Avalonia/samples/ControlCatalog/Pages/RadioButtonPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <StackPanel Orientation="Vertical" Gap="4">
    <TextBlock Classes="h1">RadioButton</TextBlock>
    <TextBlock Classes="h2">Allows the selection of a single option of many</TextBlock>

    <StackPanel Orientation="Horizontal"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Gap="16">
      <StackPanel Orientation="Vertical"
                  Gap="16">
        <RadioButton IsChecked="True">Option 1</RadioButton>
        <RadioButton>Option 2</RadioButton>
        <RadioButton IsChecked="{x:Null}">Option 3</RadioButton>
        <RadioButton IsEnabled="False">Disabled</RadioButton>
      </StackPanel>
      <StackPanel Orientation="Vertical"
                  Gap="16">
        <RadioButton IsChecked="True" IsThreeState="True">Three States: Option 1</RadioButton>
        <RadioButton IsChecked="False" IsThreeState="True">Three States: Option 2</RadioButton>
        <RadioButton IsChecked="{x:Null}" IsThreeState="True">Three States: Option 3</RadioButton>
        <RadioButton IsChecked="{x:Null}" IsThreeState="True" IsEnabled="False">Disabled</RadioButton>
      </StackPanel>
    </StackPanel>
  </StackPanel>
</UserControl>