static int instCount = 0;

    public FakeAutocompleteRepository()
    {
        instCount++;
        ...
    }

[PartCreationPolicy(CreationPolicy.Shared)]
[Export(typeof(IAutocompleteRepository))]
[ExportMetadata("IsTesting", "True")]
class FakeAutocompleteRepository : IAutocompleteRepository
{ ... }

var catalog = new AggregateCatalog();
                catalog.Catalogs.Add(new AssemblyCatalog(System.Reflection.Assembly.GetExecutingAssembly()));
                catalog.Catalogs.Add(new DirectoryCatalog("."));
                var container = new CompositionContainer(catalog);
                var batch = new CompositionBatch();
                batch.AddPart(this);
                container.Compose(batch);


                if (null != ConfigurationSettings.AppSettings["IsTesting"] && bool.Parse(ConfigurationSettings.AppSettings["IsTesting"]))
                    repository = container.GetExports<IAutocompleteRepository>().Where(expDef => expDef.Metadata.Keys.Contains("IsTesting")).Single().GetExportedObject();

[Export("/MyViewModel")]
public class MyViewModel
{
    [ImportingConstructor]
    public MyViewModel(
        [Import("/Services/LoggingService")] ILoggingService l)
    {
        logger = l;
    }

    private ILoggingService logger { get; set; }

    /* ... */
}

MyViewModel target = new MyViewModel(new MockLoggingService());