WPF中ListBox控件选择多个数据项

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <StackPanel>
            <TextBlock>在控件中一次选择多个数据项：</TextBlock>
            <ListBox SelectionMode="Multiple">
                <ListBoxItem>a</ListBoxItem>
                <ListBoxItem>b</ListBoxItem>
                <ListBoxItem>c</ListBoxItem>
                <ListBoxItem>d</ListBoxItem>
                <ListBoxItem>e</ListBoxItem>
            </ListBox>
            <TextBlock>在控件中使用Ctrl一次选择多个数据项：</TextBlock>
            <ListBox x:Name="lbx" SelectionMode="Extended">
                <ListBox.ContextMenu>
                    <ContextMenu>
                        <MenuItem Header="Show" Click="btnShowSelectItems"></MenuItem>
                    </ContextMenu>
                </ListBox.ContextMenu>
                <ListBoxItem>张三</ListBoxItem>
                <ListBoxItem>李四</ListBoxItem>
                <ListBoxItem>王五</ListBoxItem>
                <ListBoxItem>赵六</ListBoxItem>
                <ListBoxItem>孙七</ListBoxItem>
            </ListBox>
        </StackPanel>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void btnShowSelectItems(object sender, RoutedEventArgs e)
        {
            string msg=string.Empty;
            List<string> list = new List<string>();
            if (lbx.SelectedItems != null && lbx.SelectedItems.Count > 1)
            {
                //多选
                foreach (ListBoxItem lbi in lbx.SelectedItems)
                {
                    list.Add(lbi.Content.ToString());
                }
                msg = string.Join(",", list);
            }
            else if (lbx.SelectedItems != null && lbx.SelectedItems.Count == 1)
            {
                //单选
                msg = ((ListBoxItem)lbx.SelectedItem).Content.ToString();
            }
            else
            {
                //没有选中任何项
            }
            MessageBox.Show(msg);
        }
    }
}