Use MEF to dispose of dbcontext properly

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MEF

Dispose

C#6.0

I'm using MEF as the IoC container in WPF with Caliburn.Micro framework. The MEF container resides in the Bootstrapper. Bootstrapper has the method:

protected override void OnStartup(object sender, StartupEventArgs e)
{            
   DisplayRootViewFor<ViewModels.Main.MainViewModel>();
}

Now, the MainViewModel has this:

private IEnumerable<ExportFactory<Screen>> _screenList { get; set; }

[ImportingConstructor]
public MainViewModel(IEventAggregator eventAggregator, [ImportMany] IEnumerable<ExportFactory<Screen>> screenList)
{
    _screenList = screenList;
    eventAggregator = eventAggregator;
    _eventAggregator.Subscribe(this);        
}

The class basically has an IEnumerable of ExportFactory. A Screen is basically a ViewModel that inherits from VMBase which in turn inherits from Screen. Now, some of those ViewModel use a Unit of Work, which, in turn, has a DbContext injected into it via the constructor.

So basically, the dependency chain is like this:
DbContext -> UnitOfWork (IDisposable) -> ViewModelBase(IDisposable) -> ViewModels.
These ViewModel are instantiated using the ExportFactory in the MainViewModel.

The MainViewModel is subscribed to the eventAggregator, and Handler of MainViewModel calls the ExportFactory to create a new instance of a ViewModel as necessary. The problem is, when that ViewModel is closed, the DbContext is not properly disposed of.

This is the Handle method:

public void Handle(Type message)
{            
   DeactivateItem(ActiveItem, true);           
   ActivateItem(screenList.FirstOrDefault(c => c.CreateExport().Value.GetType() == message).CreateExport().Value);           
}

This creates new instances of DbContext, but doesn't call Dispose() of ViewModelBase ever, as I verified using breakpoints.

How can I properly dispose of the DbContext in my application?

What I have tried:

I tried changing the Handle method to call Dispose() like this:

DeactivateItem(ActiveItem, true);
_currentLifetimeContext?.Dispose();
_currentLifetimeContext = _screenList.FirstOrDefault(c => c.CreateExport().Value.GetType() == message).CreateExport();
ActivateItem(_currentLifetimeContext.Value);

but it throws an error "The operation cannot be completed because the DbContext has been disposed." the second time I try to create a ViewModel using the ExportFactory.
Once the DbContext has been disposed, the ExportFactory cannot create any instance of ViewModel that has the DbContext injected into its constructor anymore.

I want to dispose the DbContext every time a ViewModel is closed, and when a new ViewModel is activated, create a new instance of DbContext and inject it to the ViewModel as necessary.