Avalonia/samples/ControlCatalog/Pages/CheckBoxPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.CheckBoxPage">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h2">A check box control</TextBlock>

    <StackPanel Orientation="Horizontal"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Spacing="16">
      <StackPanel Orientation="Vertical"
                  Spacing="16">
        <CheckBox>_Unchecked</CheckBox>
        <CheckBox IsChecked="True">_Checked</CheckBox>
        <CheckBox IsChecked="{x:Null}">_Indeterminate</CheckBox>
        <CheckBox IsChecked="True" IsEnabled="False">Disabled</CheckBox>
      </StackPanel>
      <StackPanel Orientation="Vertical"
                  HorizontalAlignment="Center"
                  Spacing="16">
        <CheckBox IsChecked="False" IsThreeState="True">Three State: Unchecked</CheckBox>
        <CheckBox IsChecked="True" IsThreeState="True">Three State: Checked</CheckBox>
        <CheckBox IsChecked="{x:Null}" IsThreeState="True">Three State: Indeterminate</CheckBox>
        <CheckBox IsChecked="{x:Null}" IsThreeState="True" IsEnabled="False">Three State: Disabled</CheckBox>
      </StackPanel>
    </StackPanel>
  </StackPanel>
</UserControl>