将文本从一个堆栈面板传输到另一个

我正在开发一个 UWP 待办事项列表。

我有两个并排的堆栈面板。

<Grid x:Name="MainGrid" Background="Transparent">
    <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition  />
        <RowDefinition  />
        <RowDefinition MaxHeight="65"/>
    </Grid.RowDefinitions>
    <StackPanel Grid.Row="0" Grid.ColumnSpan="2" >
        <TextBlock Margin="10" FontSize="30">Please enter in an item you wish to complete</TextBlock>
        <StackPanel Orientation="Horizontal">
            <TextBox MinWidth="200" Name="Input"></TextBox>
            <Button Margin="10" Click="ButtonAdd_Click">Add</Button>
        </StackPanel>
        <TextBlock Margin="10" FontSize="30">Below are the Items you need to complete</TextBlock>
    </StackPanel>

    <StackPanel x:Name="outputArea" Grid.Column="0" Grid.Row="1" >

    </StackPanel>

    <StackPanel x:Name="completedList" Grid.Column="1" Grid.Row="1">

    </StackPanel>
    <Button Grid.Column="0" Grid.Row="2" HorizontalAlignment="Left"  Click="ButtonSettings_Click">Settings</Button>
    <Button Grid.Column="1" Grid.Row="2" HorizontalAlignment="Right"  Click="ButtonClear_Click">Clear done</Button>
</Grid>
</Page>

outputArea 堆栈面板包含用户输入的文本
。C# 代码

private void ButtonAdd_Click(object sender, RoutedEventArgs e)
    {
        StackPanel sp = new StackPanel(); ;
        TextBox textbox = new TextBox();
        textbox.Text += Input.Text;
        textbox.IsReadOnly = true;
        CheckBox done = new CheckBox();
        done.Content = "check when done";
        done.Checked += Done_Checked;

        sp.Orientation = Orientation.Horizontal;
        sp.Children.Add(textbox);
        sp.Children.Add(done);



        outputArea.Children.Add(sp);

    }

此代码从文本框获取输入并将其输出到输出区域我的问题是当复选框被选中时，我试图将文本从输出区域移动到完成的列表。我已经尝试了以下

 private void Done_Checked(object sender, RoutedEventArgs e)
    {
        // find the parent horiz sp, and move it.
        TextBox textbox = new TextBox();

        textbox.Text += textbox.Text;
        textbox.IsReadOnly = true;
        StackPanel stackpanel = new StackPanel();

        stackpanel.Orientation = Orientation.Horizontal;

        stackpanel.Children.Add(textbox);

    }

提前致谢。