WPF User Control extending border class. "Does not support direct content"?

I am producing graphics for a process control system and I would like to create a system border which would visually wrap the various sub system being displayed in the process mimic. I could use a regular border for this except I want it to not only changing color reflecting system status, but also popping up small "balloons" indicating the piece of the system that is in alarm state.

I created a test project with a User Control and added a ListBox (for the balloons) and a ContentPresenter element wrapped in a border control. However, whenever I use this new control in another app, it wont allow me to add content to it. I've tried messing some with the ContentPropertyAttribute and properties of the ContentPresenter element, but I feel I am in the blind here.

<UserControl x:Class="SystemStatusBorder.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Canvas Height="290" Width="303">        
        <Border HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
            <ContentPresenter/>
        </Border>
        <ListBox Canvas.Right="0" Canvas.Bottom="0">
            <ListBox.RenderTransform>
                <TranslateTransform X="20"></TranslateTransform开发者_JAVA技巧>
            </ListBox.RenderTransform>
            <ListBoxItem>TagA</ListBoxItem>
            <ListBoxItem>TagB</ListBoxItem>
        </ListBox>
    </Canvas>
</UserControl>

I don't get it. What more should it need other than just the existence of a contentpresenter? UserControl subclasses ContentControl so I would have thought the wiring was in place. Eventually, I want it to be used something like this:

<SystemBorder>
  <SystemBorder.MonitoredTags>
        <List of relevant tags for the specific sub system goes here>
  </SystemBorder.MonitoredTags>
  <regular content goes here>  
</SystemBorder>

---

To create your own container control, you must create a new custom control (not a UserControl).

Make your new control inherit from ContentControl.

Custom Controls don't have their own XAML. Instead, they are assigned a ControlTemplate.

When you create your first Custom Control, the IDE will create a new file Themes\Generic.xaml.

This is where the template for your control is. You can modify this template to match the XAML in your question. This will support the ContentPresenter element.

I found a very good walkthrough here.