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 (NorthwindEntities ocNwind = new
NorthwindEntities("name=NorthwindEntities"))
{
List<Order> orderList = null;
ObjectQuery<Order> orders = ocNwind.Orders;
orders.MergeOption = MergeOptions.AppendOnly;
var orderQuery = 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 (Order order in orderQuery)
{
// Materialize the object
orderList.Add(order);
}
}
VB 9.0
Using ocNwind As New NorthwindEntities("name=NorthwindEntities")
Dim orderList As List(Of Order) = Nothing
Dim oqOrders As ObjectQuery(Of Order) = ocNwind.Orders
orders.MergeOption = MergeOptions.AppendOnly
Dim orderQuery = 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)
For Each oqOrder As Order In orderQuery
' Materialize the object
orderList.Add(oqOrder)
Next oqOrder
End Using

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 = (from o in orders
where o.Customer.Country == "Brazil"
orderby o.OrderID descending
select o).Take(5);
VB 9.0
oqOrdersoqOrders = oqOrders.Include("Order_Details")
.Include("Customer")
.Include("Employee")
.Include("Shipper")
orderQuery = (From o In oqOrders _
Where o.Customer.Country = "Brazil" _
Order By o.OrderID Descending _
Select o).Take(5)

可选地，可以使用下面这样更为易读的语法：

C# 3.0
orderQuery = (from o in orders.Include("Order_Details")
.Include("Customer")
.Include("Employee")
.Include("Shipper")
where o.Customer.Country == "Brazil"
orderby o.OrderID descending
select o).Take(5);
VB 9.0
orderQuery = (From o In oqOrders.Include("Order_Details" _
.Include("Customer") _
.Include("Employee") _
.Include("Shipper") _
Where o.Customer.Country = "Brazil" _
Order By o.OrderID Descending _
select o).Take(5);