Windows Phone ContentProperty

Question

我在Windows Phone 8项目中使用UserControl，我想放一些像这样的控件

<MyUserControl>
    <Grid Name="MyGrid">
        ...
    </Grid>
</MyUserControl>

我写这段代码：

[ContentProperty("MyContent")]
public partial class MyUserControl : UserControl
{

    public static readonly DependencyProperty MyContentProperty = DependencyProperty.Register(
        "MyContent",
        typeof(object),
        typeof(MyUserControl),
        new PropertyMetadata(null));

    public object MyContent
    {
        get { return GetValue(MyContentProperty); }
        set
        {
            SetValue(MyContentProperty, value);
        }
    }
}

MyUserControl的XAML文件：

<UserControl x:Name="RootUserControl"
    DataContext="{Binding RelativeSource={RelativeSource Self}}"
    ...>
    <Grid x:Name="LayoutRoot">
        <ContentPresenter Name="Presenter" Content="{Binding MyContent, ElementName=RootUserControl}"/>
    </Grid>
</UserControl>

它的工作，但部分工作。当我尝试将MyGrid或其他内容用于MyUserControl时，它返回null。怎么了？