Question

I em using Linq to EF and trying to get FirstOrDefault entitiy from ObjectSet.
The query looks like this :

        Notification not = new Notification();
        ........
        //not.SubTypeID = null;

        var elem = ent.Notifications.FirstOrDefault(p =>                     
                    p.ID == not.ID &&
                    p.SubTypeID == not.SubTypeID &&
                    p.Location == not.Location &&
                    p.TypeID == ns.TypeID
            );

Sometimes SubTypeID can be null and in this case nothing is returnced in elem althogh p.SubTypeID and not.SubTypeID are both null.

But the strange thing is that when I run this query :

        var elem = ent.Notifications.FirstOrDefault(p =>                     
                    p.ID == not.ID &&
                    p.SubTypeID == null &&
                    p.Location == not.Location &&
                    p.TypeID == ns.TypeID
            );

everything works as expected and I see data in elem.

So what I am doing wrong and what is the difference between these 2 queries when not.SubTypeID is null.

Answer 1

In C#, the result of null == null is true. But in SQL, which this expression tree is getting compiled into, the result of NULL = NULL is NULL. Example on PostgreSQL:

SELECT 1 WHERE NULL = NULL;
 ?column?
----------
(0 rows)

You might consider using an expression like:

p.SubTypeID == not.SubTypeID ||
(p.SubTypeID == null && not.SubTypeID == null)

This is kind of a hack, and there may be a better solution than this. But the NULL = NULL test is likely the root of your problem.