使用ToList（）避免DbSet对象计数

Question

Using C# Entity Framework v6.1.1, I was trying to make a Count operation on a DbSet :

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

I did not use Contains, as there is a known EF issue on its use with Where Count etc.

Now, the above line throws an NullReferenceException, telling me that object reference is not set to an instance of object.

Changing it to:

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

Works as expected.

Now, my assumption is that DbSet is working as a type of a proxy, loading lazily objects only when requested, something the ToList() forces it to.

I am concerned though about the performance of all this. Is there a better way of making the count of a DbSet ? Am I really enforced to do the ToList() call everywhere?

I noticed that DbSet is not an IEnumerable .

UPDATE: I forgot to mention that I have Lazy Loading disabled, and that I am invoking this code without having applied Eager Loading to the Items collection, which probably explains a lot.

Answer 1

It seems you have a materialization problem but to answer your title question, I think that

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

should do

For investigation:

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

leads to:

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]

when

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

leads to:

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]

But in my case I have no exception in none case.