HOW TO：儲存和重複使用查詢 (LINQ to SQL)
當您的應用程式會多次執行結構類似的查詢時，您藉由編譯查詢一次並使用不同的參數加以執行數次，通常可以提高效能。 例如，應用程式可能必須擷取位於特定城市的所有客戶，該城市是使用者於執行階段在表單中指定。 LINQ to SQL 支援就此目的使用「已編譯查詢」(Compiled Query)。
.gif "注意事項") 注意事項 |
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這種模式的使用方式代表已編譯查詢的最常見用途。其他方法也有可能。例如，在可擴充設計工具所產生之程式碼的部分類別中，已編譯查詢可以儲存為靜態成員。 |
範例
在許多情況下，您會想要跨越執行緒界限重複使用查詢。 在此情況下，以靜態變數儲存已編譯查詢特別有效。 下列程式碼範例採用了設計用來儲存已編譯查詢的 Queries 類別，並採用表示強型別 DataContext 的 Northwind 類別。
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
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));
' 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
// 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);
}
您目前無法 (以靜態變數) 儲存會傳回「匿名型別」(anonymous type) 的查詢，因為該型別沒有名稱可提供做為泛型引數。 下列範例顯示如何藉由建立可表示結果的型別並以它做為泛型引數，來解決這個問題。
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
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 });
}