C#のWPFでCollectionViewを使ってリスト表示をソート

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="250" Width="250">
    <Grid Margin="10">
        <DataGrid ItemsSource="{Binding TestList}">
        </DataGrid>
    </Grid>
</Window>

    public partial class MainWindow : Window
    {
        public class TestItem
        {
            public string Item1 { get; set; }
            public string Item2 { get; set; }
            public string Item3 { get; set; }
        }


        public ObservableCollection<TestItem> TestList { get; set; }


        public MainWindow()
        {
            InitializeComponent();

            TestList = new ObservableCollection<TestItem>();
            TestList.Add(new TestItem() { Item1 = "q", Item2 = "7", Item3 = "い" });
            TestList.Add(new TestItem() { Item1 = "w", Item2 = "4", Item3 = "ろ" });
            TestList.Add(new TestItem() { Item1 = "e", Item2 = "1", Item3 = "は" });
            TestList.Add(new TestItem() { Item1 = "r", Item2 = "2", Item3 = "に" });
            TestList.Add(new TestItem() { Item1 = "t", Item2 = "3", Item3 = "ほ" });

            var cv = CollectionViewSource.GetDefaultView(TestList);
            cv.SortDescriptions.Clear();
            cv.SortDescriptions.Add(new SortDescription("Item2", ListSortDirection.Descending));

            DataContext = this;
        }
    }