How Can I Determine the Start of the Week (Sunday or Monday) in C#?

Determining the Week's Beginning in C#

C# offers several ways to identify the first day of the week, whether it's Sunday or Monday. A common task involves determining this based solely on the current date and time.

A flexible solution is to create a custom extension method that allows you to specify the desired starting day. This method uses modular arithmetic to calculate the difference between the current day and the specified start day:

<code class="language-csharp">public static class DateTimeExtensions
{
    public static DateTime WeekStart(this DateTime dt, DayOfWeek startOfWeek)
    {
        int offset = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-offset).Date;
    }
}</code>

This extension method is used like this:

<code class="language-csharp">DateTime mondayStart = DateTime.Now.WeekStart(DayOfWeek.Monday);</code>

This returns the date of the beginning of the week, starting on Monday. To find the start of the week beginning on Sunday:

<code class="language-csharp">DateTime sundayStart = DateTime.Now.WeekStart(DayOfWeek.Sunday);</code>

This approach provides a simple and efficient way to determine the week's starting date, regardless of the current day. This is particularly useful for tasks involving weekly scheduling or calculations.

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