================================================================================
КАК ВЫУЧИТЬ ЗА ~2 ЧАСА (ПОРЯДОК ПАМЯТИ)
================================================================================

Час 1 — «скелет»
  1) SQL: две таблицы users + products, поля как в schema.sql.
  2) Модели User и Product (имена полей = как в SELECT).
  3) DbService: строка подключения, Login (одна строка из users), GetProducts (цикл while Read).

Час 2 — «окна»
  4) App: старт с LoginWindow как MainWindow.
  5) LoginWindow: два TextBox, кнопки, вызов db.Login, OpenByRole — if по Role,
     ОБЯЗАТЕЛЬНО desktop.MainWindow = новое окно, иначе после Close() приложение умрёт.
  6) BaseProductsWindow: UserName + Products, DataContext = this, ItemsControl ItemsSource.
  7) Картинка: строка ImagePath из БД → конвертер → Bitmap (avares://ИМЯ_СБОРКИ/...).
  8) Admin / Manager / Client — три класса, только Title и : base(user).

Запомнить наизусть: порядок колонок в reader (0,1,2,3), SQL с @параметрами,
и три роли: admin, manager, client.

================================================================================
БИБЛИОТЕКИ (NuGet) — КОРОТКО
================================================================================

  Avalonia              — UI
  Avalonia.Desktop      — настольный запуск
  Avalonia.Themes.Fluent — тема оформления
  Npgsql                — PostgreSQL

Все Avalonia.* — одна версия. Подробнее: файл Packages.txt

================================================================================
ВЕРСИЯ AVALONIA
================================================================================

Код рассчитан на Avalonia 11.x. На экзамене поменяйте только номера пакетов
в .csproj на те, что в шаблоне (главное — одинаковые между собой).

В XAML: ItemsSource у списка (не Items). Имя сборки в конвертере должно совпадать
с именем проекта (здесь: AvaloniaApplication1).

================================================================================
ФАЙЛ: AvaloniaApplication1.csproj
================================================================================

(см. реальный файл в проекте — там PackageReference и AvaloniaResource для Assets)

================================================================================
ФАЙЛ: Program.cs
================================================================================

using System;
using Avalonia;

namespace AvaloniaApplication1;

internal static class Program
{
    [STAThread]
    public static void Main(string[] args) => BuildAvaloniaApp()
        .StartWithClassicDesktopLifetime(args);

    public static AppBuilder BuildAvaloniaApp()
        => AppBuilder.Configure<App>()
            .UsePlatformDetect()
            .LogToTrace();
}

================================================================================
ФАЙЛ: App.axaml
================================================================================

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="AvaloniaApplication1.App"
             RequestedThemeVariant="Default">
  <Application.Styles>
    <FluentTheme />
  </Application.Styles>
</Application>

================================================================================
ФАЙЛ: App.axaml.cs
================================================================================

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using AvaloniaApplication1.Views;

namespace AvaloniaApplication1;

public partial class App : Application
{
    public override void Initialize()
    {
        AvaloniaXamlLoader.Load(this);
    }

    public override void OnFrameworkInitializationCompleted()
    {
        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            desktop.MainWindow = new LoginWindow();

        base.OnFrameworkInitializationCompleted();
    }
}

================================================================================
ФАЙЛ: Models/User.cs
================================================================================

namespace AvaloniaApplication1.Models;

public sealed class User
{
    public int Id { get; set; }
    public string Login { get; set; } = "";
    public string Password { get; set; } = "";
    public string Role { get; set; } = "";
    public string FullName { get; set; } = "";
}

================================================================================
ФАЙЛ: Models/Product.cs
================================================================================

namespace AvaloniaApplication1.Models;

public sealed class Product
{
    public string Name { get; set; } = "";
    public decimal Price { get; set; }
    public int Discount { get; set; }
    public string ImagePath { get; set; } = "";

    public string FinalPrice =>
        (Price * (100 - Discount) / 100m).ToString("F2");
}

