title: Problem with MEF import
body: <p>I have problem with import class into interfaces in my view model class with MEF.</p>
  
  <p>I view model I create property:</p>
  
  <pre><code>    [Import]
      private IEventAggregator EventAgg { get; set; }
  </code></pre>
  
  <p>and I want subcribe this event aggregator in constructor of view model:</p>
  
  <p>So I try this:</p>
  
  <pre><code>    public MessengerViewModel()
      {
          EventAgg.Subscribe(this);
      }
  </code></pre>
  
  <p>I run app but app freeze.</p>
  
  <p>So I try subscribe event aggregator in OnImportsSatisfied method:</p>
  
  <pre><code>    public void OnImportsSatisfied()
      {
          EventAgg.Subscribe(this);
      }
  </code></pre>
  
  <p>But this method is never calling when is view model class created. </p>
  
  <p>Only solution which work is:</p>
  
  <pre><code>    [ImportingConstructor]
      public MessengerViewModel(IEventAggregator eventAggregator)
      {
          EventAgg = eventAggregator;
          EventAgg.Subscribe(this);
      }
  </code></pre>
  
  <p>What is bad in first two ways?</p>