MEF field imports not being resolved

Question

I have MEF/Prism 4 project for which I can resolve imports via the ImportingConstructor, but not via field imports in the same class.

In the sample code below, myDataService is correctly resolved in the constructor. But _myDataServiceFieldImport isn't resolved, despite the Import attribute. Same result whether it's a field or property.

Anything obvious I'm missing here?

[ModuleExport(typeof(TestModule))] 
public class TestModule : IModule
{
    private IMyDataService _myDataService;

    [Import]
    private IMyDataService _myDataServiceFieldImport;

    [ImportingConstructor]
    public TestModule(IMyDataService myDataService)
    {
        _myDataService = myDataService;
    }
}

[PartCreationPolicy(CreationPolicy.Shared)]
[Export(typeof(IMyDataService))]
public class MyDataService : IMyDataService 
{

}

Answer 1

Change the access modifier from private to public and check if that works.

Answer 2

Turns out it was just me being dumb - I was checking the property/field values in the constructor, whereas they're only ever going to be resolved once the constructor has completed.