Dependency Injection

Core Principles

• Use constructor injection exclusively — avoid service locator pattern
• Choose the correct lifetime: Singleton, Scoped, or Transient
• Organize registrations in IServiceCollection extension methods
• Use keyed services (.NET 8+) for multiple implementations of the same interface
• Validate registrations at startup where possible
• Never resolve scoped services from a singleton

Patterns

Service Registration Extension Methods

// Organize by layer or feature
public static class DependencyInjection
{
    public static IServiceCollection AddApplicationServices(
        this IServiceCollection services)
    {
        services.AddMediatR(cfg =>
            cfg.RegisterServicesFromAssembly(typeof(DependencyInjection).Assembly));

        services.AddValidatorsFromAssembly(
            typeof(DependencyInjection).Assembly);

        return services;
    }

    public static IServiceCollection AddInfrastructureServices(
        this IServiceCollection services, IConfiguration configuration)
    {
        // Scoped — one instance per HTTP request
        services.AddScoped<IOrderRepository, OrderRepository>();
        services.AddScoped<IUnitOfWork, UnitOfWork>();

        // Open generics
        services.AddScoped(typeof(IRepository<>), typeof(EfRepository<>));

        // Singleton — one instance for the app lifetime
        services.AddSingleton<IDateTimeProvider, DateTimeProvider>();

        // Transient — new instance every time
        services.AddTransient<IEmailSender, SmtpEmailSender>();

        return services;
    }
}

// Program.cs
builder.Services
    .AddApplicationServices()
    .AddInfrastructureServices(builder.Configuration);

Lifetime Decision Guide

Singleton  — stateless services, caches, configuration wrappers
             Example: IDateTimeProvider, HybridCache, IOptions<T>
             Warning: must be thread-safe

Scoped     — per-request state, DbContext, UnitOfWork, repositories
             Example: AppDbContext, ICurrentUserService
             Default choice for most services

Transient  — lightweight stateless services, factory output
             Example: IEmailSender, validators
             Warning: do not hold expensive resources

Keyed Services (.NET 8+)

// Registration
services.AddKeyedSingleton<IPaymentGateway, StripeGateway>("stripe");
services.AddKeyedSingleton<IPaymentGateway, PayPalGateway>("paypal");
services.AddKeyedSingleton<IPaymentGateway, SquareGateway>("square");

// Injection via attribute
public sealed class CheckoutService(
    [FromKeyedServices("stripe")] IPaymentGateway gateway)
{
    public Task ChargeAsync(decimal amount) => gateway.ChargeAsync(amount);
}

// Resolution from provider
public sealed class PaymentRouter(IServiceProvider provider)
{
    public IPaymentGateway GetGateway(string name)
        => provider.GetRequiredKeyedService<IPaymentGateway>(name);
}

Decorator Pattern

// Base service
services.AddScoped<IOrderService, OrderService>();

// Decorate with cross-cutting concerns (outermost last)
services.Decorate<IOrderService, CachedOrderService>();
services.Decorate<IOrderService, LoggingOrderService>();

// Decorator implementation
public sealed class CachedOrderService(
    IOrderService inner,
    HybridCache cache) : IOrderService
{
    public async Task<Order?> GetOrderAsync(Guid id, CancellationToken ct)
    {
        return await cache.GetOrCreateAsync(
            $"orders:{id}",
            async token => await inner.GetOrderAsync(id, token),
            cancellationToken: ct);
    }
}

Factory Delegates

services.AddScoped<IReportGenerator>(sp =>
{
    var config = sp.GetRequiredService<IOptions<ReportOptions>>().Value;
    return config.Format switch
    {
        "pdf" => sp.GetRequiredService<PdfReportGenerator>(),
        "csv" => sp.GetRequiredService<CsvReportGenerator>(),
        _ => throw new InvalidOperationException(
            $"Unknown report format: {config.Format}")
    };
});

Background Service Scoping

// Background services are singletons — create scopes for scoped deps
public sealed class OrderProcessor(
    IServiceScopeFactory scopeFactory,
    ILogger<OrderProcessor> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            using var scope = scopeFactory.CreateScope();
            var repository = scope.ServiceProvider
                .GetRequiredService<IOrderRepository>();
            var unitOfWork = scope.ServiceProvider
                .GetRequiredService<IUnitOfWork>();

            await ProcessPendingOrdersAsync(repository, unitOfWork, ct);
            await Task.Delay(TimeSpan.FromSeconds(30), ct);
        }
    }
}

Options Pattern Registration

// Strongly-typed options with validation
services.AddOptions<DatabaseOptions>()
    .BindConfiguration(DatabaseOptions.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart();

services.AddOptions<JwtOptions>()
    .BindConfiguration("Jwt")
    .ValidateDataAnnotations()
    .ValidateOnStart();

Anti-Patterns

// BAD: Service locator — hides dependencies
public class OrderService(IServiceProvider provider)
{
    public void Process()
    {
        var repo = provider.GetRequiredService<IOrderRepository>();
    }
}

// BAD: Captive dependency — scoped inside singleton
services.AddSingleton<MySingleton>(); // holds IOrderRepository (scoped)

// BAD: Resolving in constructor
public class OrderService(IServiceProvider provider)
{
    private readonly IOrderRepository _repo =
        provider.GetRequiredService<IOrderRepository>(); // resolves too early
}

// BAD: new-ing up services manually
public class OrderController
{
    private readonly OrderService _service = new OrderService(
        new OrderRepository(new AppDbContext())); // defeats DI
}

Detect Existing Patterns

1. Search for builder.Services.Add calls in Program.cs
2. Look for IServiceCollection extension methods in the codebase
3. Check for [FromKeyedServices] usage (indicates .NET 8+ keyed DI)
4. Search for AddScoped, AddTransient, AddSingleton registration patterns
5. Look for IServiceScopeFactory usage in background services
6. Check for Scrutor package (provides Decorate<> extension)

Adding to Existing Project

1. Group registrations — move scattered builder.Services.Add* calls into extension methods per layer
2. Audit lifetimes — verify no captive dependencies (scoped inside singleton)
3. Add ValidateOnStart — add ValidateOnStart() to all Options registrations
4. Migrate to keyed services — replace manual factory delegates with keyed services if on .NET 8+
5. Use Scrutor for decorator support if not already available:

   <PackageReference Include="Scrutor" />
   

Decision Guide

Scenario	Lifetime	Notes
DbContext	Scoped	One per request
Repository	Scoped	Matches DbContext lifetime
HttpClient handler	Transient	Via IHttpClientFactory
Cache wrapper	Singleton	Thread-safe required
Options	Singleton	Via IOptions<T>
Current user	Scoped	Per-request context
Background service	Singleton	Use IServiceScopeFactory inside

References

• https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection
• https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection-guidelines