WPF的一个小疑惑

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;   public class CheckTheWindowStyle : Window
    {
        MenuItem itemChecked;        [STAThread]
        public static void Main()
        {
            Application app = new Application();
            app.Run(new CheckTheWindowStyle());
        }
        public CheckTheWindowStyle()
        {
            Title = "Check the Window Style";            // Create DockPanel.
            DockPanel dock = new DockPanel();
            Content = dock;            // Create Menu docked at top.
            Menu menu = new Menu();
            dock.Children.Add(menu);
            DockPanel.SetDock(menu, Dock.Top);            // Create TextBlock filling the rest.
            TextBlock text = new TextBlock();
            text.Text = Title;
            text.FontSize = 32;
            text.TextAlignment = TextAlignment.Center;
            dock.Children.Add(text);            // Create MenuItem objects to change WindowStyle.
            MenuItem itemStyle = new MenuItem();
            itemStyle.Header = "_Style";
            menu.Items.Add(itemStyle);            itemStyle.Items.Add(
                CreateMenuItem("_No border or caption", WindowStyle.None));            itemStyle.Items.Add(
                CreateMenuItem("3_D-border window",WindowStyle.ThreeDBorderWindow));
            
            
            itemStyle.Items.Add(
                CreateMenuItem("_Single-border window",
                               WindowStyle.SingleBorderWindow));
                        itemStyle.Items.Add(
                CreateMenuItem("_Tool window",
                               WindowStyle.ToolWindow));       
        }
        MenuItem CreateMenuItem(string str, WindowStyle style)
        {
            MenuItem item = new MenuItem();
            item.Header = str;
            item.Tag = style;
            item.IsChecked = (style == WindowStyle);
            item.Click += StyleOnClick;            if (item.IsChecked)
                itemChecked = item;            return item;
        }
        void StyleOnClick(object sender, RoutedEventArgs args)
        {
            itemChecked.IsChecked = false;
            itemChecked = args.Source as MenuItem;
            itemChecked.IsChecked = true;            WindowStyle = (WindowStyle)itemChecked.Tag;
        }
    }运行出来以后为啥是默认的"Single-border window".
为神马不是其他三个中的一个呢？
itemStyle.Items.Add(
                CreateMenuItem("_No border or caption", WindowStyle.None));            itemStyle.Items.Add(
                CreateMenuItem("_Single-border window",
                               WindowStyle.SingleBorderWindow));            itemStyle.Items.Add(
                CreateMenuItem("3_D-border window",
                               WindowStyle.ThreeDBorderWindow));
            itemStyle.Items.Add(
                CreateMenuItem("_Tool window",
                               WindowStyle.ToolWindow));
为啥没WPF专区哈。。.