ASP.Net Entity Framework Repository & Linq

Question

My scenario:

This is an ASP.NET 4.0 web app programmed via C#

I implement a repository pattern. My repositorys all share the same ObjectContext, which is stored in httpContext.Items. Each repository creates a new ObjectSet of type E. Heres some code from my repository:

public class Repository<E> : IRepository<E>, IDisposable
    where E : class
{
    private DataModelContainer _context = ContextHelper<DataModelContainer>.GetCurrentContext();
    private IObjectSet<E> _objectSet;
    private IObjectSet<E> objectSet
    {
        get
        {
            if (_objectSet == null)
            {
                _objectSet = this._context.CreateObjectSet<E>();
            }
            return _objectSet;
        }
    }

    public IQueryable<E> GetQuery()
    {
        return objectSet;
    }

Lets say I have 2 repositorys, 1 for states and 1 for countrys and want to create a linq query against both. Note that I use POCO classes with the entity framework. State and Country are 2 of these POCO classes.

Repository stateRepo = new Repository<State>();
Repository countryRepo = new Repository<Country>();

IEnumerable<State> states = (from s in _stateRepo.GetQuery()
                             join c in _countryRepo.GetQuery() on s.countryID equals c.countryID
                             select s).ToList();
Debug.WriteLine(states.First().Country.country)

essentially, I want to retrieve the state and the related country entity. The query only returns the state data... and I get a null argument exception on the Debug.WriteLine

LazyLoading is disabled in my .edmx... thats the way I want it.

Answer 1

Your repository implementation is very similar to mine, especially the way you are storing the ObjectContext. It works fine for me, so I don't think it's a conceptual problem.

Try using a static objectcontext (no wrapper) just to see if that fixes the problem. Perhaps there is a bug in your ContextHelper which causes your context to get disposed and recreated.

Answer 2

You're doing a join without retrieving anything from it. There are multiple solutions to your problem:

• Use Include to load the dependent entities: from s in ((ObjectSet<State>) _stateRepo.GetQuery).Include("Country"). The problem with this approach is that you should expose the ObjectSet directly rather than as a IQueryable if you want to avoid casting.
• Use context.LoadProperty(states.First(), s => s.Country) to explicitly load the Country from the database for a given state.
• Select both entities in the query: from s in ... join c ... select new { s, c }. You won't be able to access directly the state's Country property but you have it in the anonymous type.
• Enable lazy loading.