Dotnet Extensions Dependency Injection

Dependency Injection Patterns

Organize Microsoft.Extensions.DependencyInjection (DI) registrations into composable IServiceCollection extension methods. Eliminates massive Program.cs files, enables service reuse in tests, and enforces correct lifetime management.

Acronyms: DI (Dependency Injection), DML (Data Manipulation Language).

When to Use This Skill

• Organizing service registrations in ASP.NET Core applications to avoid bloated Program.cs files
• Designing composable Add* extension methods that group related services into cohesive units
• Making service configuration reusable between production application and integration test projects
• Choosing correct service lifetimes (Singleton, Scoped, Transient) based on state and threading
• Fixing scope-related bugs where scoped services are injected into singletons or background services
• Implementing factory-based registration with IServiceProvider for complex initialization logic
• Creating conditional service registration that varies by environment (development vs production)

Related Skills

Core Principles

1. Compose via Extension Methods — Group related registrations into Add{Feature}Services() methods on IServiceCollection. Why: keeps Program.cs clean and enables reuse.
2. Return for Chaining — Every extension method returns IServiceCollection to support fluent composition. Why: consistent API that reads as a pipeline.
3. Correct Lifetime by Default — Choose Singleton for stateless/thread-safe, Scoped for per-request state, Transient for lightweight/cheap. Why: wrong lifetimes cause stale data or memory leaks.
4. Explicit Configuration — Accept configuration parameters instead of hardcoding connection strings or secrets. Why: hidden settings cause deployment failures.
5. Scope Per Unit of Work — In background services and actors, create a scope for each unit of work. Why: scoped services require an explicit scope outside HTTP request pipelines.

Values: 基礎と型の追求（Add* メソッドという「型」を徹底することで、どのプロジェクトでも再利用可能な DI 構造の基盤を作る）, 成長の複利（テストでの再利用を設計に組み込み、実装と品質が同時に成長する構造を作る）

Workflow: Organize DI Registrations

Step 1: Create Feature Extension Methods

Group related services into a single Add{Feature}Services() extension method placed near the services it registers. Why: co-location makes registrations discoverable.

namespace MyApp.Users;

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

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

        // Return for chaining
        return services;
    }
}

File placement convention: {Feature}ServiceCollectionExtensions.cs next to the feature's services.

src/
  MyApp.Api/
    Program.cs                                  # Composes all Add* methods
  MyApp.Users/
    Services/
      UserService.cs
    UserServiceCollectionExtensions.cs          # AddUserServices()
  MyApp.Email/
    EmailServiceCollectionExtensions.cs         # AddEmailServices()

Values: 基礎と型の追求（命名規則とファイル配置の「型」が、チーム全体の発見可能性を支える）

Step 2: Add Configuration Binding

Use IOptions<T> with BindConfiguration for feature-specific settings. Accept the config section name as a parameter for flexibility. Why: explicit configuration prevents hidden deployment failures.

namespace MyApp.Email;

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

        // Register services
        services.AddSingleton<IMjmlTemplateRenderer, MjmlTemplateRenderer>();
        services.AddScoped<IUserEmailComposer, UserEmailComposer>();
        services.AddScoped<IEmailSender, SmtpEmailSender>();

        return services;
    }
}

Values: ニュートラルな視点（設定を外部パラメータ化し、環境に依存しない普遍的な設計を保つ）

Step 3: Choose Correct Lifetimes

Select lifetimes based on state and thread-safety. Why: wrong lifetimes cause the most common DI bugs — stale DbContext, captive dependencies, and memory leaks.

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

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

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

Values: 温故知新（DI コンテナの基本原則を正しく理解し、.NET の進化した機能と組み合わせる）

Step 4: Compose in Program.cs

Chain all Add* calls in Program.cs for a clean, scannable entry point. Why: composition at the top level reveals the application's dependency structure at a glance.

