Get Week Commencing from DateTime in C#

public static DateTime GetWeekCommencing(DateTime date)
{
  CultureInfo info = Thread.CurrentThread.CurrentCulture;
  DayOfWeek firstday = info.DateTimeFormat.FirstDayOfWeek;
  DayOfWeek today = info.Calendar.GetDayOfWeek(date);

  int diff = today - firstday;
  DateTime firstDate = date.AddDays(-diff);
  return firstDate;
}