Avalonia/samples/ControlCatalog/Pages/ComboBoxPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.ComboBoxPage"
             xmlns:sys="clr-namespace:System;assembly=netstandard">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">ComboBox</TextBlock>
    <TextBlock Classes="h2">A drop-down list.</TextBlock>

    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Margin="0 16 0 0" Spacing="8">
      <ComboBox PlaceholderText="Pick an Item">
        <ComboBoxItem>Inline Items</ComboBoxItem>
        <ComboBoxItem>Inline Item 2</ComboBoxItem>
        <ComboBoxItem>Inline Item 3</ComboBoxItem>
        <ComboBoxItem>Inline Item 4</ComboBoxItem>
      </ComboBox>

      <ComboBox SelectedIndex="0">
        <ComboBoxItem>
          <Panel>
            <Rectangle Fill="{DynamicResource SystemAccentColor}"/>
            <TextBlock Margin="8">Control Items</TextBlock>
          </Panel>
        </ComboBoxItem>
        <ComboBoxItem>
          <Ellipse Width="50" Height="50" Fill="Yellow"/>
        </ComboBoxItem>
        <ComboBoxItem>
          <TextBox Text="TextBox"/>
        </ComboBoxItem>
      </ComboBox>

       <ComboBox x:Name="fontComboBox"  SelectedIndex="0">
         <ComboBox.ItemTemplate>
           <DataTemplate>
             <TextBlock Text="{Binding Name}" FontFamily="{Binding}" />
           </DataTemplate>
         </ComboBox.ItemTemplate>
       </ComboBox>
      
      <ComboBox PlaceholderText="Pick an Item">
        <ComboBoxItem>Inline Items</ComboBoxItem>
        <ComboBoxItem>Inline Item 2</ComboBoxItem>
        <ComboBoxItem>Inline Item 3</ComboBoxItem>
        <ComboBoxItem>Inline Item 4</ComboBoxItem>
        <DataValidationErrors.Error>
          <sys:Exception /> 
        </DataValidationErrors.Error>
      </ComboBox>
    </StackPanel>

  </StackPanel>
</UserControl>