public ProxyReadOnlyFreeRepository(IServiceProvider serviceProvider)
        {
            var factory = serviceProvider.GetService <IFreeRepositoryFactory <TEntity, TKey> >() ??
                          throw new NullReferenceException($"Cannot get a {nameof(IFreeRepositoryFactory<TEntity, TKey>)} instance.");

            _inner = factory.CreateReadOnlyFreeRepository();
        }