using System;

using static System.Console;

​

WriteLine(DateTime.Now.StartOfWeek(DayOfWeek.Monday));

WriteLine(DateTime.Now.StartOfWeek());

​

public static class DateTimeExtensions {

    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek = DayOfWeek.Sunday) => dt.AddDays(-(7 + (dt.DayOfWeek - startOfWeek)) % 7).Date;

}