================================================================================
ФАЙЛ: Services/DbService.cs  (папка может называться services — без разницы)
================================================================================

using System.Collections.Generic;
using AvaloniaApplication1.Models;
using Npgsql;

namespace AvaloniaApplication1.Services;

public sealed class DbService
{
    private readonly string connString =
        "Host=localhost;Port=5432;Username=postgres;Password=ВАШ_ПАРОЛЬ;Database=ВАША_БД";

    public User? Login(string login, string password)
    {
        using var conn = new NpgsqlConnection(connString);
        conn.Open();

        using var cmd = new NpgsqlCommand(
            "SELECT id, login, password, role, fullname FROM users WHERE login=@l AND password=@p",
            conn);

        cmd.Parameters.AddWithValue("l", login);
        cmd.Parameters.AddWithValue("p", password);

        using var reader = cmd.ExecuteReader();

        if (reader.Read())
        {
            return new User
            {
                Id = reader.GetInt32(0),
                Login = reader.GetString(1),
                Role = reader.GetString(3),
                FullName = reader.GetString(4)
            };
        }

        return null;
    }

    public List<Product> GetProducts()
    {
        var list = new List<Product>();

        using var conn = new NpgsqlConnection(connString);
        conn.Open();

        using var cmd = new NpgsqlCommand(
            "SELECT name, price, discount, image FROM products", conn);
        using var reader = cmd.ExecuteReader();

        while (reader.Read())
        {
            var img = reader.IsDBNull(3) ? "" : reader.GetString(3);

            if (string.IsNullOrWhiteSpace(img))
                img = "Assets/no_image.png";
            else
                img = "Assets/" + img;

            list.Add(new Product
            {
                Name = reader.GetString(0),
                Price = reader.GetDecimal(1),
                Discount = reader.GetInt32(2),
                ImagePath = img
            });
        }

        return list;
    }
}

================================================================================
ФАЙЛ: Converters/AssetPathToBitmapConverter.cs
================================================================================

using System;
using System.Globalization;
using Avalonia.Data.Converters;
using Avalonia.Media.Imaging;
using Avalonia.Platform;

namespace AvaloniaApplication1.Converters;

public sealed class AssetPathToBitmapConverter : IValueConverter
{
    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        var relative = value as string;
        if (string.IsNullOrWhiteSpace(relative))
            relative = "Assets/no_image.png";

        relative = relative.Replace('\\', '/');
        var uri = new Uri("avares://AvaloniaApplication1/" + relative);

        try
        {
            using var stream = AssetLoader.Open(uri);
            return new Bitmap(stream);
        }
        catch
        {
            var fallback = new Uri("avares://AvaloniaApplication1/Assets/no_image.png");
            using var stream = AssetLoader.Open(fallback);
            return new Bitmap(stream);
        }
    }

    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
        => throw new NotSupportedException();
}

================================================================================
ФАЙЛ: Views/LoginWindow.axaml
================================================================================

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="AvaloniaApplication1.Views.LoginWindow"
        Width="400" Height="280"
        Title="Вход">
  <StackPanel Margin="20" Spacing="10">
    <TextBox x:Name="loginBox" Watermark="Логин"/>
    <TextBox x:Name="passwordBox" PasswordChar="*"/>
    <TextBlock x:Name="errorText" Foreground="Red" TextWrapping="Wrap"/>
    <Button Content="Войти" Click="Login_Click"/>
    <Button Content="Гость" Click="Guest_Click"/>
  </StackPanel>
</Window>

================================================================================
ФАЙЛ: Views/LoginWindow.axaml.cs
================================================================================

using System;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Interactivity;
using AvaloniaApplication1.Models;
using AvaloniaApplication1.Services;

namespace AvaloniaApplication1.Views;

public partial class LoginWindow : Window
{
    private readonly DbService db = new DbService();

    public LoginWindow()
    {
        InitializeComponent();
    }

    private void Login_Click(object? sender, RoutedEventArgs e)
    {
        errorText.Text = "";
        var login = loginBox.Text?.Trim() ?? "";
        var password = passwordBox.Text ?? "";

        try
        {
            var user = db.Login(login, password);
            if (user == null)
            {
                errorText.Text = "Неверный логин или пароль.";
                return;
            }

            OpenByRole(user);
        }
        catch (Exception ex)
        {
            errorText.Text = "Ошибка БД: " + ex.Message;
        }
    }

    private void Guest_Click(object? sender, RoutedEventArgs e)
    {
        errorText.Text = "";
        OpenByRole(new User { Role = "client", FullName = "Гость" });
    }

    private void OpenByRole(User user)
    {
        Window w;
        if (user.Role == "admin")
            w = new AdminWindow(user);
        else if (user.Role == "manager")
            w = new ManagerWindow(user);
        else
            w = new ClientWindow(user);

        if (Avalonia.Application.Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            desktop.MainWindow = w;

        w.Show();
        Close();
    }
}

================================================================================
ФАЙЛ: Views/BaseProductsWindow.axaml
================================================================================

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:conv="clr-namespace:AvaloniaApplication1.Converters"
        x:Class="AvaloniaApplication1.Views.BaseProductsWindow"
        Width="900" Height="600">

