NHibernate IList to List

Question

Hi I am trying to return a collection of building domain.

private long _id;
private string _buildingName;
private IList<BasicRoom> _rooms;

I am using NHibernate and this mapping for the rooms

<bag name="rooms" table="tb_rooms" lazy="false">
<key column="buildingID"/>
<one-to-many class="Room"/>
</bag>

And I am calling the db like this;

Buildings = (List<Building>)session.CreateCriteria(typeof(Building)).AddOrder(Order.Asc("buildingName")).List<Building>();

The problem is I don't want the _rooms collection to be an IList but I need it to be a List. Alas, NHibernate requires that I use an Interface for collections. Any idea how to do this. I am new to .Net and thinking that maybe dropping NHibernate might be the way forward. I just need to get the collection typed as an List so I can move on. Any help much appreciated.

Answer 1

Why do you need a concrete List object, when you can do everything you want with an IList?

If it absolutely has to be a concrete list, you have two options.

First, simply cast the IList<Building> to List<Building>. This will only work if the underlying type is a List<Building>, which I personally doubt.

Second, call the ToList() extension method on it:

Buildings = (List<Building>)session.CreateCriteria(typeof(Building)).AddOrder(Order.Asc("buildingName")).List<Building>().ToList();

I personally recommend that you do neither and use the IList instead.

Answer 2

How about using the constructor of List<T> that takes an IEnumerable<T>? Then you can use:

Buildings = new List<Building>(session.CreateCriteria(typeof(Building)).AddOrder(Order.Asc("buildingName")).List<Building>());

Answer 3

That's not how NHibernate works. You don't get to tell it what the concrete types of your domain model objects are. You only get to tell it what the base types and interfaces are. Then it gets to choose how to implement the domain types efficiently.

For example, NHibernate uses its own internal implementation of IList<T> that knows how to do lazy loading.

Moreover, NHibernate uses its own internal implementation of Building from your domain model. It is not necessarily the case that all the Building objects you will be using with NHibernate will have, as their concrete type, Building. It is only true that they will be Buildings, whether Building is the concrete type of your object, or whether it is the base type of the concrete type of your object.

But this is the right thing to do, since you are supposed to be using interfaces and base types instead of concrete types anyway. NHibernate helps you use all the right approaches to programming.

Now, why do you need a List<Building> per se? Why will IList<Building> not suffice?

Answer 4

One reason would be if Silverlight is the Client.