An example for @grumpydev

Contrived - not actually *that* bad

    [TestFixture]
    public class When_the_selection_manager_has_its_search_results_updated
    {
        private FakeSearchService searchService;
        private SelectionManager selectionManager;

        [TestFixtureSetUp]
        public void ArrangeAndAct()
        {
            searchService = new FakeSearchService()
                                    {
                                        SelectableItemsResult = new[]
                                        {
                                            new SelectableItem()
                                                {Id = "One", Selected = false},
                                            new SelectableItem()
                                                {Id = "Two", Selected = false},
                                            new SelectableItem()
                                                {Id = "Three", Selected = false}
                                        };
                                    };

            selectionManager = new SelectionManager(searchService)
            {
                SelectableItems = new[]
                                        {
                                            new SelectableItem() {Id = "One", Selected = true},
                                            new SelectableItem() {Id = "Four", Selected = true},
                                            new SelectableItem() {Id = "Five", Selected = false}
                                        };
            };

            selectionManager.PerformAdditionalSearch("searchTerm");

        }

        [Test]
        public void Items_which_are_selected_in_both_lists_are_selected_in_the_merged_output()
        {
            var item = selectionManager.SelectableItems
                 .Where(x => x.Id == "One")
                 .Single();

            Assert.That(item.Selected);
        }

        [Test]
        public void Items_only_present_in_the_search_results_appear_unselected_in_the_merged_output()
        {
            var item = selectionManager.SelectableItems
                 .Where(x => x.Id == "Two")
                 .Single();

            Assert.That(item.Selected, Is.False);
        }

        [Test]
        public void Items_which_are_selected_in_the_current_list_without_being_present_in_the_returned_list_are_kept_selected_in_the_current_list()
        {
            var item = selectionManager.SelectableItems
             .Where(x => x.Id == "Four")
             .Single();

            Assert.That(item.Selected);
        }

        [Test]
        public void Items_which_are_not_selected_in_the_current_list_without_being_present_in_the_returned_list_are_removed_from_the_current_list()
        {
            var item = selectionManager.SelectableItems
              .Where(x => x.Id == "Five")
              .FirstOrDefault();

            Assert.That(item, Is.Null);
        }
    }

Contrived - z a tad better

   [TestFixture]
    public class When_the_selection_manager_has_its_search_results_updated2
    {
        private FakeSearchService searchService;
        private SelectionManager selectionManager;

        private const string ItemPresentInBothLists = "One";
        private const string ItemOnlyPresentInSearchResults = "Two";
        private const string AlreadySelectedItem = "Four";
        private const string UnselectedItem = "Five";

        [TestFixtureSetUp]
        public void ArrangeAndAct()
        {
            searchService = new FakeSearchServiceBuilder()
                .WithUnselectedItem(ItemPresentInBothLists)
                .WithUnselectedItem(ItemOnlyPresentInSearchResults)
                .Get();

            selectionManager = new SelectionManagerBuilder()
                .UsingSearchService(searchService)
                .WithSelectedItem(ItemPresentInBothLists)
                .WithSelectedItem(AlreadySelectedItem)
                .WithUnselectedItem(UnselectedItem)
                .Get();

            selectionManager.PerformAdditionalSearch("searchTerm");
        }

        [Test]
        public void Items_which_are_selected_in_both_lists_are_selected_in_the_merged_output()
        {
            Assert.That(selectionManager.HasSelectedItemWithId(ItemPresentInBothLists));
        }

        [Test]
        public void Items_only_present_in_the_search_results_appear_unselected_in_the_merged_output()
        {
            Assert.That(selectionManager.HasUnselectedItemWithId(ItemOnlyPresentInSearchResults));
        }

        [Test]
        public void Items_which_are_selected_in_the_current_list_without_being_present_in_the_returned_list_are_kept_selected_in_the_current_list()
        {
            Assert.That(selectionManager.HasSelectedItemWithId(AlreadySelectedItem));
        }

        [Test]
        public void Items_which_are_not_selected_in_the_current_list_without_being_present_in_the_returned_list_are_removed_from_the_current_list()
        {
            Assert.That(selectionManager.DoesNotHaveItemWithId(UnselectedItem));
        }
    }