Many companies are building their own base classes wrapping the native UI classes (in order to be able to customize them). 

Here a proof-of-concept how you can do Window and UserControl inheritance in WPF.

- The DemoWPFBaseClassesLib : contain the 2 base class for the Window and for the UserControl (both are changing the background color)
public class MyBaseWindow : Window
public class MyBaseUserControl : System.Windows.Controls.UserControl
- The UserControls library DemoWpfBaseClassesUCLibrary with one UserControl derived for the base one
public partial class UserControl1 : MyBaseUserControl
- The standalone WPF application DemoWPFBaseClasses with the main window derived from the base one
public partial class Window1 : MyBaseWindow
There are also changes to do in the XAML files (corresponding to the derived classes) - see the text in yellow:

- For the Window:
<src:MyBaseWindow x:Class="DemoWPFBaseClasses.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:src="clr-namespace:DemoWPFBaseClassesLib;assembly=DemoWPFBaseClassesLib" 
    xmlns:my="clr-namespace:DemoWpfBaseClassesUCLibrary;assembly=DemoWpfBaseClassesUCLibrary"
    Title="DemoWPFBaseClasses" Height="300" Width="300">
    <Grid>
        <Button Width="74" Height="24" Content="Test" HorizontalAlignment="Left" Margin="12,12,0,0" VerticalAlignment="Top"></Button>
        <my:UserControl1 Width="200" Height="100" HorizontalAlignment="Left" Margin="12,51,0,0" VerticalAlignment="Top"></my:UserControl1>
    </Grid>
</src:MyBaseWindow>

- For the UserControl:
<runtime:MyBaseUserControl x:Class="DemoWpfBaseClassesUCLibrary.UserControl1"
    xmlns:runtime="clr-namespace:DemoWPFBaseClassesLib;assembly=DemoWPFBaseClassesLib"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="100" Width="200">
    <Grid>
        <TextBox Height="23" Margin="12,12,0,0" Name="textBox1" VerticalAlignment="Top" HorizontalAlignment="Left" Width="120" />
    </Grid>
</runtime:MyBaseUserControl>