编译的查询 (LINQ to Entities)
如果应用程序需要在实体框架中多次执行结构类似的查询，通常可以通过仅编译查询一次并在每次执行时使用不同参数的方法来提高性能。 例如，某应用程序要检索特定城市的所有客户，而该城市是运行时由用户在窗体中指定的。 LINQ to Entities 支持将已编译的查询用于此目的。
从 .NET Framework 4.5 开始，LINQ 查询是自动缓存的。 但是，您仍可以使用已编译的 LINQ 查询来降低后续执行中的这一开销，编译的查询比自动缓存的 LINQ 查询效率更高。 不自动缓存将 Enumerable.Contains 运算符应用到内存中集合的 LINQ to Entities 查询。 此外，不允许在已编译的 LINQ 查询中参数化内存中的集合。
CompiledQuery 类提供查询的编译和缓存以供重复使用。 从概念上讲，此类包含 CompiledQuery 的 Compile 方法以及若干重载。 调用 Compile 方法可以创建新的委托来表示已编译的查询。 给定 Compile 及其参数值，ObjectContext 方法将返回生成某个结果的委托（例如 IQueryable<T> 实例）。 只在第一次执行的过程中查询才编译一次。 编译时为查询设置的合并选项在以后无法更改。 对查询进行编译后，你可以仅提供基元类型的参数，但不能替换将更改生成的 SQL 的查询部分。 有关详细信息，请参阅 EF 合并选项和编译查询。
CompiledQuery 的 Compile 方法编译的 LINQ to Entities 查询表达式由 Func 泛型委托之一表示，例如 Func<T1,T2,T3,T4,TResult>。 查询表达式最多可以包装一个 ObjectContext 参数、一个返回参数和 16 个查询参数。 如果需要的查询参数不止 16 个，则可以创建一个结构并用其属性表示查询参数。 然后，该结构上的这些属性在经过设置之后就可以用于查询表达式中。
示例 1
下面的示例将编译并调用一个查询，该查询接受 Decimal 输入参数，并返回一个应付款总额大于或等于 200.00 美元的订单序列：
static readonly Func<AdventureWorksEntities, Decimal, IQueryable<SalesOrderHeader>> s_compiledQuery2 =
CompiledQuery.Compile<AdventureWorksEntities, Decimal, IQueryable<SalesOrderHeader>>(
(ctx, total) => from order in ctx.SalesOrderHeaders
where order.TotalDue >= total
select order);
static void CompiledQuery2()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
Decimal totalDue = 200.00M;
IQueryable<SalesOrderHeader> orders = s_compiledQuery2.Invoke(context, totalDue);
foreach (SalesOrderHeader order in orders)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}",
order.SalesOrderID,
order.OrderDate,
order.TotalDue);
}
}
}
ReadOnly s_compQuery2 As Func(Of AdventureWorksEntities, Decimal, IQueryable(Of SalesOrderHeader)) = _
CompiledQuery.Compile(Of AdventureWorksEntities, Decimal, IQueryable(Of SalesOrderHeader))( _
Function(ctx As AdventureWorksEntities, total As Decimal) _
From order In ctx.SalesOrderHeaders _
Where (order.TotalDue >= total) _
Select order)
Sub CompiledQuery2()
Using context As New AdventureWorksEntities()
Dim totalDue As Decimal = 200.0
Dim orders As IQueryable(Of SalesOrderHeader) = s_compQuery2.Invoke(context, totalDue)
For Each order In orders
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", _
order.SalesOrderID, _
order.OrderDate, _
order.TotalDue)
Next
End Using
End Sub
示例 2
下面的示例将编译并调用一个返回 ObjectQuery<T> 实例的查询：
static readonly Func<AdventureWorksEntities, ObjectQuery<SalesOrderHeader>> s_compiledQuery1 =
CompiledQuery.Compile<AdventureWorksEntities, ObjectQuery<SalesOrderHeader>>(
ctx => ctx.SalesOrderHeaders);
static void CompiledQuery1_MQ()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
IQueryable<SalesOrderHeader> orders = s_compiledQuery1.Invoke(context);
foreach (SalesOrderHeader order in orders)
Console.WriteLine(order.SalesOrderID);
}
}
ReadOnly s_compQuery1 As Func(Of AdventureWorksEntities, ObjectQuery(Of SalesOrderHeader)) = _
CompiledQuery.Compile(Of AdventureWorksEntities, ObjectQuery(Of SalesOrderHeader))( _
Function(ctx) ctx.SalesOrderHeaders)
Sub CompiledQuery1_MQ()
Using context As New AdventureWorksEntities()
Dim orders As ObjectQuery(Of SalesOrderHeader) = s_compQuery1.Invoke(context)
For Each order In orders
Console.WriteLine(order.SalesOrderID)
Next
End Using
End Sub
示例 3
下面的示例将编译并调用一个返回 Decimal 形式产品标价平均值的查询：
