public class TestFilter : ActionFilterAttribute
{
    // this is always null
    [Dependency]
    public IUserService UserService { get; set; }

    // other members
}

public static void Start()
{
    var container = UnityConfig.GetConfiguredContainer();

    FilterProviders.Providers.Remove(FilterProviders.Providers.OfType<FilterAttributeFilterProvider>().First());
    FilterProviders.Providers.Add(new UnityFilterAttributeFilterProvider(container));

    DependencyResolver.SetResolver(new UnityDependencyResolver(container));

    Microsoft.Web.Infrastructure.DynamicModuleHelper.DynamicModuleUtility.RegisterModule(typeof(UnityPerRequestHttpModule));
}