已编译的查询 (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 支持使用已编译的查询用于此目的。
CompiledQuery 类提供查询的编译和缓存以供重复使用。从概念上讲，此类包含具有若干重载的 Compile 方法。调用 Compile 方法可以创建新的委托来表示已编译的查询。给定 ObjectContext 及其参数值，Compile 方法将返回生成某个结果的委托（例如 IQueryable 实例）。首次调用该委托时，将转换并缓存查询。
Compile 方法编译的 LINQ to Entities 查询表达式由一个泛型 Func 委托（如 Func）表示。查询表达式通常都可以封装一个 ObjectContext 参数、一个返回参数和三个查询参数。如果需要的查询参数不止三个，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
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：
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.SalesOrderHeader _
Where (order.TotalDue >= total) _
Select order)
Sub CompiledQuery2()
Using AWEntities As New AdventureWorksEntities()
Dim totalDue As Decimal = 200.0
Dim orders As IQueryable(Of SalesOrderHeader) = s_compQuery2.Invoke(AWEntities, 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
static readonly Func<AdventureWorksEntities, Decimal, IQueryable<SalesOrderHeader>> s_compiledQuery2 =
CompiledQuery.Compile<AdventureWorksEntities, Decimal, IQueryable<SalesOrderHeader>>(
(ctx, total) => from order in ctx.SalesOrderHeader
where order.TotalDue >= total
select order);
static void CompiledQuery2()
{
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
Decimal totalDue = 200.00M;
IQueryable<SalesOrderHeader> orders = s_compiledQuery2.Invoke(AWEntities, totalDue);
foreach (SalesOrderHeader order in orders)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}",
order.SalesOrderID,
order.OrderDate,
order.TotalDue);
}
}
}
示例
下面的示例将编译并调用一个返回 ObjectQuery 实例的查询：
ReadOnly s_compQuery1 As Func(Of AdventureWorksEntities, ObjectQuery(Of SalesOrderHeader)) = _
CompiledQuery.Compile(Of AdventureWorksEntities, ObjectQuery(Of SalesOrderHeader))( _
Function(ctx) ctx.SalesOrderHeader)
Sub CompiledQuery1_MQ()
Using AWEntities As New AdventureWorksEntities()
Dim orders As ObjectQuery(Of SalesOrderHeader) = s_compQuery1.Invoke(AWEntities)
For Each order In orders
Console.WriteLine(order.SalesOrderID)
Next
End Using
End Sub
static readonly Func<AdventureWorksEntities, ObjectQuery<SalesOrderHeader>> s_compiledQuery1 =
CompiledQuery.Compile<AdventureWorksEntities, ObjectQuery<SalesOrderHeader>>(
ctx => ctx.SalesOrderHeader);
static void CompiledQuery1_MQ()
{
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
IQueryable<SalesOrderHeader> orders = s_compiledQuery1.Invoke(AWEntities);
foreach (SalesOrderHeader order in orders)
Console.WriteLine(order.SalesOrderID);
}
}
示例
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：
Using AWEntities As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, Decimal)( _
Function(ctx) ctx.Product.Average(Function(Product) Product.ListPrice))
Dim averageProductPrice As Decimal = compQuery.Invoke(AWEntities)
Console.WriteLine("The average of the product list prices is $: {0}", averageProductPrice)
End Using
static readonly Func<AdventureWorksEntities, Decimal> s_compiledQuery3MQ = CompiledQuery.Compile<AdventureWorksEntities, Decimal>(
ctx => ctx.Product.Average(product => product.ListPrice));
static void CompiledQuery3_MQ()
{
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
Decimal averageProductPrice = s_compiledQuery3MQ.Invoke(AWEntities);
Console.WriteLine("The average of the product list prices is $: {0}", averageProductPrice);
}
}
示例
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：
Using AWEntities As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, String, Contact)( _
Function(ctx, name) ctx.Contact.First(Function(contact) contact.EmailAddress.StartsWith(name)))
Dim contactName As String = "caroline"
Dim foundContact As Contact = compQuery.Invoke(AWEntities, contactName)
Console.WriteLine("An email address starting with 'caroline': {0}", _
foundContact.EmailAddress)
End Using
static readonly Func<AdventureWorksEntities, string, Contact> s_compiledQuery4MQ =
CompiledQuery.Compile<AdventureWorksEntities, string, Contact>(
(ctx, name) => ctx.Contact.First(contact => contact.EmailAddress.StartsWith(name)));
