Loading parts from unreferenced assembly

Question

Hi,

This may be a silly question to ask, but I'm new to MEF and struggling a bit on this issue. Hope to find some help here.

This is what I'm trying to accomplish:

• I have a main application (say a console app), in which I would like to consume extension assemblies that are not to be referenced by the main app.
• I have created a separate assembly containing interfaces only (e.g. ILogger, ICacheManager, IMessageSender,... You get the idea). The main app references this assembly.
• I have a third assembly, which references and implements those interfaces. The idea is that it should be possible to change e.g. the logger implementation (e.g. EntLib-based to log4net-based) without having to recompile the main app. (Recompilation should only be necessary if I add interfaces).

Now, in my console app, I start with the following code:

class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        p.Run();
    }

    private void Run()
    {
        Compose();
    }

    private void Compose()
    {

        var catalog = new AggregateCatalog(
            new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()),
            new DirectoryCatalog("Extensions", "Extension.dll")
            );

        var container = new CompositionContainer(catalog);
                       
        container.ComposeParts(this);

        // I'd expect to see my parts here.
    }
}

I would expect that I would find my exported part in the catalog/container after completing the Compose method. The catalog does find and load the Extensions library.

For some obscure reason (well, obscure to me), I find no parts.

Can anyone tell me why this is, and/or what I should do to accomplish my goal?

What I do find puzzling as well is that, if a add a reference to extension assembly, and add an AssemblyCatalog to my AggregateCatalog, it does work. Like so:

private void Compose()
{

    var catalog = new AggregateCatalog(
        new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()),
        new AssemblyCatalog(typeof(Extension.EMailSender).Assembly)
        );

    var container = new CompositionContainer(catalog);
                       
    container.ComposeParts(this);
}

I think I'm missing something really simple here, but I need a pointer.

I've posted my sample solution here: http://sdrv.ms/MSKI6f

Hope to find some help.

Thanks,

Fréderic

Answer 1

Was hoping I would have gotten some feedback by now... Anybody?

Answer 2

Catalog is like a source of extension components. You can explicitly say: "here is the assembly. take parts here", or you can say: "here is file system directory, search parts in assemblies there".

---

Developer

Answer 3

Not sure exactly what you're expecting here.

In order to use a IMessageSender, you're going to need a reference to the interfaces assembly.

I ran your sample with slightly modified Compose() method:

            var catalog = new AggregateCatalog(
                new DirectoryCatalog("Extensions", "Extension.dll")
                );

            var container = new CompositionContainer(catalog);
                       
            var sender = container.GetExportedValue<IMessageSender>();

And it works fine:  sender is an EmailSender.