Dependency Injection Patterns

When to Use This Skill

Use this skill when:

• Organizing service registrations in ASP.NET Core applications
• Avoiding massive Program.cs/Startup.cs files with hundreds of registrations
• Making service configuration reusable between production and tests
• Designing libraries that integrate with Microsoft.Extensions.DependencyInjection

---

The Problem

Without organization, Program.cs becomes unmanageable:

// BAD: 200+ lines of unorganized registrations
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddScoped<IUserRepository, UserRepository>();
builder.Services.AddScoped<IOrderRepository, OrderRepository>();
builder.Services.AddScoped<IProductRepository, ProductRepository>();
builder.Services.AddScoped<IUserService, UserService>();
builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.AddScoped<IEmailSender, SmtpEmailSender>();
builder.Services.AddScoped<IEmailComposer, MjmlEmailComposer>();
builder.Services.AddSingleton<IEmailLinkGenerator, EmailLinkGenerator>();
builder.Services.AddScoped<IPaymentProcessor, StripePaymentProcessor>();
builder.Services.AddScoped<IInvoiceGenerator, InvoiceGenerator>();
// ... 150 more lines ...

Problems:

• Hard to find related registrations
• No clear boundaries between subsystems
• Can't reuse configuration in tests
• Merge conflicts in team settings
• No encapsulation of internal dependencies

---

The Solution: Extension Method Composition

Group related registrations into extension methods:

// GOOD: Clean, composable Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Services
    .AddUserServices()
    .AddOrderServices()
    .AddEmailServices()
    .AddPaymentServices()
    .AddValidators();

var app = builder.Build();

Each Add* method encapsulates a cohesive set of registrations.

---

Extension Method Pattern

Basic Structure

namespace MyApp.Users;

public static class UserServiceCollectionExtensions
{
    public static IServiceCollection AddUserServices(this IServiceCollection services)
    {
        // Repositories
        services.AddScoped<IUserRepository, UserRepository>();
        services.AddScoped<IUserReadStore, UserReadStore>();
        services.AddScoped<IUserWriteStore, UserWriteStore>();

        // Services
        services.AddScoped<IUserService, UserService>();
        services.AddScoped<IUserValidationService, UserValidationService>();

        // Return for chaining
        return services;
    }
}

With Configuration

namespace MyApp.Email;

public static class EmailServiceCollectionExtensions
{
    public static IServiceCollection AddEmailServices(
        this IServiceCollection services,
        string configSectionName = "EmailSettings")
    {
        // Bind configuration
        services.AddOptions<EmailOptions>()
            .BindConfiguration(configSectionName)
            .ValidateDataAnnotations()
            .ValidateOnStart();

        // Register services
        services.AddSingleton<IMjmlTemplateRenderer, MjmlTemplateRenderer>();
        services.AddSingleton<IEmailLinkGenerator, EmailLinkGenerator>();
        services.AddScoped<IUserEmailComposer, UserEmailComposer>();
        services.AddScoped<IOrderEmailComposer, OrderEmailComposer>();

        // SMTP client depends on environment
        services.AddScoped<IEmailSender, SmtpEmailSender>();

        return services;
    }
}

With Dependencies on Other Extensions

namespace MyApp.Orders;

public static class OrderServiceCollectionExtensions
{
    public static IServiceCollection AddOrderServices(this IServiceCollection services)
    {
        // This subsystem depends on email services
        // Caller is responsible for calling AddEmailServices() first
        // Or we can call it here if it's idempotent

        services.AddScoped<IOrderRepository, OrderRepository>();
        services.AddScoped<IOrderService, OrderService>();
        services.AddScoped<IOrderEmailNotifier, OrderEmailNotifier>();

        return services;
    }
}

---

File Organization

Place extension methods near the services they register:

src/
  MyApp.Api/
    Program.cs                    # Composes all Add* methods
  MyApp.Users/
    Services/
      UserService.cs
      IUserService.cs
    Repositories/
      UserRepository.cs
    UserServiceCollectionExtensions.cs   # AddUserServices()
  MyApp.Orders/
    Services/
      OrderService.cs
    OrderServiceCollectionExtensions.cs  # AddOrderServices()
  MyApp.Email/
    Composers/
      UserEmailComposer.cs
    EmailServiceCollectionExtensions.cs  # AddEmailServices()

Convention: {Feature}ServiceCollectionExtensions.cs next to the feature's services.

---

Naming Conventions

Pattern	Use For
Add{Feature}Services()	General feature registration
Add{Feature}()	Short form when unambiguous
Configure{Feature}()	When primarily setting options
Use{Feature}()	Middleware (on IApplicationBuilder)

// Feature services
services.AddUserServices();
services.AddEmailServices();
services.AddPaymentServices();

// Third-party integrations
services.AddStripePayments();
services.AddSendGridEmail();

// Configuration-heavy
services.ConfigureAuthentication();
services.ConfigureAuthorization();

---

Testing Benefits

The main advantage: reuse production configuration in tests.

