Semi.Avalonia/demo/Semi.Avalonia.Demo/Pages/ComboBoxDemo.axaml

<UserControl
    x:Class="Semi.Avalonia.Demo.Pages.ComboBoxDemo"
    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"
    xmlns:pages="clr-namespace:Semi.Avalonia.Demo.Pages"
    d:DesignHeight="800"
    d:DesignWidth="800"
    mc:Ignorable="d">
    <Design.DataContext>
        <pages:ComboBoxDemoViewModel />
    </Design.DataContext>
    <StackPanel Spacing="20">
        <StackPanel.Styles>
            <Style Selector="ComboBox">
                <Setter Property="Width" Value="300" />
                <Setter Property="ItemsSource" Value="{Binding Items}" />
            </Style>
        </StackPanel.Styles>

        <ComboBox />
        <ComboBox Classes="ClearButton" />
        <ComboBox PlaceholderText="Please Select" />
        <ComboBox IsEnabled="False" />
        <ComboBox Classes="Large" IsEnabled="False" />
        <ComboBox Classes="Small" />
        <ComboBox Classes="Bordered" />
        <ComboBox Classes="Bordered" IsEnabled="False" />

        <StackPanel Orientation="Horizontal">
            <ComboBox Width="100" Classes="Large" PlaceholderText="Large" />
            <ComboBox Width="100" PlaceholderText="Default" />
            <ComboBox Width="100" Classes="Small" PlaceholderText="Small" />
        </StackPanel>

        <StackPanel Orientation="Horizontal">
            <ComboBox Width="100" IsEnabled="False" PlaceholderText="Disabled" />
            <ComboBox Width="100" Classes="Bordered" PlaceholderText="Bordered" />
            <ComboBox Width="100" Classes="Bordered" IsEnabled="False" />
        </StackPanel>
    </StackPanel>
</UserControl>