Avalonia/samples/ControlCatalog/Pages/RectanglePage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.RectanglePage">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">Rectangle</TextBlock>
    <TextBlock Classes="h2">A control which represents a rectangle with a fill and a stroke.</TextBlock>

    <StackPanel Orientation="Vertical"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Spacing="16">
      <Rectangle Width="200" Height="32" Fill="Red" Stroke="Blue" StrokeThickness="{Binding #StrokeThickness.Value}"/>

      <Grid ColumnDefinitions="Auto,*">
          <TextBlock Grid.Column="0" Text="Stroke thickness"/>
          <Slider Grid.Column="1"  Name="StrokeThickness" Maximum="10" Value="1"/>
      </Grid>
      
    </StackPanel>
  </StackPanel>
</UserControl>