How about this?
public static class ExtendDateTime
public int CountWeekDays(this DateTime thisdate, DateTime thatDate)
int days = Math.Abs((thisDate - thatDate).Days) + 1;
return = ((days/7) * 5) + (days % 7);
Usage would be like this:
DateTime startDate = new DateTime(2011, 11, 1);
int weekdays = startDate.CountWeekDays(new DateTime(2011, 11, 30));
BTW, if you haven't noticed, this extension method works whether the date being compared occurs in the future OR in the past.