testexam/exam/UsersWindow.axaml

<Window 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:vm="clr-namespace:exam.Models"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="exam.UsersWindow"
        Title="Пользователи">
    <Grid>
        <DataGrid x:Name="UsersDataGrid" IsReadOnly="True" Items="Model">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Фамилия" Binding="{Binding Lastname}"/>
                <DataGridTextColumn Header="Имя" Binding="{Binding Firstname}"/>
                <DataGridTextColumn Header="Логин" Binding="{Binding Loginuser}"/>
                <DataGridTextColumn Header="Страна" Binding="{Binding IdcountryNavigation.Country1}"/>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>