static void Main(string[] args)
        {
            // 创建一个网关接口,TestData是数据库名
            TestDataEntities td = new TestDataEntities();
            
            // 创建一个实体对象,Student是表映射过来的对象,将其赋值
            Student st1 = new Student();
            st1.StudentID = "s4";
            st1.StudentName = "test1";
            st1.Age = 20;

            // 将实体对象添加到网关接口，插入操作
            td.Student.AddObject(st1);
            // 网关保存并改变
            td.SaveChanges();

            Console.WriteLine("添加成功！");
        }

    class Program
    {
        // 创建一个网关接口,TestData是数据库名,静态方法只能调用静态类
        static TestDataEntities stuEntities = new TestDataEntities();

        static void Main(string[] args)
        {
            // 创建一个实体对象,Student是表映射过来的对象,将其赋值
            Student st1 = new Student();
            st1.StudentID = "s4";
            st1.StudentName = "小明";
            st1.Age = 20;

            //InsertStu(st1);
            //DeleteStu("s4");

            //Student stu2 = FindStudentByID("s4");
            //stu2.StudentName = "小明";
            //UpdateStu(stu2);
            //Console.WriteLine(stu2.StudentName);
            

        }
        // 添加操作
        public static void InsertStu(Student stu)
        {
            // 将实体对象添加到网关接口,插入操作
            stuEntities.Student.AddObject(stu);
            // 网关保存并改变
            stuEntities.SaveChanges();
            Console.WriteLine("添加成功！ID:"+stu.StudentID);
        }

        // 删除操作
        public static void DeleteStu(string ID)
        {
            Student stu = FindStudentByID(ID);
            stuEntities.Student.DeleteObject(stu);
            stuEntities.SaveChanges();
            Console.WriteLine("删除成功！ID:" + stu.StudentID);

        }

        // 更新操作
        public static void UpdateStu(Student stu)
        {
            // 其中 var 为 IQueryable<Student>类型,继承IEnumerable<Student>
            // 继承IEnumerable的类都能实现 foreach
            var student = from s in stuEntities.Student
                          where s.StudentID == stu.StudentID
                          select s;
            // 得到student集合里面的单一实体
            var oldStu = student.SingleOrDefault();
            // 修改对应的属性值
            oldStu.StudentName = stu.StudentName;
            oldStu.Age = stu.Age;
            
            // 保存修改
            stuEntities.SaveChanges();
            Console.WriteLine("更新成功！ID:" + stu.StudentID);
        }

    // 更新数据 新方法
        public bool UpdateEntity(MvcHotel.Model.Customer entity)
        {
            bool result = false;
            // 添加对象到上下文
            he.Attach(entity);
            // 改变新加入对象的状态,设置为 已修改
            he.ObjectStateManager.ChangeObjectState(entity, System.Data.EntityState.Modified);
            if (he.SaveChanges() > 0)
            {
                result = true;
            }
            return result;
        }

        // 查询操作
        public static Student FindStudentByID(string ID)
        {
        // 方式1： Linq to EF
            // 根据ID查询Student,并得到集合中的单一实体
            var stu = (from s in stuEntities.Student
                       where s.StudentID == ID
                       select s).SingleOrDefault();

        // 方式2： Entity SQL
            string sql = "select Value c from TestDataEntities.Student as c ";
            ObjectQuery<Student> query = stuEntities.CreateQuery<Student>(sql);
            ObjectResult<Student> results = query.Execute(MergeOption.NoTracking);

            //return stu;
            return query.SingleOrDefault();
        }
    }