static readonly Func<AdventureWorksEntities, Decimal> s_compiledQuery3MQ = CompiledQuery.Compile<AdventureWorksEntities, Decimal>(
ctx => ctx.Products.Average(product => product.ListPrice));
static void CompiledQuery3_MQ()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
Decimal averageProductPrice = s_compiledQuery3MQ.Invoke(context);
Console.WriteLine("The average of the product list prices is $: {0}", averageProductPrice);
}
}
Using context As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, Decimal)( _
Function(ctx) ctx.Products.Average(Function(Product) Product.ListPrice))
Dim averageProductPrice As Decimal = compQuery.Invoke(context)
Console.WriteLine("The average of the product list prices is $: {0}", averageProductPrice)
End Using
示例 4
下面的示例将编译并调用一个查询，该查询接受 String 输入参数，并返回电子邮件地址以指定字符串开始的 Contact：
static readonly Func<AdventureWorksEntities, string, Contact> s_compiledQuery4MQ =
CompiledQuery.Compile<AdventureWorksEntities, string, Contact>(
(ctx, name) => ctx.Contacts.First(contact => contact.EmailAddress.StartsWith(name)));
static void CompiledQuery4_MQ()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
string contactName = "caroline";
Contact foundContact = s_compiledQuery4MQ.Invoke(context, contactName);
Console.WriteLine("An email address starting with 'caroline': {0}",
foundContact.EmailAddress);
}
}
Using context As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, String, Contact)( _
Function(ctx, name) ctx.Contacts.First(Function(contact) contact.EmailAddress.StartsWith(name)))
Dim contactName As String = "caroline"
Dim foundContact As Contact = compQuery.Invoke(context, contactName)
Console.WriteLine("An email address starting with 'caroline': {0}", _
foundContact.EmailAddress)
End Using
示例 5
下面的示例将编译并调用一个查询，该查询接受 DateTime 和 Decimal 输入参数，并返回一个订单日期晚于 2003 年 3 月 8 日且应付款总额少于 300.00 美元的订单序列：
static readonly Func<AdventureWorksEntities, DateTime, Decimal, IQueryable<SalesOrderHeader>> s_compiledQuery5 =
CompiledQuery.Compile<AdventureWorksEntities, DateTime, Decimal, IQueryable<SalesOrderHeader>>(
(ctx, orderDate, totalDue) => from product in ctx.SalesOrderHeaders
where product.OrderDate > orderDate
&& product.TotalDue < totalDue
orderby product.OrderDate
select product);
static void CompiledQuery5()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
DateTime date = new DateTime(2003, 3, 8);
Decimal amountDue = 300.00M;
IQueryable<SalesOrderHeader> orders = s_compiledQuery5.Invoke(context, date, amountDue);
foreach (SalesOrderHeader order in orders)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", order.SalesOrderID, order.OrderDate, order.TotalDue);
}
}
}
ReadOnly s_compQuery5 = _
CompiledQuery.Compile(Of AdventureWorksEntities, DateTime, Decimal, IQueryable(Of SalesOrderHeader))( _
Function(ctx, orderDate, totalDue) From product In ctx.SalesOrderHeaders _
Where product.OrderDate > orderDate _
And product.TotalDue < totalDue _
Order By product.OrderDate _
Select product)
Sub CompiledQuery5()
Using context As New AdventureWorksEntities()
Dim orderedAfterDate As DateTime = New DateTime(2003, 3, 8)
Dim amountDue As Decimal = 300.0
Dim orders As IQueryable(Of SalesOrderHeader) = _
s_compQuery5.Invoke(context, orderedAfterDate, amountDue)
For Each order In orders
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", _
order.SalesOrderID, order.OrderDate, order.TotalDue)
Next
