Linq expression with nullable

Question

Looks like stupid question, but I just dont get it. My entity:

public class Page
{
    public int Id { get; set; }
    //...
    public int? ParentId { get; set; }
}

In controller:

db.Pages.First(x => x.ParentId == null);

Works as expected (returns some element). But:

int? test = null;
db.Pages.First(x => x.ParentId == test);

Throws Sequence contains no elements

What do I miss?

Are you using linq-to-entities? This has nothing specific to do with asp.net-mvc? — gideon
– gideon, Commented Feb 14, 2012 at 9:56

Jon Skeet · Accepted Answer · 2012-02-14 09:59:13Z

10

I believe there's an oddity around nulls with some LINQ providers. Try:

var query = db.Pages.First(x => (test != null && x.ParentId == test) ||
                                (test == null && x.ParentId == null));

Alternatively, use different queries for the different situations:

var query = test == null ? db.Pages.First(x => x.ParentId == null)
                         : db.Pages.First(x => x.ParentId == test);

Basically this is because SQL treats NULL as unequal to itself, so:

WHERE X = Y

will still fail if both X and Y are null values. Using the == null part (with a literal null) forces a conversion to ISNULL or whatever the SQL equivalent is.

I agree it's a pain, and someone else may have a better workaround, but this may help you get going.

answered Feb 14, 2012 at 9:59

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

gideon · Accepted Answer · 2012-02-14 10:01:03Z

2

You could do something like this as a workaround:

int? test = null;
if(test.HasValue) {
 db.Pages.First(x => x.ParentId == test.Value);
} else {
 db.Pages.First(x => x.ParentId == null);
}

I'm assuming since int? is actually a Nullable<int> our linq-to-entities provider isn't comparing things right.

answered Feb 14, 2012 at 10:01

gideon

19.5k11 gold badges74 silver badges114 bronze badges

Comments

Dennis Traub · Accepted Answer · 2012-02-14 10:04:54Z

1

Try this (modified according to gdoron's comment. It now is exactly what gideon posted, so please accept his instead of mine):

int? test = null;
if(test.HasValue) {
    db.Pages.First(x => x.ParentId == test.Value);
} else {
    db.Pages.First(x => x.ParentId == null);
}

edited Feb 14, 2012 at 10:04

answered Feb 14, 2012 at 9:59

Dennis Traub

51.9k7 gold badges100 silver badges113 bronze badges

2 Comments

gdoron Over a year ago

Won't work.

Value - The value of the current Nullable(Of T) object if the HasValue property is true. ==> An exception is thrown if the HasValue property is false. <==

Wonder Over a year ago

thx for this solution, but i just wanted to understand that oddity of LINQ

Collectives™ on Stack Overflow

Linq expression with nullable

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related