LINQ to Entities 常用查询语句

要使用关联实体返回局部或完整的对象图，需要在LINQ 标准查询操作符之前执行Include()操作符，如下列查询所示，它将返回和前面“使用Include()操作符进行预先加载”一节同样的结果：

 
 

  
  
C#3.0 
  
  
using(NorthwindEntitiesocNwind=new
  
  
NorthwindEntities("name=NorthwindEntities")) 
  
  
{ 
  
  
List<Order>orderList=null; 
  
  
ObjectQuery<Order>orders=ocNwind.Orders; 
  
  
orders.MergeOption=MergeOptions.AppendOnly; 
  
  
varorderQuery=orders.Include("Order_Details") 
  
  
.Include("Customer") 
  
  
.Include("Employee") 
  
  
.Include("Shipper") 
  
  
.Where(o=>o.Customer.Country=="Brazil") 
  
  
.OrderByDescending(o=>o.OrderID) 
  
  
.Select(o=>o) 
  
  
.Take(5); 
  
  
foreach(OrderorderinorderQuery) 
  
  
{ 
  
  
//Materializetheobject 
  
  
orderList.Add(order); 
  
  
} 
  
  
} 
  
  
VB9.0 
  
  
UsingocNwindAsNewNorthwindEntities("name=NorthwindEntities") 
  
  
DimorderListAsList(OfOrder)=Nothing 
  
  
DimoqOrdersAsObjectQuery(OfOrder)=ocNwind.Orders 
  
  
orders.MergeOption=MergeOptions.AppendOnly 
  
  
DimorderQuery=oqOrders.Include("Order_Details")_ 
  
  
.Include("Customer")_ 
  
  
.Include("Employee")_ 
  
  
.Include("Shipper")_ 
  
  
.Where(Function(o)o.Customer.Country="Brazil")_ 
  
  
.OrderByDescending(Function(o)o.OrderID)_ 
  
  
.Select(Function(o)o)_ 
  
  
.Take(5) 
  
  
ForEachoqOrderAsOrderInorderQuery 
  
  
'Materializetheobject 
  
  
orderList.Add(oqOrder) 
  
  
NextoqOrder 
  
  
EndUsing
 
 

LINQ 表达式语法和串联方法调用区别较大。在4 个标准查询操作符执行之后，要将ObjectQuery<Order>改变为IQueryable<Order>类型，需要把Include()操作符和LINQ 表达式相分隔，如下所示：

 
 

  
  
C#3.0 
  
  
ordersorders=orders.Include("Order_Details") 
  
  
.Include("Customer") 
  
  
.Include("Employee") 
  
  
.Include("Shipper"); 
  
  
orderQuery=(fromoinorders 
  
  
whereo.Customer.Country=="Brazil" 
  
  
orderbyo.OrderIDdescending 
  
  
selecto).Take(5); 
  
  
VB9.0 
  
  
oqOrdersoqOrders=oqOrders.Include("Order_Details") 
  
  
.Include("Customer") 
  
  
.Include("Employee") 
  
  
.Include("Shipper") 
  
  
orderQuery=(FromoInoqOrders_ 
  
  
Whereo.Customer.Country="Brazil"_ 
  
  
OrderByo.OrderIDDescending_ 
  
  
Selecto).Take(5)
 
 

可选地，可以使用下面这样更为易读的语法：

 
 

  
  
C#3.0 
  
  
orderQuery=(fromoinorders.Include("Order_Details") 
  
  
.Include("Customer") 
  
  
.Include("Employee") 
  
  
.Include("Shipper") 
  
  
whereo.Customer.Country=="Brazil" 
  
  
orderbyo.OrderIDdescending 
  
  
selecto).Take(5); 
  
  
VB9.0 
  
  
orderQuery=(FromoInoqOrders.Include("Order_Details"_ 
  
  
.Include("Customer")_ 
  
  
.Include("Employee")_ 
  
  
.Include("Shipper")_ 
  
  
Whereo.Customer.Country="Brazil"_ 
  
  
OrderByo.OrderIDDescending_ 
  
  
selecto).Take(5);
 
 

（编辑：李大同）

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