private void CommandExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            //当触发CommandManager.PreviewExecuted事件时,需要确定准备执行的命令是否是我们所关心的,如果是,可创建CommandHistoryItem对象,并将
            //其添加到Undo堆栈中

            //注意两个潜在问题:
            //1.当单击工具栏按钮以在文本框上执行命令时,CommandManager.PreviewExecuted事件被引发了两次——一次是针对工具栏按钮,另一次是针对文本框
            //下面的代码通过忽略发送者是ICommandSource的命令,避免在Undo历史中重复条目
            //2.需要明确忽略不希望添加到Undo历史中的命令
            //例如ApplicationUndo命令,通过该命令可翻转上一步操作


            // Ignore menu button source.
            if (e.Source is ICommandSource)
            {
                return;
            }

            // Ignore the ApplicationUndo command.
            if (e.Command == MonitorCommands.ApplicationUndo)
            {
                return;
            }

            // Could filter for commands you want to add to the stack (for example, not selection events)

            TextBox txt = e.Source as TextBox;

            if (txt != null)
            {
                RoutedCommand cmd = (RoutedCommand)e.Command;

                CommandHistoryItem historyItem = new CommandHistoryItem(cmd.Name, txt, "Text", txt.Text);

                ListBoxItem item = new ListBoxItem();
                item.Content = historyItem;
                lstHistory.Items.Add(historyItem);
                //本例在ListBox控件中存储所有CommandHistoryItem对象。ListBox控件的DisplayMember属性被设置为Name,因而会显示每个条目的CommandHistoryItem.Name属性

                //CommandManager.InvalidateRequerySuggested();
            }
        }