AMessanger-Avalonia-Client/AMessanger/Views/MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:AMessanger.ViewModels"
        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="AMessanger.Views.MainWindow"
        x:DataType="vm:MainViewModel"
        Icon="/Assets/avalonia-logo.ico"
        Title="AMessanger">

    <!-- <Design.DataContext> -->
        <!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
        <!-- <vm:MainViewModel />
    </Design.DataContext> -->

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <ListBox Grid.Row="0"
                 ItemsSource="{Binding Messages}" />

        <Grid Grid.Row="1" Margin="5">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition />
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>

            <TextBox Text="{Binding UserName}" />
            <TextBox Grid.Column="1" Text="{Binding Message}" />
            <Button Grid.Column="2" Command="{Binding SendButtonCommand}">Send</Button>
        </Grid>
    </Grid>

</Window>