ActualItemService.cs

    [Export(typeof(IItemService))]
    [Export(typeof(IService))]
    public class ActualItemService : IItemService, IService { }

gistfile1.txt

    [Export(typeof(IItemProcessingService))]
    [Export(typeof(IService))]
    public class ItemProcessingService : IItemProcessingService, IService
    {
        [Import]
        privte IItemService itemService;
    }

MockItemService.cs

    [Export(typeof(IItemService))]
    [Export(typeof(IService))]
    public class MockItemService : IItemService, IService { }

ServiceManager.cs

public class ServiceManager
{
    [NotNull]
    public static ServiceManager Instance { get; } = new ServiceManager();

#pragma warning disable 649
    // ReSharper disable CollectionNeverUpdated.Local
    [ImportMany(typeof(IService))]
    private List<IService> importedServices;
    // ReSharper restore CollectionNeverUpdated.Local
#pragma warning restore 649

    private readonly Dictionary<Type, object> services;

    private ServiceManager()
    {
        Log.Information($"Assembling services.");
        var catalog = new AssemblyCatalog(typeof(ServiceManager).Assembly);
        var container = new CompositionContainer(catalog);
        container.SatisfyImportsOnce(this);
        foreach (var importedService in this.importedServices)
            Log.Information($"Found '{importedService.ServiceType.FullName}' service.");
        Log.Information($"Found {this.importedServices.Count} service(s).");
        this.services = this.importedServices.ToDictionary(e => e.ServiceType, e => (object)e);
    }

    public void Add([NotNull]Type type, [NotNull]object service) => this.services.Add(type ?? throw new ArgumentNullException(nameof(type)), service ?? throw new ArgumentNullException(nameof(service)));

    public void Add<T>([NotNull]object service) => this.services.Add(typeof(T), service);

    [NotNull]
    public T Get<T>() => (T)this.services[typeof(T)];

    [CanBeNull]
    public T GetOrDefault<T>()
    {
        if (this.services.TryGetValue(typeof(T), out var service))
            return (T)service;
        return default(T);
    }
}