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如何：存储和重复使用查询

  • 项目

当您的应用程序多次执行结构上相似的查询时，您通常可以通过如下方法提高性能：编译此查询一次，然后用不同的参数执行它若干次。 例如，应用程序可能需要检索位于特定城市的所有客户，其中此城市是在运行时由用户在窗体中指定的。 LINQ to SQL 支持使用已编译的查询来实现此目的。

备注

这种使用模式代表了已编译查询最常见的用途。 也可以使用其他方法。 例如，已编译查询可以存储为扩展设计器所生成代码的分部类的静态成员。

示例 1

在很多情况下，您可能需要跨线程边界重复使用查询。 在这种情况下，将已编译查询存储在静态变量中特别有效。 下面的代码示例采用设计用于存储已编译查询的 Queries，并采用表示强类型化 DataContext 的 Northwind 类。

public static Func<Northwnd, string, IQueryable<Customer>>
    CustomersByCity =
        CompiledQuery.Compile((Northwnd db, string city) =>
            from c in db.Customers where c.City == city select c);

public static Func<Northwnd, string, IQueryable<Customer>>
    CustomersById = CompiledQuery.Compile((Northwnd db,
    string id) => db.Customers.Where(c => c.CustomerID == id));

Class Queries

    Public Shared CustomersByCity As _
        Func(Of Northwnd, String, IQueryable(Of Customer)) = _
            CompiledQuery.Compile(Function(db As Northwnd, _
    city As String) _
        From c In db.Customers Where c.City = city Select c)

    Public Shared CustomersById As _
        Func(Of Northwnd, String, IQueryable(Of Customer)) = _
            CompiledQuery.Compile(Function(db As Northwnd, _
    id As String) _
        db.Customers.Where(Function(c) c.CustomerID = id))

End Class

// The following example invokes such a compiled query in the main
// program.

public IEnumerable<Customer> GetCustomersByCity(string city)
{
    var myDb = GetNorthwind();
    return Queries.CustomersByCity(myDb, city);
}

' The following example invokes such a compiled query in the main
' program
Public Function GetCustomersByCity(ByVal city As String) As _
    IEnumerable(Of Customer)

    Dim myDb = GetNorthwind()
    Return Queries.CustomersByCity(myDb, city)
End Function

示例 2

目前你无法（在静态变量中）存储返回匿名类型的查询，因为类型没有可作为泛型自变量提供的名称。 下面的示例演示如何创建可表示结果的类型，然后将其用作泛型自变量，从而解决该问题。

class SimpleCustomer
{
    public string ContactName { get; set; }
}

class Queries2
{
    public static Func<Northwnd, string, IEnumerable<SimpleCustomer>> CustomersByCity =
        CompiledQuery.Compile<Northwnd, string, IEnumerable<SimpleCustomer>>(
        (Northwnd db, string city) =>
        from c in db.Customers
        where c.City == city
        select new SimpleCustomer { ContactName = c.ContactName });
}

Class SimpleCustomer
    Private _ContactName As String
    Public Property ContactName() As String
        Get
            Return _ContactName
        End Get
        Set(ByVal value As String)
            _ContactName = value
        End Set
    End Property
End Class

Class Queries2
    Public Shared CustomersByCity As Func(Of Northwnd, String, IEnumerable(Of SimpleCustomer)) = _
        CompiledQuery.Compile(Of Northwnd, String, IEnumerable(Of SimpleCustomer))( _
        Function(db As Northwnd, city As String) _
        From c In db.Customers _
        Where c.City = city _
        Select New SimpleCustomer With {.ContactName = c.ContactName})
End Class

请参阅