public Volunteer Add(Volunteer volunteer)
 {
     volunteer.id = _volunteerList.Max(v => v.id) + 1;
     _volunteerList.Add(volunteer);
     return(volunteer);
 }
        public Volunteer GetVolunteerbyUsername(string userName)
        {
            Volunteer result = _volunteerList.FirstOrDefault(v => v.Username == userName);

            return(result);
        }