WPF + MEF communication between Modules

Hi,

I'm new to MEF and would like to do a software where the host needs to communicate properties to each modules. I created a Small project as a test. A HostApp, an Interface and a PlugIn. The HostApp shows the PlugIn and a Button. When I click on the button, it send a list of string to the PlugIn. The PlugIn contains a listbox and I want that ListBox to be refreshed and show the list of string. The problem I have, is that after sending the list to the PlugIn, the properties contains all the strings, but the ListBox do not update.

Interface

Namespace API
   Public Interface IPlugIn
      Property Files As System.Collections.ObjectModel.ObservableCollection(Of String)
      ReadOnly Property PlugInUI As System.Windows.Controls.UserControl       
   End Interface    
End Namespace

PlugIn

Imports System.ComponentModel.Composition <Export(GetType(Contract.API.IPlugIn))>
Public Class UserControl1    Implements Contract.API.IPlugIn, System.ComponentModel.INotifyPropertyChanged,  
System.Collections.Specialized.INotifyCollectionChanged

Private _files As System.Collections.ObjectModel.ObservableCollection(Of String)
Public Property Files As System.Collections.ObjectModel.ObservableCollection(Of String) Implements Contract.API.IPlugIn.Files
   Get
      If IsNothing(_files) Then                
         _files = New System.Collections.ObjectModel.ObservableCollection(Of String)
      End If
      Return _files
   End Get
   Set(value As System.Collections.ObjectModel.ObservableCollection(Of String))
      _files = value
      OnPropertyChanged("Files")
      OnCollectionChanged(New Collections.Specialized.NotifyCollectionChangedEventArgs _ 
      (Specialized.NotifyCollectionChangedAction.Reset))
   End Set
End Property

Public ReadOnly Property PlugInUI As System.Windows.Controls.UserControl Implements     Contract.API.IPlugIn.PlugInUI
   Get
      Dim myUI As New UserControl1
      Return myUI
   End Get
End Property
Public Event PropertyChanged(sender As Object, e As         System.ComponentModel.PropertyChangedEventArgs) Implements              System.ComponentModel.INotifyPropertyChanged.PropertyChanged
Public Sub OnPropertyChanged(Optional propertyName As String = Nothing)
   RaiseEvent PropertyChanged(Me, New           ComponentModel.PropertyChangedEventArgs(propertyName))
End Sub
Public Event CollectionChanged(sender As Object, e As   System.Collections.Specialized.NotifyCollectionChangedEventArgs) Implements       System.Collections.Specialized.INotifyCollectionChanged.CollectionChanged
Public Sub OnCollectionChanged(args As System.Collections.Specialized.NotifyCollectionChangedEventArgs)
   RaiseEvent CollectionChanged(Me, args)
End Sub
End Class

PlugIn XAML

<Grid><ListBox Height="248" HorizontalAlignment="Left"   
   Margin="30,31,0,0" Name="ListBox1"  
   VerticalAlignment="Top" Width="241" 
   ItemsSource="{Binding Files}"></Grid>

Host App

Imports System.ComponentModel.Composition
Imports System.ComponentModel.Composition.Hosting    

Public Class HostApp <ImportMany(GetType(Contract.API.IPlugIn))> _
Public Property PlugIns As List(Of Contract.API.IPlugIn)     
Private _container As CompositionContainer
   Public Sub Compose()
      Dim catalog As New AggregateCatalog         
      catalog.Catalogs.Add(New           
         DirectoryCatalog("pluginfolder"))      _container = New CompositionContainer(catalog)         _container.ComposeParts(Me)
End Sub
Public Sub LoadPlugIns()
   Compose()
   For Each item In PlugIns
      Desktop.Children.Add(item.PlugInUI)
   Next
End Sub
Private Sub HostApp_Loaded(sender As Object, e As System.Windows.RoutedEventArgs) Handles Me.Loaded      
   LoadPlugIns()
End Sub
Private Sub Button1_Click(sender As System.Object, e As System.Windows.RoutedEventArgs)     Handles Button1.Click
   Dim fileList As New 
      Collections.ObjectModel.ObservableCollection(Of    
        String)
      For Each fileItem As String In   
        IO.Directory.GetFiles("Somefolder")
         fileList.Add(fileItem)
      Next
      PlugIns.Item(0).Files = fileList
End Sub
End Class

I need the Files property to be listed in the PlugIn ListBox when I click the HostApp Button.

I continue trying many things and found something really weird.
I added a button on the HostApp that get the count of string in Files PlugIn and it returns the quantity of string correctly.
I also added a button on the PlugIn that get the count of string in Files and it returns 0!

Why does the PlugIn exists in "two different instance" one in the host and one plugin-only?

Thanks for your help!