Avalonia/samples/ControlCatalog/MainView.xaml

<UserControl x:Class="ControlCatalog.MainView"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:controls="clr-namespace:ControlSamples;assembly=ControlSamples"
             xmlns:models="clr-namespace:ControlCatalog.Models"
             xmlns:pages="clr-namespace:ControlCatalog.Pages">
  <Border Background="Red">
    
  </Border>
</UserControl>