How Can I Get the Start of the Week in C#?

Calculating the Week's Start Date in C#

Many programming tasks require identifying the start of a week. While C# doesn't offer a built-in function for this, we can easily create one using an extension method.

Extension methods enhance existing classes without altering their original code. We'll define our extension method within a static class to extend the DateTime class:

<code class="language-csharp">public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek = DayOfWeek.Monday)
    {
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-1 * diff).Date;
    }
}</code>

The StartOfWeek method generates a new DateTime object representing the week's beginning, based on the provided startOfWeek parameter. The default is Monday. To use Sunday as the start, pass DayOfWeek.Sunday as an argument.

Using the extension method is straightforward:

<code class="language-csharp">DateTime dt = DateTime.Now.StartOfWeek(); // Defaults to Monday</code>

This returns the start of the current week (Monday). To begin the week on Sunday:

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

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