Getting the first day in a week with C#

I recently found myself having a DateTime value and needing to calculate the date of the first day in the week that my known date was in using C#. While this is one of those things that are pretty simple to do once you know the solution it did take me a couple of hours to get it right. It turns out that something as simple as calculating a the first day of a week is trickier than one might expect, at least if we want a solution that works for all cultures.

My first instinct was to calculate the first day of a week by subtracting dayInWeek.DayOfWeek - firstDayInWeek days from dayInWeek where dayInWeek is a DateTime specifying a date in the week. That worked fine as long as I did the calculations in an environment where the culture was English, where the first day of the week is Sunday which is enumerated as 0 in DayOfWeek (against international standards which says the first day of the week is monday). As I shifted to using a Swedish culture it broke down and I had to find a better solution.

Heres what I finally came up with:

01.using System;

02.using System.Globalization;

03. 

04.public static class FirstDayOfWeekUtility

05.{

06.///

07./// Returns the first day of the week that the specified

08./// date is in using the current culture.

09.///

10.public static DateTime GetFirstDayOfWeek(DateTime dayInWeek)

11.{

12.CultureInfo defaultCultureInfo = CultureInfo.CurrentCulture;

13.return GetFirstDateOfWeek(dayInWeek, defaultCultureInfo);

14.}

15. 

16.///

17./// Returns the first day of the week that the specified date

18./// is in.

19.///

20.public static DateTime GetFirstDayOfWeek(DateTime dayInWeek, CultureInfo cultureInfo)

21.{

22.DayOfWeek firstDay = cultureInfo.DateTimeFormat.FirstDayOfWeek;

23.DateTime firstDayInWeek = dayInWeek.Date;

24.while (firstDayInWeek.DayOfWeek != firstDay)

25.firstDayInWeek = firstDayInWeek.AddDays(-1);

26. 

27.return firstDayInWeek;

28.}

29.}