GoF паттерны, которые реально встречаются в .NET: Factory, Decorator, Strategy, Observer, Proxy?

// IHttpClientFactory — фабрика с пулом обработчиков и конфигурацией по имени
builder.Services.AddHttpClient("payments", c => c.BaseAddress = new Uri("https://pay.example.uz"));

public sealed class PaymentService(IHttpClientFactory factory)
{
    public Task<HttpResponseMessage> PingAsync() => factory.CreateClient("payments").GetAsync("/ping");
}

// Собственная фабрика, выбирающая реализацию по данным
public interface INotificationSenderFactory { INotificationSender Create(Channel channel); }

public sealed class NotificationSenderFactory(IServiceProvider sp) : INotificationSenderFactory
{
    public INotificationSender Create(Channel channel) => channel switch
    {
        Channel.Email => sp.GetRequiredService<EmailSender>(),
        Channel.Sms => sp.GetRequiredService<SmsSender>(),
        Channel.Telegram => sp.GetRequiredService<TelegramSender>(),
        _ => throw new ArgumentOutOfRangeException(nameof(channel))
    };
}
// В .NET 8 то же удобнее через keyed services: sp.GetRequiredKeyedService<INotificationSender>(channel)

public sealed class CachedProductRepository(IProductRepository inner, HybridCache cache) : IProductRepository
{
    public async Task<Product?> GetAsync(int id, CancellationToken ct) =>
        await cache.GetOrCreateAsync($"product:{id}", async t => await inner.GetAsync(id, t), cancellationToken: ct);

    public Task SaveAsync(Product p, CancellationToken ct) => inner.SaveAsync(p, ct);   // + инвалидация кэша
}

builder.Services.AddScoped<IProductRepository, EfProductRepository>();
builder.Services.Decorate<IProductRepository, CachedProductRepository>();   // Scrutor

public sealed class ApiKeyHandler(IOptions<PartnerOptions> options) : DelegatingHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken ct)
    {
        request.Headers.Add("X-Api-Key", options.Value.ApiKey);
        return base.SendAsync(request, ct);        // следующий обработчик в цепочке
    }
}
builder.Services.AddTransient<ApiKeyHandler>();
builder.Services.AddHttpClient<PartnerClient>().AddHttpMessageHandler<ApiKeyHandler>();

public interface IShippingCostStrategy
{
    bool CanHandle(ShippingMethod method);
    Money Calculate(Order order);
}

public sealed class CourierStrategy : IShippingCostStrategy { /* ... */ }
public sealed class PickupPointStrategy : IShippingCostStrategy { /* ... */ }
public sealed class PostStrategy : IShippingCostStrategy { /* ... */ }

public sealed class ShippingCalculator(IEnumerable<IShippingCostStrategy> strategies)
{
    public Money Calculate(Order order) =>
        strategies.First(s => s.CanHandle(order.ShippingMethod)).Calculate(order);
}

// 1. События C#
public event EventHandler<OrderPlacedEventArgs>? OrderPlaced;

// 2. IObservable (Rx.NET) — композиция потоков событий
IDisposable sub = priceTicks
    .Where(t => t.Symbol == "USD")
    .Throttle(TimeSpan.FromSeconds(1))
    .Subscribe(t => UpdateUi(t));

// 3. Изменение конфигурации
monitor.OnChange(flags => logger.LogInformation("Флаги обновлены"));

public interface IGitHubApi
{
    [Get("/users/{user}")]
    Task<GitHubUser> GetUserAsync(string user);
}
builder.Services.AddRefitClient<IGitHubApi>().ConfigureHttpClient(c => c.BaseAddress = new Uri("https://api.github.com"));