Thanks, but I think but are not sure that this function counts the days if the start and enddate falls within a weekday wich is a problem:
"// This function includes the start and end date if it falls on a weekday
Though I found another approach to simply test each day in a loop like this and do a count wich will work fine.
Though a bit manually but should work.
DateTime dt = new DateTime(2009, 11, 27);
if (dt.DayOfWeek == DayOfWeek.Friday)