MEFfing up a MVVM/CSLA example (Part 1)

MEF appears set to be the next big player in the .Net toolset. The next drop of PRISM looks like it will integrate MEF more, making the question of when to use an IoC and when to use MEF all the more unclear. A quick search with Google or StackOverflow returns many results but here is one that summarizes it quite well.

So it would look like in an MVVM scenario, the IoC approach is more appropriate. But I wanted to see the difference in action, in order to compare results. So lets see one possible way of building a View and associating a ViewModel and Model using MEF and how it fares against a possible IoC approach.

Requirements

For this test I set out the following requirements:

• Start building the chain of objects (parts) using View first.
• Use interfaces throughout.
• Make the view Blendable.

So the idea is that our main Window will import a UserControl, the UserControl will import the ViewModel and the ViewModel imports the model.

Setting everything up

First, lets create a simple CSLA readonly list to serve as out Model. Notice that what is exported is a factory delegate rather than the type itself (just one way of doing this), an idea inspired by a question asked at StackOverflow:



On the UI side, the contracts for the View and the ViewModel:



The implementation of the ViewModel:



A couple of things to note here. First, the way we load the model using the imported factory (similar idea used in a previous IoC example), asking MEF to use the constructor with the import by attributing it with ImportingConstructor. The second thing is the default constructor. If that was not in place, we would not have a fully blendable view. I will explain this more later but for now just remember that the Model will need to be loaded properly in the default constructor and this is done purely for design time convenience.

Where it starts to get a bit ugly.

At this point, we need to somehow import an instance of the ViewModel into our View (in XAML), which first needs to import the TelephoneList factory. It appears some functionality to facilitate and improve this process is in the pipeline for MEF, but for now the best approach I could come up with is to use a locator (idea inspired by this blog post). The locator will return a ViewModel instance depending on the mode (design or runtime) we are currently in. The full source of this class is:



This allows us to extend the locator class once per ViewModel (the ugly bit) but also let us import the instance at runtime or create a "dummy" instance at design time (the nice bit). Following from where we left the example, the implementation of an ITelephonesViewModel locator would look like this:



Where is PartInitializer?

Not sure if I mentioned this thus far, but the example was build in WPF (I am going completely SL when VS2010 and SL4 are properly released!). But the latest release of MEF for the desktop does not contain the PartInitializer class so what happened here?

It turns out that Glenn Block provided a desktop implementation. You can download the sample solution and include the required files (CompositionHost.cs and PartInitializer.cs) in your solution. Not sure if this will eventually be introduced in MEF but it works fine nevertheless.

And at this point I will leave you on a cliffhanger ending. The post is quite long so will break it in 2 parts. In Part 2 we will get a complete working example and compare it with the more "traditional" IoC approach.