Question

I’m having an issue selecting data in my data context in Entity Framework and I’ve narrowed it down to querying for null values. I have a method like this:

public void DoStuff(int? someInt)
{
    var someData = dataContext.MyEntities.Where(x => x.SomeProperty == someInt);
    // someData always yields no results if someInt is null, even though
    // there are rows in the table that have null for that column.
}

The above method fails if someInt is null. But this line works:

var someData = dataContext.MyEntities.Where(x => x.SomeProperty == null);

Why do I get data in the second one but not the first one?

Answer 1

I guess, then, that it is generating and using a SQL query of the form that expects a non-null value:

where x.SomeProperty = @param

Instead of the SQL to show the c# null-equality semantic:

where x.SomeProperty is null

(the key point here being that in c#, null equals null; in ANSI-SQL, null neither equals null nor (confusingly) not-equals null – different syntax is need to test nulls)

I’ve seen LINQ-to-SQL do the same thing, and agree that it is counter-intuitive. The only suggestion I have is: test the candidate parameter for null yourself and do the constant/literal == null test instead. It would also probably be able to do the same by inspecting and expression tree and re-writing it, if you are into expression trees – but special-casing the null is simpler.