public List<ClassDTO> GetList()
 {
     _db = new StudentDbEntities();
     var list =  _db.Classes.ToList();
     var listDto = new List<ClassDTO>();
     foreach (var @class in list)
     {
         var classDto = new ClassDTO {Id = @class.Id, Name = @class.Name, Code = @class.Code};
         var listStudentDto = new List<StudentDTO>();
         foreach (var student in @class.Students)
         {
             var studentDto = new StudentDTO {Id = student.Id, Name = student.Name};
             var classDal = new ClassDAL();
             studentDto.Class = classDal.GetById(student.ClassId);
             if (student.Mark1 != null) studentDto.Mark1 = (double) student.Mark1;
             if (student.Mark2 != null) studentDto.Mark2 = (double) student.Mark2;
             if (student.Mark3 != null) studentDto.Mark3 = (double) student.Mark3;
             listStudentDto.Add(studentDto);
         }
         classDto.Students = listStudentDto;
         listDto.Add(classDto);
     }
     return listDto;
 }
 public void Update(ClassDTO classDto)
 {
     _classDal = new ClassDAL();
     _classDal.Update(classDto);
 }
 public void Save(Data_Transfer_Objects.ClassDTO classDto)
 {
     _classDal = new ClassDAL();
     _classDal.Save(classDto);
 }
 public List<Data_Transfer_Objects.ClassDTO> GetList()
 {
     _classDal = new ClassDAL();
     List<ClassDTO> list = _classDal.GetList();
     return list;
 }
 public Data_Transfer_Objects.ClassDTO GetById(int id)
 {
     _classDal = new ClassDAL();
     ClassDTO classDto = _classDal.GetById(id);
     return classDto;
 }
 public void Delete(int id)
 {
     _classDal = new ClassDAL();
     _classDal.Delete(id);
 }