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LINQ to 實體中的已知問題和考慮

本節提供 LINQ to Entities 查詢已知問題的相關信息。

無法快取的LINQ查詢

從 .NET Framework 4.5 開始，會自動快取 LINQ to Entities 查詢。 不過，將 Enumerable.Contains 運算子套用至記憶體集合的 LINQ to Entities 查詢不會自動快取。 此外，不允許在編譯的 LINQ 查詢中參數化記憶體內部集合。

訂單信息遺失

將欄位投影至匿名類型，會導致某些執行於設定了「80」相容性層級的 SQL Server 2005 資料庫的查詢中，排序資訊遺失。 當依順序列表的數據行名稱符合選取器中的數據行名稱時，就會發生這種情況，如下列範例所示：

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類型，因為 Entity Framework 不支援無符號整數。 如果您指定不帶正負號的整數，查詢表達式轉譯期間將會擲回 ArgumentException 例外狀況，如下列範例所示。 此範例會查詢標識碼為 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: {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

類型轉換錯誤

在 Visual Basic 中，當屬性使用 CByte 函式對應至值為 1 的 SQL Server 位類型數據行時，會擲回具有「算術溢位錯誤」訊息的 SqlException。 下列範例會查詢 AdventureWorks 範例資料庫中的 Product.MakeFlag 欄位，並在遍歷查詢結果時引發例外。

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

不支援參考非標量變數

不支援在查詢中參考非純量變數，例如實體。 當這類查詢執行時，會擲回 NotSupportedException 例外狀況，並顯示「無法建立類型的常數值 EntityType。 在此內容中只支援基本類型 （例如 Int32、String 和 Guid'）。

備註

支持參考純量變數集合。

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: ");
        }
    }
    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

巢狀查詢可能會因為 SQL Server 2000 而失敗

使用 SQL Server 2000 時，如果 LINQ to Entities 查詢產生巢狀 Transact-SQL 查詢，其深度為三個或多個層級，則可能會失敗。

投影至匿名類型

如果您在 Include 上使用 ObjectQuery<T> 方法定義包含相關物件的初始查詢路徑，然後使用 LINQ 將傳回的物件投影至匿名類型，則 include 方法中指定的物件不會包含在查詢結果中。

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，請勿將傳回的類型投影到匿名型別。

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

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