Avalonia/samples/ControlCatalog/Pages/DropDownPage.xaml

<UserControl xmlns="https://github.com/avaloniaui">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">DropDown</TextBlock>
    <TextBlock Classes="h2">A drop-down list.</TextBlock>

    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Margin="0 16 0 0" Spacing="8">
      <DropDown SelectedIndex="0">
        <DropDownItem>Inline Items</DropDownItem>
        <DropDownItem>Inline Item 2</DropDownItem>
        <DropDownItem>Inline Item 3</DropDownItem>
        <DropDownItem>Inline Item 4</DropDownItem>
      </DropDown>

      <DropDown SelectedIndex="0">
        <DropDownItem>
          <Panel>
            <Rectangle Fill="{DynamicResource ThemeAccentBrush}"/>
            <TextBlock Margin="8">Control Items</TextBlock>
          </Panel>
        </DropDownItem>
        <DropDownItem>
          <Ellipse Width="50" Height="50" Fill="Yellow"/>
        </DropDownItem>
        <DropDownItem>
          <TextBox Text="TextBox"/>
        </DropDownItem>
      </DropDown>
    </StackPanel>

  </StackPanel>
</UserControl>