C#ServiceRegistration.cs
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using Microsoft.Extensions.DependencyInjection;
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public static class ServiceCollectionExtensions
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{
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    public static IServiceCollection AddMyFeature(this IServiceCollection services)
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    {
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        // Singleton: one instance for the entire app lifetime
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        services.AddSingleton<IConnectionPool, ConnectionPool>();
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        // Scoped: one instance per HTTP request
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        services.AddScoped<IOrderService, OrderService>();
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        services.AddScoped<IOrderRepository, OrderRepository>();
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        // Transient: new instance every time it's requested
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        services.AddTransient<IEmailSender, SmtpEmailSender>();
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        // Keyed services (NET 8+)
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        services.AddKeyedScoped<IPaymentProvider, StripeProvider>("stripe");
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        services.AddKeyedScoped<IPaymentProvider, PayPalProvider>("paypal");
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        // Factory registration
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        services.AddSingleton<IReportGeneratorFactory>(sp =>
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            new ReportGeneratorFactory(
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                sp.GetRequiredService<IConnectionPool>(),
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                sp.GetRequiredService<ILogger<ReportGeneratorFactory>>()
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            ));
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        return services;
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    }
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}
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// Safe background service using IServiceScopeFactory
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public class BackgroundWorker : BackgroundService
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{
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    private readonly IServiceScopeFactory _scopeFactory;
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    public BackgroundWorker(IServiceScopeFactory scopeFactory)
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    {
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        _scopeFactory = scopeFactory;
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    }
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    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
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    {
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        while (!stoppingToken.IsCancellationRequested)
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        {
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            // Create a scope for each unit of work to get Scoped services safely
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            using var scope = _scopeFactory.CreateScope();
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            var orderService = scope.ServiceProvider.GetRequiredService<IOrderService>();
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            await orderService.ProcessPendingOrdersAsync(stoppingToken);
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            await Task.Delay(TimeSpan.FromMinutes(1), stoppingToken);
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        }
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    }
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}
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