static void CompiledQuery4_MQ()
{
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
string contactName = "caroline";
Contact foundContact = s_compiledQuery4MQ.Invoke(AWEntities, contactName);
Console.WriteLine("An email address starting with 'caroline': {0}",
foundContact.EmailAddress);
}
}
示例
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：
ReadOnly s_compQuery5 = _
CompiledQuery.Compile(Of AdventureWorksEntities, DateTime, Decimal, IQueryable(Of SalesOrderHeader))( _
Function(ctx, orderDate, totalDue) From product In ctx.SalesOrderHeader _
Where Product.OrderDate > orderDate _
And Product.TotalDue < totalDue _
Order By Product.OrderDate _
Select Product)
Sub CompiledQuery5()
Using AWEntities 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(AWEntities, 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
static readonly Func<AdventureWorksEntities, DateTime, Decimal, IQueryable<SalesOrderHeader>> s_compiledQuery5 =
CompiledQuery.Compile<AdventureWorksEntities, DateTime, Decimal, IQueryable<SalesOrderHeader>>(
(ctx, orderDate, totalDue) => from product in ctx.SalesOrderHeader
where product.OrderDate > orderDate
&& product.TotalDue < totalDue
orderby product.OrderDate
select product);
static void CompiledQuery5()
{
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
DateTime date = new DateTime(2003, 3, 8);
Decimal amountDue = 300.00M;
IQueryable<SalesOrderHeader> orders = s_compiledQuery5.Invoke(AWEntities, date, amountDue);
foreach (SalesOrderHeader order in orders)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", order.SalesOrderID, order.OrderDate, order.TotalDue);
}
}
}
示例
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晚于 2004 年 3 月 8 日的订单序列。此查询以匿名类型序列形式返回订单信息。匿名类型由编译器推断，因此不能在 Compile 方法中指定类型参数，该类型只能在查询本身中定义。
Using AWEntities As New AdventureWorksEntities()
Dim compQuery = CompiledQuery.Compile( _
Function(ctx As AdventureWorksEntities, orderDate As DateTime) _
From order In ctx.SalesOrderHeader _
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(AWEntities, 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
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
var compiledQuery = CompiledQuery.Compile((AdventureWorksEntities ctx, DateTime orderDate) =>
from order in ctx.SalesOrderHeader
where order.OrderDate > orderDate
select new {order.OrderDate, order.SalesOrderID, order.TotalDue});
DateTime date = new DateTime(2004, 3, 8);
var results = compiledQuery.Invoke(AWEntities, date);
foreach (var order in results)
{
Console.WriteLine("ID: {0} Order date: {1} Total due: {2}", order.SalesOrderID, order.OrderDate, order.TotalDue);
}
}
示例
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。
ReadOnly s_compQuery = CompiledQuery.Compile(Of AdventureWorksEntities, MyParams, IQueryable(Of SalesOrderHeader))( _
Function(ctx, mySearchParams) _
From sale In ctx.SalesOrderHeader _
Where sale.ShipDate > mySearchParams.startDate _
And sale.ShipDate < mySearchParams.endDate _
And sale.TotalDue > mySearchParams.totalDue _
Select sale)
Sub CompiledQuery7()
Using AWEntities 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(AWEntities, 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
static Func<AdventureWorksEntities, MyParams, IQueryable<SalesOrderHeader>> s_compiledQuery =
CompiledQuery.Compile<AdventureWorksEntities, MyParams, IQueryable<SalesOrderHeader>>(
(ctx, myparams) => from sale in ctx.SalesOrderHeader
where sale.ShipDate > myparams.startDate && sale.ShipDate < myparams.endDate
&& sale.TotalDue > myparams.totalDue
select sale);
static void CompiledQuery7()
{
using (AdventureWorksEntities AWEntities = 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(AWEntities, 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);
}
}
}
定义查询参数的结构：
Public Structure MyParams
Public startDate As DateTime
Public endDate As DateTime
Public totalDue As Decimal
End Structure
struct MyParams
{
public DateTime startDate;
public DateTime endDate;
public decimal totalDue;
}