嵌套的自定义ContentControl未显示

Question

我有嵌套的自定义ContentControls，我很难让内部控件在外部控件内部渲染。以下是两个控件组成方式的示例：

<Window ...>
    <StackPanel>
        <local:Outer> <!-- Nothing in here gets displayed -->
            <local:Inner>
                <TextBlock>Fahrvergnügen</TextBlock>
            </local:Inner>
        </local:Outer>
        <local:Inner> <!-- This is displayed just fine -->
            <TextBlock>some text</TextBlock>
        </local:Inner>
    </StackPanel>
</Window>

Inner和Outer的样式定义如下：

<ResourceDictionary ...>

    <Style TargetType="{x:Type local:Outer}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:Outer}">
                    <ContentPresenter />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style TargetType="{x:Type local:Inner}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:Inner}">
                    <ContentPresenter />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>

额外的皱纹是Outer正在使用指向List的ContentPropertyAttribute：

[ContentPropertyAttribute("InnerItems")]
public class Outer : ContentControl
{
    private List<Inner> innerItems = new List<Inner>();

    static Outer()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(Outer), new FrameworkPropertyMetadata(typeof(Outer)));
    }

    public List<Inner> InnerItems { get { return this.innerItems; } }
}

为了完整性，这里是Inner的定义，这是微不足道的：

public class Inner : ContentControl
{
    static Inner()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(Inner), new FrameworkPropertyMetadata(typeof(Inner)));
    }
}

我猜测问题是Outer的内容无法用简单的<ContentPresenter />显示，但我不确定要用什么替换它。建议？