public Exam Get(Expression <Func <Exam, bool> > filter)
 {
     using (var context = new ExamSystemContext())
     {
         return(context.Set <Exam>().SingleOrDefault(filter));
     }
 }
 public IList <Exam> GetList(Expression <Func <Exam, bool> > filter = null)
 {
     using (var context = new ExamSystemContext())
     {
         return(filter == null
             ? context.Set <Exam>().ToList()
             : context.Set <Exam>().Where(filter).ToList());
     }
 }