title: Import allways returns null, but ImportConstructor works
body: <p>I am trying to use MEF in my application, but I have problem with the Import. </p>
  
  <pre><code>    [Import (typeof(IUserServices ))]
      public IUserServices UserService { get; private set; }
  </code></pre>
  
  <p>This does not work and UserService is always null.</p>
  
  <p>But using the ImportContstructor in the same class works perfectly:</p>
  
  <pre><code>    [ImportingConstructor ]
      public MainWindowVM(
          IUIVisualizerService uiVisualizer,
          IViewAwareStatus viewAwareStatus,
          IMessageBoxService messageBoxService, 
          IManager mwManager,
          TagItemModel tagModel,
          ILibraryModel  documentModel,
          ILibraryServices libraryServices,
          ILogServices logServices ,
          IUserServices userServices)
  </code></pre>
  
  <p>Can anybody help me in the issue. I already spend hours, but did not find any solution. Thanks!!!</p>