Extension method to calculate First/last day of month/week in c#

 public static partial class DateTimeExtensions
    {
        public static DateTime FirstDayOfWeek(this DateTime dt)
        {
            var culture = System.Threading.Thread.CurrentThread.CurrentCulture;
            var diff = dt.DayOfWeek - culture.DateTimeFormat.FirstDayOfWeek;
            if (diff < 0)
                diff += 7;
            return dt.AddDays(-diff).Date;
        }

        public static DateTime LastDayOfWeek(this DateTime dt)
        {
            return dt.FirstDayOfWeek().AddDays(6);
        }

        public static DateTime FirstDayOfMonth(this DateTime dt)
        {
            return new DateTime(dt.Year, dt.Month, 1);
        }

        public static DateTime LastDayOfMonth(this DateTime dt)
        {
            return dt.FirstDayOfMonth().AddMonths(1).AddDays(-1);
        }

        public static DateTime FirstDayOfNextMonth(this DateTime dt)
        {
            return dt.FirstDayOfMonth().AddMonths(1);
        }
    }


After declaring the above class ,you can use it like this:

 var firstdayofThisWeek = DateTime.Now.FirstDayOfWeek();


Happy Coding!