LINQ to Entities 的已知问题和注意事项

项目
02/28/2011

本节提供有关 LINQ to Entities 查询的已知问题的信息。

排序信息丢失
不支持无符号整数
类型转换错误
不支持引用非标量变量
使用 SQL Server 2000，嵌套查询可能会失败
投影到匿名类型

排序信息丢失

如果将列投影到匿名类型，则会导致对兼容级别设置为“80”的 SQL Server 2005 数据库所执行的某些查询中会丢失排序信息。当 order-by 列表中的列名与选择器中的列名相同时，就会发生这种情况，如下面的示例所示：

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

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);

}

不支持无符号整数

由于实体框架不支持无符号整数，因此不支持在 LINQ to Entities 查询中指定无符号整数类型。如果指定无符号整数，则在查询表达式转换过程中会引发 ArgumentException 异常，如下面的示例所示。此示例查询其 ID 为 48000 的订单。

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

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);
    }
}

类型转换错误

在 Visual Basic 中，如果使用 CByte 函数将某一属性映射到值为 1 且为 SQL Server 位类型的列，则会引发 SqlException 并显示“发生算术溢出错误”消息。下面的示例查询 AdventureWorks 示例数据库中的 Product.MakeFlag 列，在循环访问查询结果时引发一个异常。

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

不支持引用非标量变量

不支持在查询中引用非标量变量（如实体）。在此类查询执行时，会引发 NotSupportedException 异常，并显示以下消息：“无法创建类型为‘EntityType’的常量值。此上下文中仅支持基元类型(‘如 Int32、String 和 Guid’)。”[Unable to create a constant value of type 'Closure type'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.]

注意：
支持引用标量变量的集合。

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

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);
    }
}

使用 SQL Server 2000，嵌套查询可能会失败

使用 SQL Server 2000，如果 LINQ to Entities 查询生成具有三级或以上深度的嵌套 Transact-SQL 查询，则它们可能会失败。

投影到匿名类型

如果通过使用 ObjectQuery 上的 Include 方法将初始查询路径定义为包括相关对象，然后使用 LINQ 将返回的对象投影到匿名类型，则查询结果中将不包含在 Include 方法中指定的对象。

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

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.");
    }
}

若要获取相关对象，请勿将返回的类型投影到匿名类型。

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

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.");
    }
}

另请参见

概念

LINQ to Entities

通过