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

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

var app = builder.Build();

Values: 余白の設計（Program.cs を最小限に保つことで、構造の見通しという余白を確保する）

Step 5: Handle Scopes in Background Services

In background services, create a scope for each unit of work. Why: scoped services (DbContext, repositories) require an explicit scope outside ASP.NET Core's per-request pipeline.

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

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

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

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

Values: 継続は力（スコープ管理という地道な「型」をコツコツ守ることで、本番環境の安定性を積み上げる）

Step 6: Reuse Extensions in Tests

Use Add* methods in test setup to reuse production configuration. Override only external dependencies with test doubles. Why: test confidence comes from running real registrations.

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 external dependencies for testing
                services.RemoveAll<IEmailSender>();
                services.AddSingleton<IEmailSender, TestEmailSender>();

                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);
    }
}

Values: 成長の複利（テストでプロダクションコードを再利用する設計が、実装と品質を同時に成長させる）

Good Practices

• ✅ Group related services into Add{Feature}Services() methods for clear boundaries
• ✅ Place extension methods near the services they register for discoverability
• ✅ Return IServiceCollection from every extension method for fluent chaining
• ✅ Accept configuration parameters explicitly instead of hardcoding values
• ✅ Use consistent naming: Add{Feature}Services() for features, Configure{Feature}() for options
• ✅ Use IServiceScopeFactory in background services to create scopes per unit of work
• ✅ Reuse production Add* methods in test setup for realistic configuration
• ✅ Use ValidateOnStart() with IOptions<T> to catch configuration errors at startup
• ✅ Use conditional registration with IHostEnvironment for environment-specific services
• ✅ Accept CancellationToken in all async service methods

Common Pitfalls

1. Captive Dependency — Injecting a Scoped service into a Singleton captures a stale instance forever. Fix: inject IServiceProvider or IServiceScopeFactory and create scopes manually.
2. No Scope in Background Work — Directly injecting scoped services into BackgroundService throws or returns stale data. Fix: use IServiceScopeFactory.CreateScope() per iteration.
3. Hidden Configuration — Hardcoding connection strings or secrets inside extension methods. Fix: accept configuration values as method parameters or use IOptions<T>.
4. Overly Generic Extensions — Creating AddServices() that registers 50 unrelated things. Fix: split into feature-specific Add{Feature}Services() methods.
5. Missing Return Statement — Forgetting to return IServiceCollection from the extension method. Fix: always end with return services; for chaining.

Anti-Patterns

❌ Massive Program.cs → ✅ Composable Extensions

// ❌ BAD: 200+ lines of unorganized registrations
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddScoped<IUserRepository, UserRepository>();
builder.Services.AddScoped<IOrderRepository, OrderRepository>();
// ... 150 more lines of mixed registrations ...

// ✅ GOOD: Clean composition with clear structure
builder.Services
    .AddUserServices()
    .AddOrderServices()
    .AddEmailServices();

❌ Scoped into Singleton → ✅ Scope Factory

// ❌ BAD: Singleton captures scoped service — stale DbContext!
public class CacheService  // Registered as Singleton
{
    private readonly IUserRepository _repo;  // Scoped — captured at startup!
    public CacheService(IUserRepository repo) { _repo = repo; }
}

// ✅ GOOD: Create scope when needed
public class CacheService
{
    private readonly IServiceScopeFactory _scopeFactory;
    public CacheService(IServiceScopeFactory scopeFactory) { _scopeFactory = scopeFactory; }

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

❌ Hidden Connection String → ✅ Explicit Parameter

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

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

Quick Reference

Naming Convention Guide

Lifetime Decision Table

Conditional Registration Pattern

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

    if (environment.IsDevelopment())
        services.AddSingleton<IEmailSender, MailpitEmailSender>();
    else
        services.AddSingleton<IEmailSender, SmtpEmailSender>();

    return services;
}

Resources

• Microsoft.Extensions.DependencyInjection
• Service Lifetimes
• Options Pattern
• references/detailed-patterns.md — Layered composition, Akka.Hosting integration, keyed services, actor scope management