Null coalesce not working in LINQ query

问题

Take this:

int? item1 = null;
int? item2 = null;

someObjectList.Where(x => x.SomeItem1 == (item1 ?? x.SomeItem1)
                       && x.SomeItem2 == (item2 ?? x.SomeItem2) 
                    );

Where someObjectList is not empty and SomeItem1 and SomeItem2 is null in all the objects in the list.

Why is it returning nothing?

EDIT:

My Code:

public void GetPlacementsByMaterial(long clientMaterialID)
{
    ClientMaterial clientMaterial = ((ApplicationEntityModel)NavigationItem.ObjectContext).ClientMaterial.FirstOrDefault(x => x.ClientMaterialID == clientMaterialID);

    var list = GetPlacementList(supplier, mediaSpace);

    PlacementsList = list.Where(x => x.MediaCategoryFormatID == (clientMaterial.MediaCategoryFormatID ?? x.MediaCategoryFormatID)
                                                && x.MediaCategorySizeID == (clientMaterial.MediaCategorySizeID ?? x.MediaCategorySizeID) 
                             );
}

All ID's are Nullable<long>.

EDIT:

SQL Profiler:

SELECT *
  FROM [dbo].[CampaignSchedulePlacements] AS [Extent5]
WHERE ([Extent5].[MediaCategoryFormatID] = [Extent5].[MediaCategoryFormatID]) AND ([Extent5].[MediaCategorySizeID] = [Extent5].[MediaCategorySizeID])

Note: cleaned up the `SQL.

回答1:

In SQL, NULL is not equal to NULL.

You can interpret NULL as meaning: "there is value, but I don't know what it is". So if you're comparing two NULL values, you're really asking "is the first unknown value equal to the second unknown value?" Of course, there is no reason to assume they are, so SQL will say "no".

I am assuming that this is causing your problem. You can verify that by looking at the actual SQL produced. If it's using the SQL = operator, this is indeed the problem. You can verify that by running the SQL in a database tool, such as SQL Management Studio in case you're using SQL Server.

UPDATE:

The condition

([Extent5].[MediaCategoryFormatID] = [Extent5].[MediaCategoryFormatID]) 

will indeed return false when [Extent5].[MediaCategoryFormatID] is NULL.

That answers the question "Why is it returning nothing?"

However, another question come to mind: why would the entity framework generate that SQL from this linq query?

I'm afraid that linq to entities is not exactly known for the quality of its SQL generation, and this case seems to confirm that. You might consider Linq to SQL. Even if that seems to be a dead-end track in the long run, the current implementation if a lot better than linq to entities.

In either case, have you tried something like

someObjectList.Where(x => 
    !item1.hasValue || 
    x.SomeItem1.HasValue && x.SomeItem1.Value == item1.Value)

Make sure to verify that under the profiler as well though, linq to entities might mess it up too.

来源：https://stackoverflow.com/questions/11594275/null-coalesce-not-working-in-linq-query