I am using the Managed Extensibility Framework preview 4 in VB.NET, and I have to say, that it rocks!

The project I am writing is a UI container that has “add-ins” that process files and such. I am working with another programmer to develop other add-ins for the app as well.

To query for specific metadata, we first have to create an interface and a custom attribute.

Public Interface IMefMetadataView

    ReadOnly Property Name() As String

End Interface

<MetadataAttribute()> _

Public Class MefMetadata

    Inherits Attribute

    Public Sub New()

    End Sub

    Public Sub New(ByVal name As String)

        _Name = name

    End Sub

    Private _Name As String = ""

    Public ReadOnly Property Name() As String

        Get

            Return _Name

        End Get

    End Property

End Class

This attribute allows you to specify a name of the add-in. Then we create an interface for our add-in.

    Public Interface IAddin

    End Interface

Then we can create an add-in or two.

	<Export(GetType(IAddin))> _

	<AddinMetadata("Add-in one")> _

	Public Class Addin1

	    Implements IAddin

	

	End Class

	

	<Export(GetType(IAddin))> _

	<AddinMetadata("Add-in two")> _

	Public Class Addin2

	    Implements IAddin

	

	End Class

Now, in our host user interface, we create a catalog and a container. Specifically we are creating an AggregateCatalog (a catalog of catalogs if you will) and then creating a catalog for just our assembly, then we are adding it into the container.

    Dim Container as CompositionContainer

    Dim Catalog As New AggregateCatalog()

    Dim AssemCatalog As New AssemblyCatalog(GetType(Module1).Assembly)

    Catalog.Catalogs.Add(AssemCatalog)

    Container = New CompositionContainer(Catalog)

Querying:

Get a list of addins:

    Dim Addins As Generic.List(Of Export(Of IAddin, IMefMetadataView)) = _

        Container.GetExports(Of IAddin, IMefMetadataView).ToList()

Query for a specific add-in with a name in the metadata using a lambda function:

Dim x As Export(Of IAddin, IMefMetadataView) = Addins.Where(Function(p) String.Compare(p.Metadata("Name").ToString(), "Add-in one", True) = 0).Single()

Or

Dim x As Export(Of IAddin, IMefMetadataView) = Addins.Where(Function(p) String.Compare(p.MetadataView.Name, "Add-in one", True) = 0).Single()

The box directly above allows us to query strongly typed metadata, thanks to the IMefMetadataView Interface and the MefMetadata class.