如何在Windows Phone上打印List <grid>？</grid>

Question

我在Windows Phone项目中有两页：

MainPage.xaml中

    <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
                 <StackPanel x:Name="stp" Orientation="Horizontal"
 HorizontalAlignment="Left" Height="auto" VerticalAlignment="Top" Width="auto"/>
    </Grid>

MainPage.xaml.cs中

private void gotoAddPage(object sender, EventArgs e)
{
    NavigationService.Navigate(new Uri("/AddPage.xaml",
    UriKind.RelativeOrAbsolute));
}

protected override void OnNavigatedTo(NavigationEventArgs e)
{

    (App.Current as App).todoList.ForEach(delegate(Grid g)
    {
        stp.Children.Add(g);
    });

    base.OnNavigatedTo(e);
}

AddPage.xaml.cs

    private void OnAddButtonClick(object sender, RoutedEventArgs e)
    {
        RowDefinition rd = new RowDefinition();
        Grid gr = new Grid();
        gr.RowDefinitions.Add(rd);
        gr.ShowGridLines = true;

        TextBlock tb = new TextBlock();
        tb.Text = addTxtbox.Text;
        gr.Children.Add(tb);
        (App.Current as App).todoList.Add(gr);

        NavigationService.Navigate(new Uri("/MainPage.xaml",
        UriKind.RelativeOrAbsolute));
    }

我使用App作为List的存储 问题是当我尝试将列表项添加到MainPage上的堆栈面板时，它创建了一个异常（Element已经是另一个元素的子元素。）之后我在列表中添加第二个项目（在AddPage上）并返回到MainPage并尝试打印它。