WebApplicationFactory

public class ApiTests : IClassFixture<WebApplicationFactory<Program>>
{
    private readonly WebApplicationFactory<Program> _factory;

    public ApiTests(WebApplicationFactory<Program> factory)
    {
        _factory = factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureServices(services =>
            {
                // Production services already registered via Add* methods
                // Only override what's different for testing

                // Replace email sender with test double
                services.RemoveAll<IEmailSender>();
                services.AddSingleton<IEmailSender, TestEmailSender>();

                // Replace external payment processor
                services.RemoveAll<IPaymentProcessor>();
                services.AddSingleton<IPaymentProcessor, FakePaymentProcessor>();
            });
        });
    }

    [Fact]
    public async Task CreateOrder_SendsConfirmationEmail()
    {
        var client = _factory.CreateClient();
        var emailSender = _factory.Services.GetRequiredService<IEmailSender>() as TestEmailSender;

        await client.PostAsJsonAsync("/api/orders", new CreateOrderRequest(...));

        Assert.Single(emailSender!.SentEmails);
    }
}

Akka.Hosting.TestKit

public class OrderActorSpecs : Akka.Hosting.TestKit.TestKit
{
    protected override void ConfigureAkka(AkkaConfigurationBuilder builder, IServiceProvider provider)
    {
        // Reuse production Akka configuration
        builder.AddOrderActors();
    }

    protected override void ConfigureServices(IServiceCollection services)
    {
        // Reuse production service configuration
        services.AddOrderServices();

        // Override only external dependencies
        services.RemoveAll<IPaymentProcessor>();
        services.AddSingleton<IPaymentProcessor, FakePaymentProcessor>();
    }

    [Fact]
    public async Task OrderActor_ProcessesPayment()
    {
        var orderActor = ActorRegistry.Get<OrderActor>();
        orderActor.Tell(new ProcessOrder(orderId));

        ExpectMsg<OrderProcessed>();
    }
}

Standalone Unit Tests

public class UserServiceTests
{
    private readonly ServiceProvider _provider;

    public UserServiceTests()
    {
        var services = new ServiceCollection();

        // Reuse production registrations
        services.AddUserServices();

        // Add test infrastructure
        services.AddSingleton<IUserRepository, InMemoryUserRepository>();

        _provider = services.BuildServiceProvider();
    }

    [Fact]
    public async Task CreateUser_ValidData_Succeeds()
    {
        var service = _provider.GetRequiredService<IUserService>();
        var result = await service.CreateUserAsync(new CreateUserRequest(...));

        Assert.True(result.IsSuccess);
    }
}

---

Layered Extensions

For larger applications, compose extensions hierarchically:

// Top-level: Everything the app needs
public static class AppServiceCollectionExtensions
{
    public static IServiceCollection AddAppServices(this IServiceCollection services)
    {
        return services
            .AddDomainServices()
            .AddInfrastructureServices()
            .AddApiServices();
    }
}

// Domain layer
public static class DomainServiceCollectionExtensions
{
    public static IServiceCollection AddDomainServices(this IServiceCollection services)
    {
        return services
            .AddUserServices()
            .AddOrderServices()
            .AddProductServices();
    }
}

// Infrastructure layer
public static class InfrastructureServiceCollectionExtensions
{
    public static IServiceCollection AddInfrastructureServices(this IServiceCollection services)
    {
        return services
            .AddEmailServices()
            .AddPaymentServices()
            .AddStorageServices();
    }
}

---

Akka.Hosting Integration

The same pattern works for Akka.NET actor configuration:

public static class OrderActorExtensions
{
    public static AkkaConfigurationBuilder AddOrderActors(
        this AkkaConfigurationBuilder builder)
    {
        return builder
            .WithActors((system, registry, resolver) =>
            {
                var orderProps = resolver.Props<OrderActor>();
                var orderRef = system.ActorOf(orderProps, "orders");
                registry.Register<OrderActor>(orderRef);
            })
            .WithShardRegion<OrderShardActor>(
                typeName: "order-shard",
                (system, registry, resolver) =>
                    entityId => resolver.Props<OrderShardActor>(entityId),
                new OrderMessageExtractor(),
                ShardOptions.Create());
    }
}

// Usage in Program.cs
builder.Services.AddAkka("MySystem", (builder, sp) =>
{
    builder
        .AddOrderActors()
        .AddInventoryActors()
        .AddNotificationActors();
});

See akka/hosting-actor-patterns skill for complete Akka.Hosting patterns.

