public void add_log4net_with_factory_adds_provider_to_the_factory() { // Arrange ILoggerProvider provider = null; var factory = Substitute.For <ILoggerFactory>(); factory.When(x => x.AddProvider(Arg.Any <ILoggerProvider>())).Do(y => provider = y.Arg <ILoggerProvider>()); // Act Log4NetFactoryExtensions.AddLog4Net(factory, "somefile"); // Assert provider.Should().BeOfType <Log4NetProvider>(); }