Question

Why these two methods work differently:

    public List<Foo> GetFoos()
    {
        int? parentId = null;
        var l = _dataContext.Foos.Where(x => x.ParentElementId == parentId).ToList();
        return l;
    }

    public List<Foo> GetFoos()
    {
        var l = _dataContext.Foos.Where(x => x.ParentElementId == null).ToList();
        return l;
    }

The first one returns nothing. Second returns what was expected. Data comes from EF. ParentElementId is nullable.

Answer 1

That is because you can’t compare to null in SQL, it has the special IS NULL operator to check for null values.

The first query will be translated into a comparison, where the parameter is null:

WHERE ParentElementId = @param

This doesn’t work, because comparing two null values doesn’t yield true.

The second query will be translated into a null check, because the null value is a constant:

WHERE ParentElementId IS NULL

This works because EF is not fooled to translate it into a comparison.