Avalonia/samples/IntegrationTestApp/Pages/CheckBoxPage.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="IntegrationTestApp.Pages.CheckBoxPage">
  <StackPanel>
    <CheckBox Name="UncheckedCheckBox">Unchecked</CheckBox>
    <CheckBox Name="CheckedCheckBox" IsChecked="True">Checked</CheckBox>
    <CheckBox Name="ThreeStateCheckBox" IsThreeState="True" IsChecked="{x:Null}">ThreeState</CheckBox>
  </StackPanel>
</UserControl>