Question

使用C＃Entity Framework v6.1.1，我试图在DbSet上进行Count操作：

//Items is a List<Item>
int count = db.MyObjects.Count(o => o.Items.FirstOrDefault(i => i.ItemID == 1) != default(Item));

我没有使用Contains，因为在使用Where Count等时存在已知的EF问题。

现在，上面的行抛出一个NullReferenceException，告诉我对象引用没有设置为object的实例。

将其更改为：

//Items is a List<Item>
int count = db.MyObjects.ToList().Count(o => o.Items.FirstOrDefault(i => i.ItemID == 1) != default(Item));

按预期工作。

现在，我的假设是DbSet作为代理类型工作，仅在请求时加载延迟对象，ToList()迫使其加载。

我担心这一切的表现。是否有更好的方法来计算DbSet？我是否真的强制要在所有地方进行ToList()通话？

我注意到DbSet 不是IEnumerable。

更新：我忘了提到我禁用了延迟加载，并且我在没有将预先加载应用到Items集合的情况下调用此代码，这可能解释了很多。

Answer 1

您似乎遇到了实现问题，但要回答您的标题问题，我认为

int count = db.MyObjects
    .Where(x => x.Items.Any(y => y.ItemID == 1))
    .Count();

应该

进行调查：

Folders.Where(x => x.Files.Any(y => y.IdPseudoCountry == 16)).Count()
Folders.Count(x => x.Files.Any(y => y.IdPseudoCountry == 16))

导致：

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM [dbo].[tableD] AS [Extent1]
    WHERE  EXISTS (SELECT 
        1 AS [C1]
        FROM [dbo].[tableF] AS [Extent2]
        WHERE ([Extent1].[idDossier] = [Extent2].[idDossier]) AND (16 = [Extent2].[idPays])
    )
)  AS [GroupBy1]

当

Folders.Count(x => x.Files.FirstOrDefault(y => y.IdPseudoCountry == 16) != default(File))

导致：

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM ( SELECT 
        [Extent1].[idDossier] AS [idDossier], 
        (SELECT TOP (1) 
            [Extent2].[idFichier] AS [idFichier]
            FROM [dbo].[tableF] AS [Extent2]
            WHERE ([Extent1].[idDossier] = [Extent2].[idDossier]) AND (16 = [Extent2].[idPays])) AS [C1]
        FROM [dbo].[tableD] AS [Extent1]
    )  AS [Project2]
    WHERE [Project2].[C1] IS NOT NULL
)  AS [GroupBy1]

但在我的情况下，我在任何情况下都不例外。

使用ToList（）

1 个答案: