title: MEF isn't loading my plugin
body: <p>I've implemented a very small plugin system in <code>C# with MEF</code>. But my plugins won't be loaded. In the <code>Aggregate-Catalog</code> I can see <code>my plugin listed</code>. But, after I'll compose these parts, there isn't my plugin in the plugin list, what I'm doing wrong?</p>
  
  <p>Here's a snippet of my code:</p>
  
  <p>Plugin-Loader:</p>
  
  <pre><code>    [ImportMany(typeof(IFetchService))]
      private IFetchService[] _pluginList;
      private AggregateCatalog _pluginCatalog;
      private const string pluginPathKey = "PluginPath";
      ...
  
      public PluginManager(ApplicationContext context)
      {
          var dirCatalog = new DirectoryCatalog(ConfigurationManager.AppSettings[pluginPathKey]);
          //Here's my plugin listed...
          _pluginCatalog = new AggregateCatalog(dirCatalog);
  
          var compositionContainer = new CompositionContainer(_pluginCatalog);
          compositionContainer.ComposeParts(this);
       }
       ...
  </code></pre>
  
  <p>And here, the plugin itself:</p>
  
  <pre><code>[Export(typeof(IFetchService))]
  public class MySamplePlugin : IFetchService
  {
      public MySamplePlugin()
      {
          Console.WriteLine("Plugin entered");
      }
      ...
  }
  </code></pre>