---

Common Patterns

Conditional Registration

public static IServiceCollection AddEmailServices(
    this IServiceCollection services,
    IHostEnvironment environment)
{
    services.AddSingleton<IEmailComposer, MjmlEmailComposer>();

    if (environment.IsDevelopment())
    {
        // Use Mailpit in development
        services.AddSingleton<IEmailSender, MailpitEmailSender>();
    }
    else
    {
        // Use real SMTP in production
        services.AddSingleton<IEmailSender, SmtpEmailSender>();
    }

    return services;
}

Factory-Based Registration

public static IServiceCollection AddPaymentServices(
    this IServiceCollection services,
    string configSection = "Stripe")
{
    services.AddOptions<StripeOptions>()
        .BindConfiguration(configSection)
        .ValidateOnStart();

    // Factory for complex initialization
    services.AddSingleton<IPaymentProcessor>(sp =>
    {
        var options = sp.GetRequiredService<IOptions<StripeOptions>>().Value;
        var logger = sp.GetRequiredService<ILogger<StripePaymentProcessor>>();

        return new StripePaymentProcessor(options.ApiKey, options.WebhookSecret, logger);
    });

    return services;
}

Keyed Services (.NET 8+)

public static IServiceCollection AddNotificationServices(this IServiceCollection services)
{
    // Register multiple implementations with keys
    services.AddKeyedSingleton<INotificationSender, EmailNotificationSender>("email");
    services.AddKeyedSingleton<INotificationSender, SmsNotificationSender>("sms");
    services.AddKeyedSingleton<INotificationSender, PushNotificationSender>("push");

    // Resolver that picks the right one
    services.AddScoped<INotificationDispatcher, NotificationDispatcher>();

    return services;
}

---

Anti-Patterns

Don't: Register Everything in Program.cs

// BAD: Massive Program.cs
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddScoped<IUserRepository, UserRepository>();
builder.Services.AddScoped<IOrderRepository, OrderRepository>();
// ... 200 more lines ...

Don't: Create Overly Generic Extensions

// BAD: Too vague, doesn't communicate what's registered
public static IServiceCollection AddServices(this IServiceCollection services)
{
    // Registers 50 random things
}

Don't: Hide Important Configuration

// BAD: Buried important settings
public static IServiceCollection AddDatabase(this IServiceCollection services)
{
    services.AddDbContext<AppDbContext>(options =>
        options.UseSqlServer("hardcoded-connection-string"));  // Hidden!
}

// GOOD: Accept configuration explicitly
public static IServiceCollection AddDatabase(
    this IServiceCollection services,
    string connectionString)
{
    services.AddDbContext<AppDbContext>(options =>
        options.UseSqlServer(connectionString));
}

---

Best Practices Summary

Practice	Benefit
Group related services into Add* methods	Clean Program.cs, clear boundaries
Place extensions near the services they register	Easy to find and maintain
Return IServiceCollection for chaining	Fluent API
Accept configuration parameters	Flexibility
Use consistent naming (Add{Feature}Services)	Discoverability
Test by reusing production extensions	Confidence, less duplication

---

Lifetime Management

Choose the right lifetime based on state:

Lifetime	Use When	Examples
Singleton	Stateless, thread-safe, expensive to create	Configuration, HttpClient factories, caches
Scoped	Stateful per-request, database contexts	DbContext, repositories, user context
Transient	Lightweight, stateful, cheap to create	Validators, short-lived helpers

Rules of Thumb

// SINGLETON: Stateless services, shared safely
services.AddSingleton<IMjmlTemplateRenderer, MjmlTemplateRenderer>();
services.AddSingleton<IEmailLinkGenerator, EmailLinkGenerator>();

// SCOPED: Database access, per-request state
services.AddScoped<IUserRepository, UserRepository>();  // DbContext dependency
services.AddScoped<IOrderService, OrderService>();       // Uses scoped repos

// TRANSIENT: Cheap, short-lived
services.AddTransient<CreateUserRequestValidator>();

Scope Requirements

Scoped services require a scope to exist. In ASP.NET Core, each HTTP request creates a scope automatically. But in other contexts (background services, actors), you must create scopes manually.

