public void PropagateTest()
        {
            var testTreeItemViewModel = new TreeItemViewModel <string>("Root");

            testTreeItemViewModel.Children.Add(new TreeItemViewModel <string>("Child", testTreeItemViewModel));

            Assert.AreEqual(testTreeItemViewModel.Children.Count, 1);
            Assert.AreEqual(testTreeItemViewModel.Children[0].ParentItem, testTreeItemViewModel);

            testTreeItemViewModel.Propagate(t => t.IsExpanded = true);
            Assert.IsTrue(testTreeItemViewModel.IsExpanded);
            Assert.IsTrue(testTreeItemViewModel.Children[0].IsExpanded);
        }