MEF DirectoryCatalog reads the same dll many times

Question

I had a simple implementation of MEF loading some dlls (plugins) from a directory. This was running well under MEF1 but now I want to use the same functionality with MEF2 and it gives me an IEnumerable that contains the right count of dlls that are in the directory but all the assemblies the same.

For example I have two assemblies: fakeplugin1.dll and fakeplugin2.dll in the directory. They exports FakePlugin1 and FakePlugin2 classes. Now when I call container.ComposeParts() I don't have anything in the list decorated with ImportMany and container.Catalog contains two assemblies in the directory but both of them are FakePlugin1.

Here's the code:

[ImportMany(typeof (IDCPlugin))]
        IEnumerable<IDCPlugin> workplaceControllers;
var catalog = new DirectoryCatalog(AppDomain.CurrentDomain.BaseDirectory);
var agcatalogue = new AggregateCatalog(catalog);
var container = new CompositionContainer(agcatalogue);
container.ComposeParts();

I am trying to use ExportFactory and RegistrationBuilder but I've just realized that even the base functionality donesn't work as expected.

What am I doing wrong? Has something changed in MEF2 I should know? How to load the two different assemblies? :)

Thanks for your help in advance!

Edit: It always creates two instances of the first type in the folder (ascending in abc). If I put an other one in the folder it creates three of the same, etc.

Edit: I have uploaded code to pastebin that gives the same result with MEF2: http://pastebin.com/3fWcujPS

Answer 1

A catalog will contain Import and Export definitions for anything detected. Regardless of if you actually need it.

This is a 'feature' of MEF. You will need to either ImportMany and selectively filter the plugins you require.

So how do you handle multiple plugins gracefully? Try this:

[Export]
public class PluginService
{
   public const string BEST_PLUGIN = "BestPlugin";

   [ImportMany]
   public IEnumerable<Plugin> Plugins{ private get; set; }

   [Export(BEST_PLUGIN)]
   public Plugin BestPlugin{ get { return GetBestPlugin(); } }

   Plugin GetBestPlugin()
   {
       return Plugins.FirstOrDefault(); //or some other logic for selection
   }
}

If your plugins are resource intensive, you may want to consider Lazy initialization.

Lazy<T, TMetadata> is a type provided by MEF to hold indirect references to exports. Here, in addition to the exported object itself, you also get export metadata, or information that describes the exported object. Each Lazy<T, TMetadata> contains an IOperation object, representing an actual operation, and an IOperationData object, representing its metadata.

http://msdn.microsoft.com/en-us/library/dd460648.aspx#further_imports_and_importmany

MEF has strong rules on component cardinality (number of things) to ensure that there are never any surprises but this does mean you have to be careful with your deployment.