DateTime beginningOfWeekDate1 = d1.AddDays(-(int)d1.DayOfWeek);

public static bool InSameWeek(DateTime d1, DateTime d2)
{
DateTime beginningOfWeekDate1 = d2.AddDays(-(int)d2.DayOfWeek);
DateTime beginningOfWeekDate2 = d2.AddDays(-(int)d2.DayOfWeek);
return beginningOfWeekDate1 == beginningOfWeekDate2;
}

Thanks Ben!

I have now the same problem, I think I ll take a look at Ben’s solution.

Thanks Andrew for pointing me towards the GregorianCalendar class.

Ben’s answer was by far the most comprehensive and I’d lean towards that if I was making a DateTime library (which as Drew suggested, probably already exists).

Andrew’s answer however is much simpler and closer to what I need for this particular purpose.

Here’s what I came up with – essentially a cut down version of Ben’s. Note that itemDate is the date being tested:

DateTime today = DateTime.Today;

DateTime thisWeekStart = today.AddDays(
    DayOfWeek.Sunday - today.DayOfWeek);

DateTime thisWeekEnd = today.AddDays(
    DayOfWeek.Saturday - today.DayOfWeek);

if (itemDate > thisWeekStart && itemDate.Date <= thisWeekEnd)
    return "This week";

///

/// Verify two dates are within the same 7 day week where the week begins on a Sunday.
/// 

public static bool InSameWeekSimple(this DateTime d1, DateTime d2) {   

            if (d2 < d1) { Swap(ref d1, ref d2); } // ensure d1 is less than or equal to d2
            DateTime beginningOfWeekDate2 = d2.AddDays(-(int)d2.DayOfWeek);
            return d1 >= beginningOfWeekDate2 && d1 < = d2; // verify that d1 is greater than the date that begins d2s week and less than d2.
}

 private static void Swap(ref T obj1, ref T obj2) {
            var temp = obj1;
            obj1 = obj2;
            obj2 = temp;
}

This can be extended to work with a week beginning on a different day to Sunday:

public static class ExtensionMethods {

  ///

  /// Verify two dates are within the same 7 day week where the week begins on a Sunday.
  /// 

  public static bool InSameWeek(this DateTime d1, DateTime d2) {
    return InSameWeek(d1, d2, DayOfWeek.Sunday);
  }

  ///

  /// Verify two dates are within the same 7 day week, specifying which   day begins the week.
  /// 

  public static bool InSameWeek(this DateTime d1, DateTime d2, DayOfWeek weekBeginDay) {
    if (d2 < d1) { Swap(ref d1, ref d2); } // ensure d1 is less than or equal to d2
    DateTime beginningOfWeekDate2 = d2.AddDays(-(int)d2.DayOfWeek);
    //DateTime beginningOfWeekDate2 = d2.AddDays(((weekBeginDay - d2.DayOfWeek) - 7)%7);
    return d1 >= beginningOfWeekDate2 && d1 < = d2; // verify that d1 is greater than the date that begins d2s week and less than d2.
  }

  ///

  /// Verify two dates are within monday-friday of the same week.
  ///
  public static bool InSameWeekdays(this DateTime d1, DateTime d2) {
    if (IsWeekendDay(d1) || IsWeekendDay(d2)) { return false; }
    if (d2 < d1) { Swap(ref d1, ref d2); } // ensure d1 is less than or equal to d2
    return d1 >= d2.AddDays(-((int)d2.DayOfWeek - (int)DayOfWeek.Monday)) && d1 < = d2; // verify that d1 is greater than the date that begins d2s week and less than d2.
  }

  ///

  /// Does the date provided fall on a Saturday or Sunday
  ///
  public static bool IsWeekendDay(DateTime date) {
    return date.DayOfWeek == DayOfWeek.Saturday || date.DayOfWeek == DayOfWeek.Sunday;
  }

  private static void Swap(ref T obj1, ref T obj2) {
    var tempDate = obj1;
    obj1 = obj2;
    obj2 = tempDate;
  }
}

All of these functions were unit tested and can be verified.

Do I win?