MEF part unable to import Autofac autogenerated factory

Question

This is a (to me) pretty weird problem, because it was already running perfectly but went completely south after some unrelated changes.

I've got a Repository which imports in its constructor a list of IExtensions via Autofacs MEF integration. One of these extensions contains a backreference to the Repository as Lazy(Of IRepository) (lazy because of the circular reference that would occur).

But as soon as I try to use the repository, Autofac throws a ComponentNotRegisteredException with the message "The requested service 'ContractName=Assembly.IRepository()' has not been registered."

That is, however, not really correct, because when I break right after the container-build and explore the list of services, it's there - Exported() and with the correct ContractName.

I'd appreciate any help on this...
Michael

[Edit] Here's a thinned-out version of the code:

Repository

Public Class DocumentRepository
    Implements IDocumentRepository

    Private _extensions As IEnumerable(Of IRepositoryExtension)

    Public Sub New(ByVal extensions As IEnumerable(Of IRepositoryExtension))
        _extensions = extensions
    End Sub

    Public Sub AddDocument(ByVal document As Contracts.IDocument) Implements Contracts.IDocumentRepository.AddDocument
        For Each extension In _extensions
            extension.OnAdded(document.Id)
        Next
    End Sub
End Class

Plugin

<Export(GetType(IRepositoryExtension))>
<PartCreationPolicy(ComponentModel.Composition.CreationPolicy.Shared)>
Public Class PdfGenerator
    Implements IRepositoryExtension

    Private _repositoryFactory As Lazy(Of IDocumentRepository)
    
    Public Sub New(ByVal repositoryFactory As Lazy(Of IDocumentRepository))
        _repositoryFactory = repositoryFactory
    End Sub

    Public Sub CreatePdf(ByVal id As Guid) Implements Contracts.IRepositoryExtension.OnAdded
        Dim document = _repositoryFactory.Value.GetDocumentById(id)
    End Sub
End Class

Bootstrapper

Public Class EditorApplication
    Inherits System.Web.HttpApplication

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        Dim builder As New ContainerBuilder()

        Dim catalog1 As New TypeCatalog(GetType(DataRepositoryScheme))
        Dim catalog2 As New DirectoryCatalog(HttpContext.Current.Server.MapPath("/Plugins"))
        builder.RegisterComposablePartCatalog(New AggregateCatalog(catalog1, catalog2))

        builder.RegisterType(Of DocumentRepository).As(Of IDocumentRepository).SingleInstance().Exported(Function(x) x.As(Of IDocumentRepository)())

        AutofacServiceHostFactory.Container = builder.Build()
    End Sub
End Class

Answer 1

Ah immediately after I posted that last comment I think I figured it out:

The requested service 'ContractName=ConsoleApplication7.IDocumentRepository()'
has not been registered.

Note that there is a pair of parentheses after the contract name - this is because the contract is a function, i.e., this message was produced by the following constructor, which is slightly different from the one in your sample:

Public Sub New(ByVal repositoryFactory As Func(Of IDocumentRepository))
    _repositoryFactory = repositoryFactory
End Sub

Note the 'Func' in there. MEF, unlike Autofac, does not regard Func as a special type and so will not translate this into the same contract as for Lazy.

If you want to provide a Func to a MEF component, you need to export it as a Func from Autofac. This is a bit tricky:

builder.RegisterType(Of DocumentRepository).As(Of IDocumentRepository)

builder.Register(Function(c) c.Resolve(Of Func(Of IDocumentRepository))) _
    .As(New UniqueService()) _
    .Exported(Function(x) x.As(Of Func(Of IDocumentRepository))

You may need to play with the syntax a bit, my VB.NET is fairly shaky.

My guess is that there are stale binaries in your /Extensions directory that are interfering with debugging this.

Hope this is on the mark!

Nick