderWeekRule, firstDayOfWeek);
        if (!weekNumberNext.Equals(_minIso8601WeekNumber))
            // The next week has week number 2.
            // Thus, the current week number is not 53 but 1.
            weekNumber = _minIso8601WeekNumber;
    return weekNumber;


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Gustav Brock
Architect Cactus Data ApS
Denmark Denmark
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