  <Window.Resources>
    <conv:AssetPathToBitmapConverter x:Key="ImgConv"/>
  </Window.Resources>

  <DockPanel>
    <Border DockPanel.Dock="Top" Padding="12" Background="#E8E8E8">
      <Grid ColumnDefinitions="*,Auto">
        <TextBlock VerticalAlignment="Center" Text="{Binding UserName, StringFormat='Пользователь: {0}'}"/>
        <Button Grid.Column="1" Content="Выйти" Click="Logout_Click"/>
      </Grid>
    </Border>

    <ScrollViewer>
      <ItemsControl Margin="10" ItemsSource="{Binding Products}">
        <ItemsControl.ItemsPanel>
          <ItemsPanelTemplate>
            <WrapPanel/>
          </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
        <ItemsControl.ItemTemplate>
          <DataTemplate>
            <Border Margin="10" Padding="10" BorderBrush="Gray" BorderThickness="1" Width="200">
              <StackPanel Spacing="6">
                <Image Width="120" Height="120" HorizontalAlignment="Center"
                       Source="{Binding ImagePath, Converter={StaticResource ImgConv}}"/>
                <TextBlock Text="{Binding Name}" FontWeight="Bold" TextWrapping="Wrap"/>
                <TextBlock Text="{Binding Price, StringFormat='Цена: {0}'}"/>
                <TextBlock Text="{Binding FinalPrice, StringFormat='Со скидкой: {0}'}"/>
              </StackPanel>
            </Border>
          </DataTemplate>
        </ItemsControl.ItemTemplate>
      </ItemsControl>
    </ScrollViewer>
  </DockPanel>
</Window>

================================================================================
ФАЙЛ: Views/BaseProductsWindow.axaml.cs
================================================================================

using System.Collections.Generic;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Interactivity;
using AvaloniaApplication1.Models;
using AvaloniaApplication1.Services;

namespace AvaloniaApplication1.Views;

public partial class BaseProductsWindow : Window
{
    private readonly DbService db = new DbService();

    public string UserName { get; set; } = "";
    public List<Product> Products { get; set; } = new List<Product>();

    public BaseProductsWindow()
    {
        InitializeComponent();
    }

    public BaseProductsWindow(User user) : this()
    {
        UserName = user.FullName;
        Products = db.GetProducts();
        DataContext = this;
    }

    public void Logout()
    {
        var login = new LoginWindow();
        if (Application.Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            desktop.MainWindow = login;

        login.Show();
        Close();
    }

    private void Logout_Click(object? sender, RoutedEventArgs e)
    {
        Logout();
    }
}

================================================================================
ФАЙЛ: Views/AdminWindow.cs
================================================================================

using AvaloniaApplication1.Models;

namespace AvaloniaApplication1.Views;

public sealed class AdminWindow : BaseProductsWindow
{
    public AdminWindow(User user) : base(user)
    {
        Title = "Администратор";
    }
}

================================================================================
ФАЙЛ: Views/ManagerWindow.cs
================================================================================

using AvaloniaApplication1.Models;

namespace AvaloniaApplication1.Views;

public sealed class ManagerWindow : BaseProductsWindow
{
    public ManagerWindow(User user) : base(user)
    {
        Title = "Менеджер";
    }
}

================================================================================
ФАЙЛ: Views/ClientWindow.cs
================================================================================

using AvaloniaApplication1.Models;

namespace AvaloniaApplication1.Views;

public sealed class ClientWindow : BaseProductsWindow
{
    public ClientWindow(User user) : base(user)
    {
        Title = "Клиент";
    }
}

================================================================================
ФАЙЛ: Database/schema.sql
================================================================================

CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    login TEXT,
    password TEXT,
    role TEXT,
    fullname TEXT
);

INSERT INTO users (login, password, role, fullname) VALUES
('admin','123','admin','Администратор'),
('manager','123','manager','Менеджер'),
('user','123','client','Пользователь');

CREATE TABLE products (
    id SERIAL PRIMARY KEY,
    name TEXT,
    price NUMERIC,
    discount INT,
    image TEXT
);

INSERT INTO products (name, price, discount, image) VALUES
('Телефон', 50000, 10, 'phone.png'),
('Ноутбук', 90000, 20, 'laptop.png'),
('Мышка', 1500, 0, ''),
('Клавиатура', 3000, 5, NULL);

================================================================================
КОНЕЦ СБОРНИКА (дублирует рабочий проект; при правках в коде обновите этот файл)
================================================================================