Retrieving Objects from the Identity Cache (LINQ to SQL)
本主題描述 LINQ to SQL 查詢的類型，這些查詢可從 DataContext 所管理的識別快取傳回物件。
在 LINQ to SQL 中，DataContext 用來管理物件的其中一種方式就是在執行查詢時，將物件識別記錄在識別快取中。在某些情況下，LINQ to SQL 會先嘗試從識別快取擷取物件，然後再執行資料庫的查詢。
一般而言，為了讓 LINQ to SQL 查詢從識別快取傳回物件，此查詢必須以物件的主索引鍵為基礎，而且必須傳回單一物件。特別是，此查詢必須採用下列其中一種一般格式。
.gif "注意事項") 注意事項 |
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預先編譯的查詢不會從識別快取傳回物件。如需預先編譯之查詢的詳細資訊，請參閱 CompiledQuery 和 HOW TO：儲存和重複使用查詢 (LINQ to SQL)。 |
若要從識別快取擷取物件，查詢必須採用下列其中一種一般格式：
Table<TEntity>.Function1(predicate)
Table<TEntity>.Function1(predicate).Function2()
在這些一般格式中，Function1、Function2 和 predicate 的定義方式如下。
Function1 可以是下列任何項目：
Function2 可以是下列任何項目：
predicate 必須是物件之主索引鍵屬性設定為常數值的運算式。如果物件具有多個屬性所定義的主索引鍵，每個主索引鍵屬性都必須設定為常數值。下面是 predicate 必須採用的格式範例：
c => c.PK == constant_value
c => c.PK1 == constant_value1 && c=> c.PK2 == constant_value2
範例
下列程式碼將提供 LINQ to SQL 查詢類型的範例，這些查詢可從識別快取擷取物件。
Dim context As New NorthwindDataContext()
' This query does not retrieve an object from
' the query cache because it is the first query.
' There are no objects in the cache.
Dim a = context.Customers.First()
Console.WriteLine("First query gets customer {0}. ", a.CustomerID)
' This query returns an object from the query cache.
Dim b = context.Customers.Where(Function(c) c.CustomerID = a.CustomerID)
For Each customer In b
Console.WriteLine(customer.CustomerID)
Next
' This query returns an object from the identity cache.
' Note that calling FirstOrDefault(), Single(), or SingleOrDefault()
' instead of First() will also return an object from the cache.
Dim x = context.Customers. _
Where(Function(c) c.CustomerID = a.CustomerID). _
First()
Console.WriteLine(x.CustomerID)
' This query returns an object from the identity cache.
' Note that calling FirstOrDefault(), Single(), or SingleOrDefault()
' instead of First() (each with the same predicate) will also
' return an object from the cache.
Dim y = context.Customers.First(Function(c) c.CustomerID = a.CustomerID)
Console.WriteLine(y.CustomerID)
NorthwindDataContext context = new NorthwindDataContext();
// This query does not retrieve an object from
// the query cache because it is the first query.
// There are no objects in the cache.
var a = context.Customers.First();
Console.WriteLine("First query gets customer {0}. ", a.CustomerID);
// This query returns an object from the query cache.
var b = context.Customers.Where(c => c.CustomerID == a.CustomerID);
foreach (var customer in b )
{
Console.WriteLine(customer.CustomerID);
}
// This query returns an object from the identity cache.
// Note that calling FirstOrDefault(), Single(), or SingleOrDefault()
// instead of First() will also return an object from the cache.
var x = context.Customers.
Where(c => c.CustomerID == a.CustomerID).
First();
Console.WriteLine(x.CustomerID);
// This query returns an object from the identity cache.
// Note that calling FirstOrDefault(), Single(), or SingleOrDefault()
// instead of First() (each with the same predicate) will also
// return an object from the cache.
var y = context.Customers.First(c => c.CustomerID == a.CustomerID);
Console.WriteLine(y.CustomerID);