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I'm trying to have a custom WPF control whose content is defined in XAML but avoiding the use of a UserControl. This is because a UserControl makes it impossible to add x:Name to its content: https://stackoverflow.com/questions/751325/how-to-create-a-wpf-usercontrol-with-named-content/3413382

My files are laid out like this:

.
│ App.config
│ App.xaml
│ App.xaml.cs
│ MainWindow.xaml
│ MainWindow.xaml.cs
│ WpfScratch.csproj
│
├───bin
│ ├───Debug
│ │ │ WpfScratch.exe
│ │ │ WpfScratch.exe.config
│ │ │ WpfScratch.pdb
│ │ │
│ │ └───Controls
│ └───Release
├───Controls
. MyPanel.cs
. MyPanel.xaml

and the source of the relevant ones is:

MyPanel.cs

public class MyPanel : Control

 static MyPanel()
 
 DefaultStyleKeyProperty.OverrideMetadata(typeof(MyPanel), new FrameworkPropertyMetadata(typeof(MyPanel)));
 

 public MyPanel()
 
 var templateUri = new Uri("/WpfScratch;component/Controls/MyPanel.xaml", UriKind.Relative);
 Template = (ControlTemplate) Application.LoadComponent(templateUri);
 

 public static readonly DependencyProperty BodyProperty = DependencyProperty.Register("Body", typeof(object), typeof(MyPanel), new PropertyMetadata(default(object)));

 public object Body
 
 get => GetValue(BodyProperty);
 set => SetValue(BodyProperty, value);
 

 public static readonly DependencyProperty FooterProperty = DependencyProperty.Register("Footer", typeof(object), typeof(MyPanel), new PropertyMetadata(default(object)));

 public object Footer
 
 get => GetValue(FooterProperty);
 set => SetValue(FooterProperty, value);
 

MyPanel.xaml

<ControlTemplate xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:local="clr-namespace:WpfScratch.Controls" 
 TargetType="local:MyPanel" >
 <DockPanel LastChildFill="True" x:Name="Content">
 <Border BorderThickness="2"
 BorderBrush="Aqua"
 DockPanel.Dock="Bottom">
 <ContentPresenter x:Name="Footer"
 Content="Binding Footer, RelativeSource=RelativeSource TemplatedParent"
 Margin="0, 10" />
 </Border>
 <Border BorderThickness="2"
 BorderBrush="Magenta">
 <ContentPresenter x:Name="Body" 
 Content="Binding Body, RelativeSource=RelativeSource TemplatedParent" />
 </Border>
 </DockPanel>
</ControlTemplate>

MainWindow.xaml

<Window x:Class="WpfScratch.MainWindow"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:local="clr-namespace:WpfScratch"
 xmlns:controls="clr-namespace:WpfScratch.Controls"
 mc:Ignorable="d"
 Title="MainWindow" Height="450" Width="800">
 <controls:MyPanel x:Name="HelloWorldPanel">
 <controls:MyPanel.Body>
 <Label x:Name="HelloLabel">Hello!</Label>
 </controls:MyPanel.Body>
 <controls:MyPanel.Footer>
 <Label x:Name="WorldLabel">World!</Label>
 </controls:MyPanel.Footer>
 </controls:MyPanel>
</Window>

My main concern is the loading of the XAML file, i.e.:

 var templateUri = new Uri("/WpfScratch;component/Controls/MyPanel.xaml", UriKind.Relative);
 Template = (ControlTemplate) Application.LoadComponent(templateUri);

1. Is Application.Load() the way to go? UWP seems to deprecate it in favour of XAMLReader.Load(), but I'm not sure how I would get the correct stream for that. I tried digging into the sources of Application.LoadComponent and there's some logic involved with processing pack URIs


2. The URI is a hardcoded mouthful and seems not very resilient to refactoring. Is there a terse way of saying "hey my XAML file is right next to this one"?

The conventional way of attaching default template to custom control is to specify default style inside a special resource dictionary. For this approach to work three conditions should be met:

1) Resource dictionary should be located at Themes/Generic.xaml. There you should place a default style for your custom control (that in turn should set Template property to default value).

2) Your AssemblyInfo.cs file should contain ThemeInfoAttribute.

3) Your custom control should override default style key inside a static constructor, using DefaultStyleKeyProperty.OverrideMetadata method.

Alternatively, if you are not planning on reusing your custom control, you can just drop the default style into App.xaml resources and avoid above shenanigans.

For more info see: https://michaelscodingspot.com/2016/12/24/explicit-implicit-and-default-styles-in-wpf/