Multiple ViewModels request same domain object, should I give them the same/different instance(s)?

Question

newbie here, sorry if this is an obvious question, and sorry for my English. I tried to Google but didn't find an answer to my question.

I'm using Visual Studio, Autofac, EF CTP5 code only (maybe later will switch to HHibernate due to EF CTP5 does not support IoC container), and trying to use the following patterns: MVVM, DI, DDD.

My question is: When multiple ViewModels request the same domain-object from the repository, should I give them the same instance or create a new instance for each request?

If I create a new instance for each request, I guess I would write something like this:

public class PersonRepository
{
    public Person GetById(int id)
    {
        using (var dbContext = new MainDbContext())
        {
            return dbContext.Persons.Where(x => x.Id == id);
        }
    }
}

And after any ViewModle modifies its instance of the domain-object, I would have to find a way to notify all other ViweModels to update their instances, to ensure data integrity.

If I give the same instance of the domain-object for all requests, then I guess I would have to write something like this:

public class PersonRepository
{
    List<Person> _cache = new List<Person>();
    public Person GetById(int id)
    {
        var person = getFromCache(id);
        if (person == null)
        {
            person = getFromDatabase(id);
            _cache.Add(person);
        }
        return person;
    }
}

And I also need to find a way to remove instances from the cache when they are not needed by any ViewModels (or use weakreference for the cache), to avoid memory leak.

Which way should I go? Do I have better options?

Thanks!

Edit: Changed contents for better description.

(sorry for my English)

Update:

After I did some research on this, I realized this question is too confusing, so I posted a new question at here: Multiple ViewModels modify the same domain object

Answer 1

If you're thinking about multiple instances that arise because of multiple users, you likely want to instantiate one view model instance and get a handle on how to do this using your ORM and database, which involves understanding optimistic v pessimistic locking. Here's a link that has information on how EF supports this, although I personally use NHibernate. I would also recommend you prefer optimistic locking as well.

If you're talking about multiple views of the same domain object (ie, person) with a use case, you likely want different view models, although its possible to use multiple instances of the same view model. Either way, the synchronization here would not be the responsibility of the database. Here's a good link that explores various possibilities with a bias towards an event aggregator.

HTH,
Berryl

P.S. Just saw you were caching things in your repository. While this is an excellent consideration, your best off leaving cache management to your ORM (ie, EF, NHibernate), and then understanding how to fine tune it if and when necessary. Default behavior for NHib, for example, will cache intelligently and appropriately in typical scenarios.