Why does Entity Framework return null List<> instead of empty ones?

Question

I'm pretty new in the ASP .NET MVC world. Maybe, that's the reason I can't explain to myself the cause of what is, for me, an annoying problem.

I have one class with One-To-Many relashionship.

class MyClass{
    public List<OtherClass> otherClasses {get;set;}
}

When I'm persisting one instance of this class, I fill it's relationship with an empty List<>

MyClass myClass = new MyClass(){ otherClasses = new List<OtherClass>() }
context.myClass.Add(myClass);

The problem is that, when I try to retrieve that instance, and for any reason, I try to access that list, system gives me a Null Reference Exception...

My question is: why doesn't EF return empty lists instead of null ones? Especially in this case, that I'm persisting it with an empty list?

There's any way to avoid verifing if instances are null?

what is you entity framework class?

Ahmadali Shafiee
– Ahmadali Shafiee

2012-02-12 05:57:02 +00:00
Commented Feb 12, 2012 at 5:57 — Ahmadali Shafiee
– Ahmadali Shafiee, Commented Feb 12, 2012 at 5:57

Erik Funkenbusch · Accepted Answer · 2012-02-12 17:56:52Z

31

You should have your entity create those lists in the constructor. EF doesn't create dependent collections, and expects the entity to do so.

So, your case, you would make your entity like this:

class MyClass{ 
    public List<OtherClass> _otherClasses {get;set;} 

    public MyClass() {
        _otherClasses = new List<OtherClass>();
    }
}

edited Feb 12, 2012 at 17:56

answered Feb 12, 2012 at 7:01

Erik Funkenbusch

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8 Comments

igor.araujo Over a year ago

This definitely solves my problem! Thanks for your help! So the EF calls object constructor before loading it with the persisted info, don't it?

Erik Funkenbusch Over a year ago

No, EF doesn't "call" the constructor. C# does. Whenever an object is created, the constructor is called. Even when EF materializes objects from the db.

Alireza Over a year ago

@MystereMan you're totally right, but, the point is that sometimes this constructor call is somehow bypassed! For instance, in WCF binary deserialization

Erik Funkenbusch Over a year ago

@Alireza - If the constructor is not called in WCF, it's because WCF is likely not newing the object, but using another mechanism to materialize it. Serialization is a complex process and they often do things at a low level to accomplish it.

Deblaton Jean-Philippe Over a year ago

@ErikFunkenbusch I can't find any EF related documentation telling "EF expects the entity to creates dependant collections".

|

Eranga · Accepted Answer · 2012-02-12 06:27:02Z

17

Make the otherClasses collection virtual. This will enable EF to lazy load the collection.

class MyClass{
    public virtual List<OtherClass> otherClasses {get;set;}
}

Otherwise use eager loading with Include method.

context.myClass.Include(m => m.otherClasses).SingleOrDefault(m => m.Id == foo);

answered Feb 12, 2012 at 6:27

Eranga

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2 Comments

igor.araujo Over a year ago

Indeed, lazy loading resolves my problem... But I'm trying to avoid lazy loading because it was giving me Circular Reference Serialization Error when I was using JSON requests. Anyway, I'm glad for your help. Thanks!

Jess Over a year ago

For circular reference problems, you can use attributes like JsonIgnore to prevent a back-linking navigation property from being serialized.

Davin Tryon · Accepted Answer · 2012-02-12 10:11:20Z

0

So, if I understand correctly, you are adding an empty List<OtherClass> to the context and then trying to retrieve it.

I guess you have to think about how the context will track and query entities that are in its context. This is usually done by the Key of the entity. In your example, you have not given the entity a Key, therefore, the context has no handle on the entity.

Therefore, when you query, the context doesn't find an object and returns null.

If you want to initialize a new entity, I would recommend to give it at least a Key (usually the Id property), and then select by that key when you lookup later.

Hope this helps.

answered Feb 12, 2012 at 10:11

Davin Tryon

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1 Comment

igor.araujo Over a year ago

In fact, my real classes have all an ID attribute. It was only an example where I forgot to include the ID :P. Anyway, thanks for your answer!

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