LINQ To Entities, null navigation properties

Question

Apologies if this is a duplicate or if I just need to RTFM, but I couldn't find a post that exactly answered my question. I feel I'm reinventing the wheel and there must be a better way. MVC4, Entity Framework 5. I'm using LINQ to project a list of entities from my domain into a list of view models, and encountering NULL reference exceptions, in situations where the navigation property points to a single related entity; for navigation properties which are collections a single level deep; then for those where there may be a navigation property that itself has a navigation property (i.e., maybe two levels deep).

It appears to work when I'm pointing to the actual database; I believe this is because my configuration has lazy loading turned on; but then when I write unit tests I'm getting the NULL reference exceptions. I've got a solution but there's a bit of a code smell. I'd like to find a cleaner solution that works in both situations.

Please consider the following code:

var theList = DomainContext.MyParentEntities.Select(g => new SomeViewModelForAList {

        SomeStringProperty = g.SingleRelatedEntity.Name,
        SomeIntProperty = 
            g.EntitiesFromSomeOtherTable.Count(x => !x.IsDeleted),
        SomeOtherIntProperty =
            g.YetAnotherRelatedEntity.YetMoreChildren.Count(x => !x.IsDeleted)

}).ToList();

Now, it's possible that any of the following entities related to MyParentEntities instances could be NULL: "SingleRelatedEntity", "EntitiesFromSomeOtherTable", "YetAnotherRelatedEntity", and/or "YetMoreChildren". Any of those could throw an exception.

I can write all sorts of NULL coalescing code, which works, but appears redundant and ugly. I've also looked at writing extension methods as described here (although Jimmy's post looks a bit old and maybe there's a better way now):

http://lostechies.com/jimmybogard/2009/12/08/linq-query-operators-and-null-lists/

The extension method idea works, and the technique can be adapted to single related entities using generics; here are my attempts to do that:

    public static IEnumerable<TSource> NullsToEmpty<TSource>(this IEnumerable<TSource> source)
    {
        return source ?? Enumerable.Empty<TSource>();
    }

    // same idea here

    public static T NullToEmpty<T>(this T theEntity) where T : EntityBase, new()
    {
        var retVal = new T();
        if (theEntity == null)
        {
            retVal.Name = "";
        }
        else
        {
            retVal = theEntity;    
        }

        return retVal;
    }

The code then becomes:

var theList = DomainContext.MyParentEntities.Select(g => new SomeViewModelForAList {

        SomeStringProperty = g.SingleRelatedEntity.NullToEmpty().Name,
        SomeIntProperty = 
            g.EntitiesFromSomeOtherTable.NullsToEmpty().Count(x => !x.IsDeleted),
        SomeOtherIntProperty =
            g.YetAnotherRelatedEntity.NullToEmpty().
            YetMoreChildren.NullsToEmpty().Count(x => !x.IsDeleted)

}).ToList();

Again this works but also feels like it's ugly and repetitive. Any suggestions appreciated. Thank you, -dB

score 0 · Accepted Answer · edited May 23 '17 at 10:26

I've made a few additional observations. There are a number of issues:

you can't easily unit test Entity Framework unless you do integration and/or end-to-end tests, as described by Ladislov here and elsewhere:

Is UnitOfWork and GenericRepository Pattern redundant In EF 4.1 code first?

it's a matter of getting the Linq query right such that there aren't round trips to the database, avoiding N + 1 issues, grabbing the entire object graph etc (i.e. integration and unit tests are needed to make sure you've got it right
(in my case at least), the NULLs are not an issue in production as EF translates to SQL which checks IF EXISTS; thus I don't need "DefaultIfEmpty()" as described for example here:

http://www.progware.org/Blog/post/Left-Outer-Join-in-LINQ-to-Entities-%28for-Entity-Framework-4%29.aspx

in my unit tests I simulate EF's behaviour by returning (for empty sets) a new empty Collection (so avoid the NullToEmpty technique, which LINQ to Entities can't interpret anyway)

LINQ To Entities, null navigation properties

1 Answers1