Import allways returns null, but ImportConstructor works

I am trying to use MEF in my application, but I have problem with the Import.

    [Import (typeof(IUserServices ))]
    public IUserServices UserService { get; private set; }

This does not work and UserService is always null.

But using the ImportContstructor in the same class works perfectly:

    [ImportingConstructor ]
    public MainWindowVM(
        IUIVisualizerService uiVisualizer,
        IViewAwareStatus viewAwareStatus,
        IMessageBoxService messageBoxService, 
        IMan开发者_开发技巧ager mwManager,
        TagItemModel tagModel,
        ILibraryModel  documentModel,
        ILibraryServices libraryServices,
        ILogServices logServices ,
        IUserServices userServices)

Can anybody help me in the issue. I already spend hours, but did not find any solution. Thanks!!!

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The property will only be set by MEF after the constructor is fully executed. When are you checking if the property is null?

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I'm using ChinchV2 together with MefedMVVM to create the container. Here the code, which provides the export:

[PartCreationPolicy(CreationPolicy.Shared)]
[Export (typeof(IUserServices ))]
public class TestUserServices:IUserServices 
{
    public void GetSettings(Action<HubSettings, Exception> callback)
    {
        var dPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().GetName().CodeBase).Remove(0, 6);
        callback(new HubSettings {DataPath = dPath}, null);
    }
}