WpfControl.xaml

<UserControl x:Class="SlimDucky.WpfControl"
             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" 
             xmlns:local="clr-namespace:SlimDucky"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <GroupBox Header="Windows Presentation Foundation">
    </GroupBox>
</UserControl>