MEF: How to manually configure Export for a contract implementation

问题

I have a MEF's CompositionContainer, a contract (say IFoo) and a module (Prism module but it doesn't matter much here, just some component). I want in my module to register the contract's implementation (FooImpl).

If it's Unity I'd do like this:

unity.RegisterType<IFoo, FooImpl>().

That's all. With MEF I've puzzled. I have to mark my implementation with ExportAttribute, but that lead to it'll be exported automatically. I want to manage this by myself. Take a look at the code:

class MyModule: IModule {
  private CompositionContainer m_container;
  public MyModule(CompositionContainer container) {
    m_container = container;
  }
  public void Initialize() {
    ??? I want something like m_container.CreateExport<IFoo, FooImpl>()
  }
}

public interface IFoo {}
public class FooImpl : IFoo {
    //[ImportingConstructor]
    public FooImpl(ISomeService svc) {}
}

In Initialize I want manualy export FooImpl as IFoo contract not relying on ExportAttribute on FooImpl class. I understand that I just can create an instance FooImpl (in MyModule.Initialize above), but FooImpl has constructor dependencies on other component/services and I want they to be resolved on creation.

So probably I should ask: how to manually add export having a CompositionContainer instance and a contract? And then mark this export somehow as it has ImportingConstructorAttribute?

回答1:

That's not how MEF works by default. However, you can create your own ExportProvider implementation which does this, and pass it to the CompositionContainer constructor.

The MEF contrib project has a few examples of export provider implementations. The Fluent Definition Provider is probably the closest to what you describe. There is also an export provider which provides integration with unity.

回答2:

Have a look at AttributedModelServices.ComposeExportedValue. You would use it like this:

m_container.ComposeExportedValue<IFoo>(new IFooImpl());

回答3:

The latest preview version of MEF has a class called RegistrationBuilder which you can use to configure your MEF parts via code. You can get the preview version from the MEF CodePlex site.

Here's a blog post with an overview of the new features.

来源：https://stackoverflow.com/questions/3493640/mef-how-to-manually-configure-export-for-a-contract-implementation

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