I am just learning MVVM with MEF and already see the benefits but I am a little confused about some implementation details. The app I am building has several Models that do the same with with different entities (WCF RIA Services exposing a Entity framework object) and I would like to avoid implementing a similar interface/model for each view I need and the following is what I have come up with though it currently doesn't work.

The common interface has a new completed event for each model that implements the base model, this was the easiest way I could implement a common class as the compiler did not like casting from a child to the base type.

The code as it currently sits compiles and runs but the is a null IModel being passed into the [ImportingConstructor] for the FaqViewModel class.

I have a common interface (simplified for posting) defined as follows, this should look familiar to those who have seen Shawn Wildermuth's RIAXboxGames sample.

public interface IModel
{
    void GetItemsAsync();
    event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
}

A base method that implements the interface

public class ModelBase : IModel
{
    public virtual void GetItemsAsync() { }
    public virtual event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
    protected void PerformQuery<T>(EntityQuery<T> qry, EventHandler<EntityResultsArgs<T>> evt) where T : Entity
    {
        Context.Load(qry, r =>
        {
            if (evt == null) return;
            try
            {
                if (r.HasError)
                {
                    evt(this, new EntityResultsArgs<T>(r.Error));
                }
                else if (r.Entities.Count() > 0)
                {
                    evt(this, new EntityResultsArgs<T>(r.Entities));
                }
            }
            catch (Exception ex)
            {
                evt(this, new EntityResultsArgs<T>(ex));
            }
        }, null);
    }

    private DomainContext _domainContext;
    protected DomainContext Context
    {
        get
        {
            if (_domainContext == null)
            {
                _domainContext = new DomainContext();
                _domainContext.PropertyChanged += DomainContext_PropertyChanged;
            }

            return _domainContext;
        }
    }

    void DomainContext_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        switch (e.PropertyName)
        {
            case "IsLoading":
                AppMessages.IsBusyMessage.Send(_domainContext.IsLoading);
                break;
            case "IsSubmitting":
                AppMessages.IsBusyMessage.Send(_domainContext.IsSubmitting);
                break;
        }
    }
}

A model that implements the base model

[Export(ViewModelTypes.FaqViewModel, typeof(IModel))]
public class FaqModel : ModelBase
{
    public override void GetItemsAsync()
    {
        PerformQuery(Context.GetFaqsQuery(), GetFaqsComplete);
    }

    public override event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
}

A view model

[PartCreationPolicy(CreationPolicy.NonShared)]
[Export(ViewModelTypes.FaqViewModel)]
public class FaqViewModel : MyViewModelBase
{
    private readonly IModel _model;

    [ImportingConstructor]
    public FaqViewModel(IModel model)
    {
        _model = model;
        _model.GetFaqsComplete += Model_GetFaqsComplete;

        _model.GetItemsAsync(); // Load FAQS on creation
    }

    private IEnumerable<faq> _faqs;
    public IEnumerable<faq> Faqs
    {
        get { return _faqs; }
        private set
        {
            if (value == _faqs) return;

            _faqs = value;
            RaisePropertyChanged("Faqs");
        }
    }

    private faq _currentFaq;
    public faq CurrentFaq
    {
        get { return _currentFaq; }
        set
        {
            if (value == _currentFaq) return;

            _currentFaq = value;
            RaisePropertyChanged("CurrentFaq");
        }
    }

    public void GetFaqsAsync()
    {
        _model.GetItemsAsync();
    }

    void Model_GetFaqsComplete(object sender, EntityResultsArgs<faq> e)
    {
        if (e.Error != null)
        {
            ErrorMessage = e.Error.Message;
        }
        else
        {
            Faqs = e.Results;
        }
    }
}

And then finally the Silverlight view itself

public partial class FrequentlyAskedQuestions
{
    public FrequentlyAskedQuestions()
    {
        InitializeComponent();
        if (!ViewModelBase.IsInDesignModeStatic)
        {
            // Use MEF To load the View Model
            CompositionInitializer.SatisfyImports(this);
        }
    }

    [Import(ViewModelTypes.FaqViewModel)]
    public object ViewModel
    {
        set
        {
            DataContext = value;
        }
    }
}