Entity Framework常用方法及案例

⒈Skip(int count)

　　说明：跳过集合的前n个元素；延迟。即我们跳过给定的数目返回后面的结果集。

⒉Take(int count)

　　说明：获取集合的前n个元素；延迟。即只返回限定数量的结果集。

案例：分页查询

1         public IList<StudentScore> test1(int pageSize,int pageIndex)
2         {
3             //skip表示跳过多少条，Take表示取多少条
4             var list = _db.StudentScore.Skip(pageSize * (pageIndex - 1)).Take(pageSize).ToList();
5             return list;
6         }

⒊OrderBy

说明：按指定表达式对集合排序；延迟，默认是升序，加上descending表示降序，对应的扩展方法是 OrderBy和OrderByDescending

1         public IList<StudentScore> test2()
2         {
3             var list = _db.StudentScore.OrderBy(s => s.Id).ToList();
4             var list2 = _db.StudentScore.OrderByDescending(s => s.Id).ToList();
5             return list;
6         }

⒋Add

说明：添加一个实体

1         public int test4()
2         {
3             var student = new Student {SName = "fanqi",SSex = 1,SBirthday = new DateTime(1994,6,24) };
4             var add = _db.Student.Add(student);
5             _db.SaveChanges();
6             return add.Entity.Id;
7         }

⒌Remove

说明：删除一个实体

 1         public void test5()
 2         {
 3             _db.Remove(new Student { Id = 9 });
 4             _db.SaveChanges();
 5 
 6             //Student user = new Student(){ Id = 9};
 7             //将对象添加到EF管理容器
 8             //_db.Student.Attach(user);
 9             //将对象包装类的标识状态变更为删除状态
10             //_db.Student.Remove(user);
11             //通知上下文将实体的变化保存到数据库中
12             //_db.SaveChanges();
13         }

⒍Update

说明：更新一个实体

 1         public void test6()
 2         {
 3             Student student = _db.Student.FirstOrDefault(f => f.Id == 7);
 4             student.SName = "fanqi";
 5             _db.Student.Update(student);
 6             _db.SaveChanges();
 7 
 8             //声明一个实体对象并赋初始值
 9             //Student student = new Student { Id = 8, SName = "fanqi" };
10             //将实体对象加入到EF对象容器中，并获取伪包装类对象
11             //EntityEntry<Student> entry = _db.Entry<Student>(student);
12             //将包装类对象的状态设置为unchanged
13             //entry.State = EntityState.Unchanged;
14             //设置实体对象被改变的属性
15             //entry.Property(a => a.SName).IsModified = true;
16             //通知上下文将实体的变化保存到数据库中
17             //_db.SaveChanges();
18         }

⒎Join

说明：内连接

 1         public dynamic test7()
 2         {
 3             var studentList = _db.Student;
 4             var scoreList = _db.StudentScore;
 5             var newList = studentList.Join(scoreList, stu => stu.Id, sco => sco.SId, (stu, sco) => new
 6             {
 7                 studentId = stu.Id,
 8                 studentName = stu.SName,
 9                 studentSex = stu.SSex == 1?"男":"女",
10                 scoreId = sco.Id,
11                 courseId = sco.CId,
12                 courseScoreNum = sco.Score
13             });
14             return newList;
15         }

⒏GroupJoin

说明：左连接

 1         public dynamic test8()
 2         {
 3             var studentList = _db.Student;
 4             var scoreList = _db.StudentScore;
 5 
 6             var newList = studentList.GroupJoin(scoreList, stu => stu.Id, sco => sco.SId, (stu, sco) => new
 7             {
 8                 studentId = stu.Id,
 9                 studentName = stu.SName,
10                 studentSex = stu.SSex == 1 ? "男" : "女",
11                 sco
12             });
13             return newList;
14         }