Problem with derived ControlTemplates in WPF

The following xaml code works:

<Window x:Class="DerivedTemplateBug.Window1"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.micr开发者_StackOverflowosoft.com/winfx/2006/xaml"
 xmlns:local="clr-namespace:DerivedTemplateBug"
 Title="Window1" Height="300" Width="300">
 <Button>
  <Button.Template>
   <ControlTemplate>
    <Border BorderBrush="Black" BorderThickness="2">
     <TextBlock>Testing!</TextBlock>
    </Border>
   </ControlTemplate>
  </Button.Template>
 </Button>
</Window>

Now, if you define the following data template:

using System.Windows.Controls;

namespace DerivedTemplateBug
{
 public class DerivedTemplate : ControlTemplate
 {
 }
}

And then swap the ControlTemplate for the derived class:

<Window x:Class="DerivedTemplateBug.Window1"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:local="clr-namespace:DerivedTemplateBug"
 Title="Window1" Height="300" Width="300">
 <Button>
  <Button.Template>
   <local:DerivedTemplate>
    <Border BorderBrush="Black" BorderThickness="2">
     <TextBlock>Testing!</TextBlock>
    </Border>
   </local:DerivedTemplate>
  </Button.Template>
 </Button>
</Window>

You get the following error:

Invalid ContentPropertyAttribute on type 'DerivedTemplateBug.DerivedTemplate', property 'Content' not found.

Can anyone tell me why this is the case?

I get a different error when I try this:

'Border' object cannot be added to 'DerivedTemplate'. 
Object of type 'System.Windows.Controls.Border' cannot be converted 
to type 'System.Windows.FrameworkElementFactory'.

Looking up FrameworkElementFactory, it appears this was an old way of creating templates in code:

This class is a deprecated way to programmatically create templates, which are subclasses of FrameworkTemplate such as ControlTemplate or DataTemplate; not all of the template functionality is available when you create a template using this class.

My question is, why are you inheriting from ControlTemplate? I can't think of a use case for doing this. If you are simply trying to create your own templates in code-behind, MSDN recommends the following approach:

The recommended way to programmatically create a template is to load XAML from a string or a memory stream using the Load method of the XamlReader class.