// ASP.NET Controller - scope exists automatically
public class OrdersController : ControllerBase
{
    private readonly IOrderService _orderService;  // Scoped - works!

    public OrdersController(IOrderService orderService)
    {
        _orderService = orderService;
    }
}

// Background Service - no automatic scope!
public class OrderProcessingService : BackgroundService
{
    private readonly IServiceProvider _serviceProvider;

    public OrderProcessingService(IServiceProvider serviceProvider)
    {
        // Inject IServiceProvider, NOT scoped services directly
        _serviceProvider = serviceProvider;
    }

    protected override async Task ExecuteAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            // Create scope manually for each unit of work
            using var scope = _serviceProvider.CreateScope();
            var orderService = scope.ServiceProvider.GetRequiredService<IOrderService>();

            await orderService.ProcessPendingOrdersAsync(ct);
            await Task.Delay(TimeSpan.FromMinutes(1), ct);
        }
    }
}

---

Akka.NET Actor Scope Management

Actors don't have automatic DI scopes. If you need scoped services inside an actor, inject IServiceProvider and create scopes manually.

Pattern: Scope Per Message

public sealed class AccountProvisionActor : ReceiveActor
{
    private readonly IServiceProvider _serviceProvider;
    private readonly IActorRef _mailingActor;

    public AccountProvisionActor(
        IServiceProvider serviceProvider,
        IRequiredActor<MailingActor> mailingActor)
    {
        _serviceProvider = serviceProvider;
        _mailingActor = mailingActor.ActorRef;

        ReceiveAsync<ProvisionAccount>(HandleProvisionAccount);
    }

    private async Task HandleProvisionAccount(ProvisionAccount msg)
    {
        // Create scope for this message processing
        using var scope = _serviceProvider.CreateScope();

        // Resolve scoped services
        var userManager = scope.ServiceProvider.GetRequiredService<UserManager<User>>();
        var orderRepository = scope.ServiceProvider.GetRequiredService<IOrderRepository>();
        var emailComposer = scope.ServiceProvider.GetRequiredService<IPaymentEmailComposer>();

        // Do work with scoped services
        var user = await userManager.FindByIdAsync(msg.UserId);
        var order = await orderRepository.CreateAsync(msg.Order);

        // DbContext commits when scope disposes
    }
}

Why This Pattern Works

1. Each message gets fresh DbContext - No stale entity tracking
2. Proper disposal - Connections released after each message
3. Isolation - One message's errors don't affect others
4. Testable - Can inject mock IServiceProvider

Singleton Services in Actors

For stateless services, inject directly (no scope needed):

public sealed class NotificationActor : ReceiveActor
{
    private readonly IEmailLinkGenerator _linkGenerator;  // Singleton - OK!
    private readonly IActorRef _mailingActor;

    public NotificationActor(
        IEmailLinkGenerator linkGenerator,  // Direct injection
        IRequiredActor<MailingActor> mailingActor)
    {
        _linkGenerator = linkGenerator;
        _mailingActor = mailingActor.ActorRef;

        Receive<SendWelcomeEmail>(Handle);
    }
}

Akka.DependencyInjection Reference

Akka.NET's DI integration is documented at:

• Akka.DependencyInjection: https://getakka.net/articles/actors/dependency-injection.html
• Akka.Hosting: https://github.com/akkadotnet/Akka.Hosting

---

Common Mistakes

Injecting Scoped into Singleton

// BAD: Singleton captures scoped service - stale DbContext!
public class CacheService  // Registered as Singleton
{
    private readonly IUserRepository _repo;  // Scoped!

    public CacheService(IUserRepository repo)  // Captured at startup!
    {
        _repo = repo;  // This DbContext lives forever - BAD
    }
}

// GOOD: Inject factory or IServiceProvider
public class CacheService
{
    private readonly IServiceProvider _serviceProvider;

    public CacheService(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public async Task<User> GetUserAsync(string id)
    {
        using var scope = _serviceProvider.CreateScope();
        var repo = scope.ServiceProvider.GetRequiredService<IUserRepository>();
        return await repo.GetByIdAsync(id);
    }
}

No Scope in Background Work

// BAD: No scope for scoped services
public class BadBackgroundService : BackgroundService
{
    private readonly IOrderService _orderService;  // Scoped!

    public BadBackgroundService(IOrderService orderService)
    {
        _orderService = orderService;  // Will throw or behave unexpectedly
    }
}

// GOOD: Create scope for each unit of work
public class GoodBackgroundService : BackgroundService
{
    private readonly IServiceScopeFactory _scopeFactory;

    public GoodBackgroundService(IServiceScopeFactory scopeFactory)
    {
        _scopeFactory = scopeFactory;
    }

    protected override async Task ExecuteAsync(CancellationToken ct)
    {
        using var scope = _scopeFactory.CreateScope();
        var orderService = scope.ServiceProvider.GetRequiredService<IOrderService>();
        // ...
    }
}

---

Resources

• Microsoft.Extensions.DependencyInjection: https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection
• Akka.Hosting: https://github.com/akkadotnet/Akka.Hosting
• Akka.DependencyInjection: https://getakka.net/articles/actors/dependency-injection.html
• Options Pattern: See microsoft-extensions/configuration skill