AutofacCompositeExtensionsPrototype.cs

public static class AutofacCompositeExtensions {
    public static IRegistrationBuilder<TImplementer, ConcreteReflectionActivatorData, SingleRegistrationStyle> RegisterComposite<TImplementer>(this ContainerBuilder builder) {
        var registrationBuilder = RegistrationBuilder.ForType<TImplementer>();
        builder.RegisterCallback(registry => {
            registrationBuilder.OnPreparing(e => {
                foreach (var service in e.Component.Services.OfType<IServiceWithType>()) {
                    e.Parameters = e.Parameters.Concat(new[] {
                        new ResolvedParameter(
                            (p, _) => IsCollectionOf(p.ParameterType, service),
                            (_, c) => ResolveCollection(c, service, e.Component)
                        )
                    });
                }
            });

            var registration = registrationBuilder.CreateRegistration();
            registry.Register(registration);
            registry.Registered += (sender, e) => {
                foreach (var service in registration.Services) {
                    IComponentRegistration @default;
                    if (registry.TryGetRegistration(service, out @default) && @default != registration)
                        throw new InvalidOperationException($"Registration {e.ComponentRegistration.Activator.LimitType} has overriden the composite registration {registration.Activator.LimitType}. Please add PreserveExistingDefaults() to preserve the composite.");
                }
            };
        });

        return registrationBuilder;
    }

    private static bool IsCollectionOf(Type type, IServiceWithType service) {
        return service.ServiceType.MakeArrayType().IsAssignableTo(type);
    }

    private static object ResolveCollection(IComponentContext context, IServiceWithType service, IComponentRegistration exceptRegistration) {
        var otherRegistrations = context.ComponentRegistry.RegistrationsFor((Service)service).Where(r => r != exceptRegistration);

        var array = otherRegistrations.Select(r => context.ResolveComponent(r, NoParameters)).ToArray();
        var typed = Array.CreateInstance(service.ServiceType, array.Length);
        array.CopyTo(typed, 0);

        return typed;
    }
}