MEF and dynamic decision on instance creation - dependency-injection

Question

This might be a strange case but I want to sometimes reuse the same instance when getting exports with MEF and sometimes create a new.
Basicly I have a WCF service class the is instance per call. Each instance imports a RepositoryFactory which will also be new instance per service class. I return a Repository in the Factory and a repository gets a IDbContext injected.
I want each instance of the Factory to inject the same instance of IDbContext but have seperate instances between Factory instances.
So:
1) Factory1 is created
2) Factory1 creates Repository1-1 that gets IDbContext1 injected
3) Factory1 creates Repository1-2 that gets IDbContext1 injected
4) Factory2 is created
5) Factory2 creates Repository2-1 that gets IDbContext2 injected
6) Factory2 creates Repository2-2 that gets IDbContext2 injected
This should ensures that Repositories created from the same factory share a Unit of Work.
But being new to MEF I'm not sure how I would go about doing that.
EDIT
This is what I got:
public class RepositoryFactory
{
private readonly CompositionContainer _container;
[Import(RequiredCreationPolicy=CreationPolicy.NonShared)]
private readonly IDbContext _context;
public IRepository<T> CreateRepository<T>() where T : class, IEntity
{
//Somehow add the _context instance into the Repository import
return _container.GetExportedValue<EntityRepository<T>>();
}
}
and then
public class EntityRepository<T> : IRepository<T> where T : class, IEntity
{
// Perhaps a contract name might help!!
[Import(RequiredCreationPolicy=CreationPolicy.Shared)]
protected readonly IDbContext _context;

Answer 1

You cannot accomplish this with MEF; no matter what you do the MEF container will not act correctly as a Unit of Work manager for you, it just isn't made for this.
You should attempt to explicitly code a Unit of Work infrastructure for your DAL to consume. Your repositories should explicitly ask the a Unit of Work Manager to provide a current Unit of Work and with it the appropriate context.
Take a look at the code in NCommon https://github.com/riteshrao/ncommon; you can refactor the Unit of Work features to serve your needs.

Answer 2

OK here is a solution I came up with but haven't tried. It's somewhat simple and actually works around MEF but doesn't really break it, at least not in my case.
Add to IRepository class the following method:
void SetContext(IDbContext context);
or better yet
IDbContext context { set; }
and in the factory:
public class RepositoryFactory
{
private readonly CompositionContainer _container;
[Import(RequiredCreationPolicy=CreationPolicy.NonShared)]
private readonly IDbContext _context;
public IRepository<T> CreateRepository<T>() where T : class, IEntity
{
IRepository<T> repo = _container.GetExportedValue<EntityRepository<T>>();
repo.context = _context;
return repo;
}
}
And the rest should be self explanatory:
public class EntityRepository<T> : IRepository<T> where T : class, IEntity
{
protected IDbContext _context;
IDbContext context
{
set { _context = value; }
}
public virtual IQueryable<T> GetQuery()
{
return _context.Set<T>();
}
public virtual T GetById(Guid id)
{
return _context.Set<T>().Find(id);
}
public virtual void SaveOrUpdate(T entity)
{
if (_context.Set<T>().Find(entity.Id) == null)
{
_context.Set<T>().Add(entity);
}
_context.SaveChanges();
}
public virtual void Delete(T entity)
{
_context.Set<T>().Remove(entity);
_context.SaveChanges();
}
}
If you are using it in the same way as I am I can't really see a problem with this implementation. The factory is responsible for creating the class so It can be responsible for setting the context. The CreationPolicy should ensure that each Factory gets it's own instance of DbContext that it then relegates to it's Repositories so they share a context.