public void Second_Registration_Should_Override_The_First_One() { // Arrange var poorManContainer = new SimpleIocContainer(); poorManContainer.Register <IFileSystem, FileSystem>(); poorManContainer.RegisterSingleton <IFileSystem>(new MockFileSystem()); // Act var fileSystem = poorManContainer.GetInstance <IFileSystem>(); // Assert fileSystem.Should().BeOfType <MockFileSystem>(); }