Calculating the First Day of a Week in C#

Recently, I was tasked with finding the date of the first day of a week given a specific DateTime value using C#. Although it might seem like a simple task, it can be quite tricky when taking into account the various cultural differences and special cases that may arise. I initially thought of subtracting dayInWeek.DayOfWeek - firstDayInWeek days from dayInWeek, where dayInWeek is a DateTime that represents a date in the week, and firstDayInWeek is the enumerated value of the first day of the week. However, this solution only works in a "en-us" culture, where the first day of the week is Sunday (enumerated as 0 in DayOfWeek). In other cultures, the first day of the week is Monday, making this initial solution inadequate.

So, I came up with a better solution that works for all cultures. The solution is as follows:

using System;
using System.Globalization;

public class FirstDayOfWeekUtils
{
    /// <summary>
    /// This method returns the first day of the week that the specified date is in using the current culture. 
    /// </summary>
    public static DateTime GetFirstDayOfWeek(DateTime dayInWeek)
    {
        // Get the current culture information
        CultureInfo defaultCultureInfo = CultureInfo.CurrentCulture;

        // Call the overload method with the specified culture information
        return GetFirstDayOfWeek(dayInWeek, defaultCultureInfo);
    }

    /// <summary>
    /// This method returns the first day of the week that the specified date is in using the specified culture information. 
    /// </summary>
    public static DateTime GetFirstDayOfWeek(DateTime dayInWeek, CultureInfo cultureInfo)
    {
        // Get the first day of the week according to the specified culture information
        DayOfWeek firstDay = cultureInfo.DateTimeFormat.FirstDayOfWeek;

        // Store the date in a variable
        DateTime firstDayInWeek = dayInWeek.Date;

        // Subtract days from the date until it matches the first day of the week specified by the culture information
        while (firstDayInWeek.DayOfWeek != firstDay)
            firstDayInWeek = firstDayInWeek.AddDays(-1);

        // Return the first day of the week
        return firstDayInWeek;
    }
}

In the GetFirstDayOfWeek method, you can specify either the DateTime value representing a date in the week or the CultureInfo representing the culture for which you want to calculate the first day of the week. If you don't specify the CultureInfo, the method will use the current culture.

Let me know your feedback if you have a better solution.