MEF

[Peter O'Hanlon]

The more I use MEF, the more I love it. So, I've got this code in my App.xaml file:

 

protected override void OnStartup(StartupEventArgs e)
{
  base.OnStartup(e);
  AggregateCatalog catalog = new AggregateCatalog();
  catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
  string path = System.IO.Path.Combine(NativeMethods.StartupPath(), "Plugins");
  catalog.Catalogs.Add(new DirectoryCatalog(path));

  _container = new CompositionContainer(catalog);
  CompositionBatch batch = new CompositionBatch();
  // We've put an Import on the MainWindow so that we can add this to the composition batch.
  batch.AddPart(this);
  _container.Compose(batch);
  MainWindow.Show();
}

Now, all I do is drop in code into the Plugins directory and it just works. Really, that's what I want the tagline for MEF to be - It Just Works. Thanks guys, I appreciate the pain you've taken away. Now Goldlight quite happily moves task dialog and KTM code out from the main library into a services library.

--
Peter O'Hanlon

[Josh Smith]

I agree.  MEF is great.  It's non-intrusive and simple.  That's all I care about, and oh yeah, it works too ;)

[Laurent Bugnion]

Me too, very seduced by MEF. I used it in a prototype to load modules by dropping them in a folder, and also by loading XAPs with skins within, to offer multiple skins to the user.

Note that in Preview 8 for SL, you don't need to create the catalogs yourself, instead you can use PartInitializer and PackageService to make it even easier. I can pass you a sample if you want (need to blog about it, but with the book going on...)

In my prototype i used a DuplexService to notify the SL app when a new package is available in the server folder, so the SL app can initiate the download without the user having to do anything ;)

Cheers,
Laurent
--
Sent from mobile

[Peter O'Hanlon]

Sounds cool.

--
Peter O'Hanlon

[Shawn Wildermuth]

I am using the RIA Services/Silverlight 4 Exemplar I am working on and even though I am doing everything in the XAP (not loading XAPs dynamically), the hooking up of the models to the VM's to the Views is pretty awesome.  For example:

 

  [PartCreationPolicy(CreationPolicy.Shared)] // For singleton

  [Export(typeof(IGamesModel))]

  public class GamesModel : IGamesModel

  {

 

// ...

 

  [Export(typeof(IGameEditorViewModel))]

  public class GameEditorViewModel : MyViewModelBase, IGameEditorViewModel

  {

    private IGamesModel _model;

 

    [ImportingConstructor]

    public GameEditorViewModel(IGamesModel theModel)

    {

      _model = theModel

 

// ...

 

  public partial class GameListView : UserControl

  {

    public GameListView()

    {

      InitializeComponent();

 

      // Use MEF To load the View Model

      PartInitializer.SatisfyImports(this);

 

    }

 

    [Import(typeof(IGameListViewModel))]

    public IGameListViewModel ViewModel

    {

      set

      {

        DataContext = value;

      }

    }

 

// ...

[Sacha Barber]

Yeah MEF rocks, way better than MAF (al be it they are doing slightly different things)

--
Sacha Barber
sacha....@gmail.com

[Glenn Block]

Hey Shawn, by default all parts will have a creation policy of any which translates to shared. You should only need to specify the policy if you want to change to non-shared. In the case of ViewModels I would imagine in most cases you want non-shared rather than shared. i.e. if you have an OrderVM you would expect to have multiple instances (multiple orders open at the same time).

Nice article btw, I am planning to comment on it.

Regards

Glenn

[Glenn Block]

Wheew, glad you are all liking it. We definitely had a goal of NOT doing something like MAF.

Glenn

[Glenn Block]

Peter you should even have to specify the path like that if it is relative to your bin....

You could jjust do...

var catalog = new DirectoryCatalog(".\Plugins") which will take the plugins folder off of your bin.

You also don't have to use the batch directly. Instead we've added a convenience extension method on the container called ComposeParts which lives in System.ComponentModel.Composition.

With it you can change the code above to just....

_container.ComposeParts(this) ;

thus consolidating the 3 lines of non-intuitive code to one. It accepts a params arrray as well so you can pass multiple parts.

Glenn

[Peter O'Hanlon]

Glenn

 

Cool. Thanks for that. The only real problem I have with MEF right now is that the documentation is a bit sparse, and light on best practice guidance. I guess that's natural at the moment, but it can make figuring things out a bit of a pain.

--
Peter O'Hanlon

[Laurent Bugnion, GalaSoft]

Who needs documentation when you can have Glenn’s personal MSN account (I will send you his contact info against a small payment ;))

 

Just kidding. I agree that lack of doc needs to be solved to see a wide adoption. Hope the team has some time for that soon J

 

Laurent

 

From: wpf-di...@googlegroups.com [mailto:wpf-di...@googlegroups.com] On Behalf Of Peter O'Hanlon
Sent: Sunday, December 20, 2009 3:47 PM
To: wpf-di...@googlegroups.com
Subject: Re: [WPF Disciples] MEF

 

Glenn

[Peter O'Hanlon]

I shall make payment when your new SL4 comes out. I have high hopes after your last book.

--
Peter O'Hanlon

[Sacha Barber]

You haz succeeded (LOL talk, I haz it, http://icanhascheezburger.com/)

 

MEF is cool....well done

--
Sacha Barber
sacha....@gmail.com

[Peter O'Hanlon]

U haz codz skillz...

--
Peter O'Hanlon

[Sacha Barber]

--
Sacha Barber
sacha....@gmail.com

[Glenn Block]

We know. We've got much better docs in the works which will shop in MSDN. On the topic of best practices, I am starting to cover some of that in my new series on my blog. It's focused on Silverlight, but many of the practices are the same.

[Glenn Block]

As long as I get a decent cut I am ok with that :-)

[Sacha Barber]

This is what I tend to do

 

 

public MainWindow()
{

    App.ComposeMEFContainer(this);
}

 

 

public static void ComposeMEFContainer(Object part)
{
    var directory = System.IO.Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
    var container = new CompositionContainer(new DirectoryCatalog(directory));
    var batch = new CompositionBatch();
    batch.AddPart(part);
    container.Compose(batch);
}

 

 

Glenn, how could you make this better, its the same as what Peter is trying to do I think, where we have  have a Plugins folder somewhere. Except for me this was just the bin folder of my app.

--
Sacha Barber
sacha....@gmail.com

[Glenn Block]

Add a "using System.ComponentModel.Composition" in the class.

 

public static void ComposeMEFContainer(Object part)
{

    var container = new CompositionContainer(new DirectoryCatalog(@".\"));
    container.ComposeParts(part);
}

 

Should work.....

Why use a static class? Are you calling it multiple times in the app?

[Sacha Barber]

Cool.

 

No it only gets called once actually, good point, can change from static. Cheers Glenn.

--
Sacha Barber
sacha....@gmail.com

[Glenn Block]

Yeah, a common pattern is to make the method parameterless (you might not even need a method for two lines :-) ), and just call container.ComposeParts(this)

[Glenn Block]

Cheers Sacha, off to bed.

[Corrado Cavalli]

Your blog series is a invaluable source of resources about MEF,
I've learned more from them than Codeplex wiki and MSDN, keep
going! ;-)

-Corrado

On Mon Dec 21 04:04:20 CST 2009, Glenn Block

[Sacha Barber]

NIght night.

--
Sacha Barber
sacha....@gmail.com