public string Select(Course item) { CourseCheckState courseCheckState = new CourseCheckState { IsValidate = true }; Student student = _studentDao.GetAll().SingleOrDefault(x => x.Id == item.StudentId); List <Course> courseSelected = _courseDao.GetAll().Where(x => x.StudentId == student.Id).ToList(); student.Courses = courseSelected; courseCheckState.CourseSelect = new CourseSelect { Student = student, Course = item }; _courseValidation.Check(courseCheckState); if (!courseCheckState.IsValidate) { return(courseCheckState.Message); } _courseDao.Create(item); return("Course Select Success"); }