CommandBinding的问题

悬赏园豆：5 [待解决问题]

自定义了一个MyCanvas类，在其中注册了一个CommandBinding，其功能是自动添加一个TextBox控件到MyCanvas，

 1     public class MyCanvas : Canvas
 2     {
 3         public MyCanvas()
 4         {
 5             CommandManager.RegisterClassCommandBinding(typeof(MyCanvas), new CommandBinding(ApplicationCommands.New, MyExecuted));
 6         }
 7         
 8         private void MyExecuted(object sender, ExecutedRoutedEventArgs e)
 9         {
10             TextBox txt = new TextBox{ Width = 60, Height = 20 ,Text = "AAA"};
11             txt.Margin = new Thickness(10);
12             this.Children.Add(txt);
13        }
14 
15     }

在界面上，一个MyCanvas套着另一个MyCanvas，一个Button的命令目标指定为子MyCanvas，界面代码：

<Window>
    <local:MyCanvas x:Name="canvas1" Background="LightBlue" >
            <local:MyCanvas x:Name="canvas2" Background="Wheat" Width="200" Height="100" Margin="70" />
            <Button Name="btn2" Content="增加Canvas2子控件" Command="New" CommandTarget="{Binding ElementName=canvas2}"  Canvas.Left="246" Canvas.Top="228" />
    </local:MyCanvas>
</Window>