Avalonia/samples/ControlCatalog/MainWindow.xaml

<Window xmlns="https://github.com/avaloniaui" MinWidth="500" MinHeight="300"
        Title="Avalonia Control Gallery"
        Icon="resm:ControlCatalog.Assets.test_icon.ico?assembly=ControlCatalog"
        xmlns:local="clr-namespace:ControlCatalog">
    <Window.Styles>
        <Style Selector="local|MainView">
            <Setter Property="local:MyControl.Proportion" Value="{Binding Proportion}" />
        </Style>
    </Window.Styles>
    <local:MainView/>
</Window>