MEF + WPF + MVVM: Where should AggregateCatalog reside?

Question

I'm taking my first crack at using MEF, but I can't seem to determine where I should create my AggregateCatalog and CompositionContainer. I have found many examples showing what the code should be:

var moduleCatalog = new AggregateCatalog(
    new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()), 
    new DirectoryCatalog(moduleDir));
var container = new CompositionContainer(moduleCatalog);
container.ComposeParts(this);

But now I just need to know where to put it. Should it go:

1. in App.xaml.cs
2. in MainWindow.xaml
3. or in one of my ViewModels.

Edit: Should have googled just a little longer, I think I found what I was looking for.

Hosting MEF within application and libraries.

Answer 1

After reading this, Hosting MEF within application and libraries, I decided that the composition of MEF can go in App.xaml.cs.

public partial class App : Application
{
    [Import]
    public Window TheMainWindow { get; set; }

    protected override void OnStartup(StartupEventArgs e)
    {
        var moduleCatalog = new AggregateCatalog(
            new AssemblyCatalog(typeof(App).Assembly), 
            new DirectoryCatalog(moduleDir));
        var container = new CompositionContainer(moduleCatalog);
        container.ComposeParts(this);
        base.MainWindow = TheMainWindow;
        TheMainWindow.Show();
    }
}

A few things to note:

• StartupUri="MainWindow.xaml" should be removed from App.xaml
• You will need to create a property for your main window, import it, and set base.MainWindow to it.