title: Why is MEF failing on the part constructor?
body: <p>I am attempting to build a modular application using MEF.<br>
  However, while composing it seems to ignore <code>SetExportedValue(..., ...)</code>, resulting in it not finding the values/objects I need.</p>
  
  <p>Here is my (anonymized and simplified) code:</p>
  
  <pre><code>interface IPlugin {  }
  </code></pre>
  
  <hr>
  
  <pre><code>class MyClass
  {
      [ImportsMany(typeof(IPlugin))]
      public IEnumerable&lt;Lazy&lt;IPlugin&gt;&gt; Plugins;
  
      public LoadParts()
      {
          var ac = new AggregateCatalog();
          ac.Catalogs.Add(new AssemblyCatalog(typeof(PluginManager).Assembly));
  
          // imagine adding some DirectoryCatalogs here
  
          var cc = new CompositionContainer(ac);
          cc.ComposeExportedValue("MyValue", someinstance);
          cc.ComposeParts(this);
      }
  }
  </code></pre>
  
  <hr>
  
  <pre><code>[Export(typeof(IPlugin))]
  class MyPart : IPlugin
  {
      [ImportingConstructor]
      public MyPart([Import("MyValue")] sometype arg)
      { }
  }
  </code></pre>
  
  <p>This results in the following error message:</p>
  
  <blockquote>
    <p>System.ComponentModel.Composition Warning: 1 : The ComposablePartDefinition 'MyNamespace.MyPart' has been rejected. The composition remains unchanged. The changes were rejected because of the following error(s): The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.</p>
    
    <p>1) No exports were found that match the constraint: </p>
  
  <pre><code>ContractName  MyValue
  RequiredTypeIdentity  somenamespace.sometype
  </code></pre>
    
    <p>Resulting in: Cannot set import 'MyNamespace.MyPart..ctor (Parameter="...", ContractName="...")' on part 'MyNamespace.MyPart'.<br>
    Element: MyNamespace.MyPart..ctor (Parameter="...", ContractName="...") -->  MyNamespace.MyPart -->  DirectoryCatalog (Path="./")</p>
  </blockquote>
  
  <p>Am I misunderstanding how part constructors work?<br>
  How do I get it to accept the constructor parameter?</p>
  
  <hr>
  
  <p>Edit for claification: <code>someinstance</code> is a specific instance of <code>sometype</code>. I do not want to add a type to the container, but a specific class instance.</p>