LINQ to Entities 中的已知問題和考量
本節提供有關 LINQ to Entities 查詢已知問題的資訊。
不能快取的 LINQ 查詢
從 .NET Framework 4.5 開始會自動快取 LINQ to Entities 查詢。 不過，不會自動快取將 Enumerable.Contains運算子套用至記憶體中集合的 LINQ to Entities 查詢。 此外也不允許在已編譯的 LINQ 查詢中參數化記憶體中的集合。
排序資訊遺失
在針對相容性層級設定為 "80" 的 SQL Server 2005 資料庫執行的某些查詢中，將資料行投影到匿名型別將會造成排序資訊遺失。 如果 order-by 清單中的資料行名稱與 selector 中的資料行名稱相符，就會發生這種情況，如以下範例所示：
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
// Ordering information is lost when executed against a SQL Server 2005
// database running with a compatibility level of "80".
var results = context.Contacts.SelectMany(c => c.SalesOrderHeaders)
.OrderBy(c => c.SalesOrderDetails.Count)
.Select(c => new { c.SalesOrderDetails.Count });
foreach (var result in results)
Console.WriteLine(result.Count);
}
Using context As New AdventureWorksEntities()
' Ordering information is lost when executed against a SQL Server 2005
' database running with a compatibility level of "80".
Dim results = context.Contacts.SelectMany(Function(c) c.SalesOrderHeaders) _
.OrderBy(Function(c) c.SalesOrderDetails.Count) _
.Select(Function(c) New With {c.SalesOrderDetails.Count})
For Each result In results
Console.WriteLine(result.Count)
Next
End Using
不支援不帶正負號的整數
不支援在 LINQ to Entities 查詢中指定不帶正負號的整數型別，因為 Entity Framework 不支援不帶正負號的整數。 如果指定不帶正負號的整數，查詢運算式轉譯期間將會擲回 ArgumentException 例外狀況，如以下範例所示。 此範例會查詢 ID 為 48000 的訂單。
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
uint s = UInt32.Parse("48000");
IQueryable<SalesOrderDetail> query = from sale in context.SalesOrderDetails
where sale.SalesOrderID == s
select sale;
// NotSupportedException exception is thrown here.
try
{
foreach (SalesOrderDetail order in query)
Console.WriteLine("SalesOrderID: " + order.SalesOrderID);
}
catch (NotSupportedException ex)
{
Console.WriteLine("Exception: {0}", ex.Message);
}
}
Using context As New AdventureWorksEntities()
Dim saleId As UInteger = UInt32.Parse("48000")
Dim query = _
From sale In context.SalesOrderDetails _
Where sale.SalesOrderID = saleId _
Select sale
Try
' NotSupportedException exception is thrown here.
For Each order As SalesOrderDetail In query
Console.WriteLine("SalesOrderID: " & order.SalesOrderID)
Next
Catch ex As NotSupportedException
Console.WriteLine("Exception: " + ex.Message)
End Try
End Using
型別轉換錯誤
在 Visual Basic 中，使用 CByte 函式將屬性對應到值為 1 的 SQL Server 位元型別資料行時，將會擲回 SqlException 並且顯示「算術溢位錯誤」訊息。 以下範例會查詢 AdventureWorks 範例資料庫中的 Product.MakeFlag 資料行，並且在重複處理查詢結果時擲回例外狀況。
Using context As New AdventureWorksEntities()
Dim productsList = _
From product In context.Products _
Select CByte(product.MakeFlag)
' Throws an SqlException exception with a "Arithmetic overflow error
' for data type tinyint" message when a value of 1 is iterated over.
For Each makeFlag In productsList
Console.WriteLine(makeFlag)
Next
End Using
不支援參考非純量變數
不支援在查詢中參考非純量變數，例如實體。 執行這類查詢時，系統會擲回 NotSupportedException 例外狀況，並且顯示一則訊息，表示「無法建立 EntityType 型別的常數值。 在此僅支援基本型別 (『例如 Int32、String 和 Guid』)」。
注意
支援參考純量變數的集合。
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
Contact contact = context.Contacts.FirstOrDefault();
// Referencing a non-scalar closure in a query will
// throw an exception when the query is executed.
IQueryable<string> contacts = from c in context.Contacts
where c == contact
select c.LastName;
try
{
foreach (string name in contacts)
{
Console.WriteLine("Name: ", name);
}
}
catch (NotSupportedException ex)
{
Console.WriteLine(ex.Message);
}
}
Using context As New AdventureWorksEntities()
Dim contact As Contact = context.Contacts.FirstOrDefault()
' Referencing a non-scalar closure in a query will
' throw an exception when the query is executed.
Dim contacts = From c In context.Contacts _
Where c.Equals(contact) _
Select c.LastName
Try
For Each name As String In contacts
Console.WriteLine("Name: ", name)
Next
Catch ex As Exception
Console.WriteLine(ex.Message)
End Try
End Using
使用 SQL Server 2000 的巢狀查詢可能失敗
使用 SQL Server 2000 時，如果 LINQ to Entities 查詢產生三層或更多層深度的巢狀 Transact-SQL 查詢，則查詢可能會失敗。
投影至匿名型別
如果您透過在 Include 上使用 ObjectQuery<T> 方法，將初始查詢路徑定義為包括相關物件，然後使用 LINQ 將傳回的物件投影至匿名型別，則包括方法中所指定的物件不會包括在查詢結果中。
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
var resultWithoutRelatedObjects =
context.Contacts.Include("SalesOrderHeaders").Select(c => new { c }).FirstOrDefault();
if (resultWithoutRelatedObjects.c.SalesOrderHeaders.Count == 0)
{
Console.WriteLine("No orders are included.");
}
}
Using context As New AdventureWorksEntities()
Dim resultWithoutRelatedObjects = context.Contacts. _
Include("SalesOrderHeaders"). _
Select(Function(c) New With {c}).FirstOrDefault()
If resultWithoutRelatedObjects.c.SalesOrderHeaders.Count = 0 Then
Console.WriteLine("No orders are included.")
End If
End Using
若要取得相關物件，請不要將傳回的型別投影至匿名型別。
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
var resultWithRelatedObjects =
context.Contacts.Include("SalesOrderHeaders").Select(c => c).FirstOrDefault();
if (resultWithRelatedObjects.SalesOrderHeaders.Count != 0)
{
Console.WriteLine("Orders are included.");
}
}
Using context As New AdventureWorksEntities()
Dim resultWithRelatedObjects = context.Contacts. _
Include("SalesOrderHeaders"). _
Select(Function(c) c).FirstOrDefault()
If resultWithRelatedObjects.SalesOrderHeaders.Count <> 0 Then
Console.WriteLine("Orders are included.")
End If
End Using