End Using
End Sub
示例 6
下面的示例将编译并调用一个查询，该查询接受 DateTime 输入参数，并返回一个订单日期迟于 2004 年 3 月 8 日的订单序列： 此查询将订单信息返回为匿名类型序列。 匿名类型由编译器推断，因此不能在 CompiledQuery 的 Compile 方法中指定类型参数，该类型只能在查询本身中定义。
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
var compiledQuery = CompiledQuery.Compile((AdventureWorksEntities ctx, DateTime orderDate) =>
from order in ctx.SalesOrderHeaders
where order.OrderDate > orderDate
select new {order.OrderDate, order.SalesOrderID, order.TotalDue});
DateTime date = new DateTime(2004, 3, 8);
var results = compiledQuery.Invoke(context, date);
foreach (var order in results)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", order.SalesOrderID, order.OrderDate, order.TotalDue);
}
}
Using context As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile( _
Function(ctx As AdventureWorksEntities, orderDate As DateTime) _
From order In ctx.SalesOrderHeaders _
Where order.OrderDate > orderDate _
Select New With {order.OrderDate, order.SalesOrderID, order.TotalDue})
Dim orderedAfterDate As DateTime = New DateTime(2004, 3, 8)
Dim orders = compQuery.Invoke(context, orderedAfterDate)
For Each order In orders
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", _
order.SalesOrderID, order.OrderDate, order.TotalDue)
Next
End Using
示例 7
下面的示例编译并调用一个查询，该查询接受用户定义的结构输入参数并返回一个订单序列。 该结构定义开始日期、结束日期和应付款总额查询参数，该查询返回在 2003 年 3 月 3 日到 3 月 8 日之间发货的应付款总额超过 700.00 美元的订单。
static Func<AdventureWorksEntities, MyParams, IQueryable<SalesOrderHeader>> s_compiledQuery =
CompiledQuery.Compile<AdventureWorksEntities, MyParams, IQueryable<SalesOrderHeader>>(
(ctx, myparams) => from sale in ctx.SalesOrderHeaders
where sale.ShipDate > myparams.startDate && sale.ShipDate < myparams.endDate
&& sale.TotalDue > myparams.totalDue
select sale);
static void CompiledQuery7()
{
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
MyParams myParams = new MyParams();
myParams.startDate = new DateTime(2003, 3, 3);
myParams.endDate = new DateTime(2003, 3, 8);
myParams.totalDue = 700.00M;
IQueryable<SalesOrderHeader> sales = s_compiledQuery.Invoke(context, myParams);
foreach (SalesOrderHeader sale in sales)
{
Console.WriteLine("ID: {0}", sale.SalesOrderID);
Console.WriteLine("Ship date: {0}", sale.ShipDate);
Console.WriteLine("Total due: {0}", sale.TotalDue);
}
}
}
ReadOnly s_compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, MyParams, IQueryable(Of SalesOrderHeader))( _
Function(ctx, mySearchParams) _
From sale In ctx.SalesOrderHeaders _
Where sale.ShipDate > mySearchParams.startDate _
And sale.ShipDate < mySearchParams.endDate _
And sale.TotalDue > mySearchParams.totalDue _
Select sale)
Sub CompiledQuery7()
Using context As New AdventureWorksEntities()
Dim myParams As MyParams = New MyParams()
myParams.startDate = New DateTime(2003, 3, 3)
myParams.endDate = New DateTime(2003, 3, 8)
myParams.totalDue = 700.0
Dim sales = s_compQuery.Invoke(context, myParams)
For Each sale In sales
Console.WriteLine("ID: {0}", sale.SalesOrderID)
Console.WriteLine("Ship date: {0}", sale.ShipDate)
Console.WriteLine("Total due: {0}", sale.TotalDue)
Next
End Using
End Sub
定义查询参数的结构：
struct MyParams
{
public DateTime startDate;
public DateTime endDate;
public decimal totalDue;
}
Public Structure MyParams
Public startDate As DateTime
Public endDate As DateTime
Public totalDue As Decimal
End Structure