You must use DateTime.Subtract() method.
Then you must get the TotalHours property of the TimeSpan.
DateTime enterDate = new DateTime(2014, 12, 15, 21, 0, 0);
DateTime leaveDate = new DateTime(2014, 12, 16, 11, 0, 0);
TimeSpan duration = leaveDate.Subtract(enterDate);
double workingHours = duration.TotalHours;