MEF in MVVM

I read a post about MEF vs. IOC which cleared the confusion: MEF should be used external of app, while IOC should be used internally.

Good point. I have been struggling with how to separate my view and view model correctly in WPF app for a long time.   In fact Bil Simer already suggested that MEF is perfect fit in MVVM/Presentation Model last year during the Edmonton Code Camp 2008.

Ideally, all view models can be compiled into a separated assembly and put into specific folder to be imported by UI views!

Some code changes during my code conversion include:

  • Remove ctor args from view model, I haven’t figure out how to feed ctor args when MEF importing. This is not easy as auto-wiring in IOC. I ended up with a separate initialize method in each view model, which is not too bad, because I have to call wireUpViewModel from view anyway, this Initialize method has a perfect place to put.
  • I’m still not sure where should I put Container.Compose() method, because this method needs the instance the object to be imported into, I think compose as needed is better for WPF app.
  • Shared mode is the default behavior for export and import, it messed up my event handling code, (true, I didn’t do -= before +=). I had to explicitly add this everywhere.
  • AllowRecomposition is a pain for me, couldn’t get it work at all. I have tried many different ways to create a second instance of the exported class. PartCreationPolicy? AllowRecomposition attribute? Maybe because I’m working on a beta version?   Recomposition doesn’t work in contrustor paramteters, see Glen’s post.
  • Too many attributes kinds of pollute my code. Adding those export/import attributes just like playing ping-pong. If you are not good at it, you become dizzy very quickly.
    [Export]
    [PartCreationPolicy(CreationPolicy.NonShared)]
    
    [Import(typeof(NewRequestTypeViewModel), RequiredCreationPolicy = CreationPolicy.NonShared)]
    
    [ImportingConstructor]
    
    [Import(AllowRecomposition=true)] //this can apply directly on ctor parameters, but recompositorion won't work in ctor (for now)
  • I still feel using StructureMap to hookup viewmodel inside view is better and more comfortable. Even this violates some principals,  but in shell to open the view is much simpler, just new view(). ViewModel should sit behind the view, those UI developers/designers shouldn’t be ware of the exist of any view modles.
public RequestDetailsView(RequestDto requestDto)
{
  InitializeComponent();
  _viewModel = ObjectFactory.With("requestDto").EqualTo(requestDto).GetInstance<MyViewModel>();
  DataContext = _viewModel;
}
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