缺点：Entity FrameWork 没有提供获取不同层上的对象的方式，无助于创建多层上的解决方案,可以结合WCF实现分层

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核心概念：

逻辑层，SSDL store schema definition language描述数据库表及其关系

概念层，CSDL conceptual schema definition L  描述 实体及关系

映射层 MSL  mapping Specification L  把CSDL实体类型定义映射到SSDL

查询对象的三种方法

1. Entity SQL

using (var data = new Formula1Entities())

            {

                string country = "Brazil";

                ObjectQuery racers = data.CreateQuery(

                   "SELECT VALUE it FROM ([Formula1Entities].[Racers]) AS it WHERE it.Country = @Country",

                   new ObjectParameter("Country", country));

                foreach (var r in racers)

                {

                    Console.WriteLine("{0} {1}", r.Firstname, r.Lastname);

                }

            }

2.QueryBuilder

类似于LINQ,但Linq的参数是委托，而QueryBuilder的参数是字符串

可以用Find(),GroupBy(),OrderBy()等方法，

  using (var data = new Formula1Entities())

            {

                string country = "Brazil";

                ObjectQuery racers = data.Racers.Where("it.Country = @Country", new ObjectParameter("Country", country));

                Console.WriteLine(racers.CommandText);

                Console.WriteLine(racers.ToTraceString());

            }

3. Linq to Entities

因为实体对象都继承于ObjectQuery，而该类实现了IQueryable接口

using (var data = new Formula1Entities())

            {

                var racers = from r in data.Racers

                             where r.Wins > 40

                             orderby r.Wins descending

                             select r;

                foreach (Racer r in racers)

                {

                    Console.WriteLine("{0} {1}", r.Firstname, r.Lastname);

                }

            }

增删改查CRUD的实现

  using (var data = new Formula1Entities())

            {

                var jaime = new Racer

                {

                    Firstname = "Jaime",

                    Lastname = "Alguersuari",

                    Country = "Spain",

                    Starts = 0

                };

                data.Racers.AddObject(jaime);

           //添加

Racer niki1 = data.Racers.Where("it.Country='Austria' && it.Lastname='Berger'").First();

   data.Racers.DeleteObject( niki1 );

//删除

  Racer niki2 = data.Racers.Where("it.Country='Austria'").OrderBy("it.Wins DESC").First();

niki2.Stars++;//修改

                int changes = 0;

                try

                {

                    changes += data.SaveChanges();

//提交到数据源

                }

   catch (OptimisticConcurrencyException ex)

                {

                    data.Refresh(RefreshMode.ClientWins, ex.StateEntries);

                    changes += data.SaveChanges();

                }

}

//懒惰加载

using (var data = new NorthwindEntities())

            {

                data.ContextOptions.LazyLoadingEnabled = false;

                int a = data.Customers.Count();

                foreach (Customer customer in data.Customers.Include("Orders"))

                {

                    Console.WriteLine("{0}", customer.CompanyName);

                    bool a2 = customer.Orders.IsLoaded;

              //   if (!customer.Orders.IsLoaded)

              //      {

              //          customer.Orders.Load();

                    }

                    foreach (Order order in customer.Orders)

                    {

                        Console.WriteLine("\t{0} {1:d}", order.OrderID, order.OrderDate);

                    }

                }

            }

data.ContextOptions.LazyLoadingEnabled = false;

LazyLoadingEnabled  懒惰加载，意思是在访问对象时，会自动加载对象的相关属性   

foreach (Customer customer in data.Customers)

LazyLoadingEnabled  默认为true,若设为false,则需要使用

Include("Orders")) 称为预先加载

     customer.Orders.Load();（延迟加载

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