List<ServiceItem> serviceItems;
List<ServiceItemDetails> serviceItemDetails;
var result = serviceItemDetails.Where(sid => serviceItems.Any(si => si.ID == sid.ID));
// The collection of ServiceItemDetails where the list of service items has an item with the same ID
// 1. get all the students
List<Student> students = _studentService.GetStudents((int)org.CurrentSchoolYearId).Where(q => q.IsActive).ToList();
// 2. get students requested
List<StudentSession> ss = _sessionService.GetStudentSessions(session.OrgSessionId).Where(q => q.IsActive && q.Status == "AC").ToList();
// check if students are requested
students.RemoveAll(x => ss.Exists(y => y.StudentId == x.StudentId));