/microsoft-extensions-dependency-injection

name: dependency-injection-patterns
description: Organize DI registrations using IServiceCollection extension methods. Group related services into composable Add* methods for clean Program.cs and reusable configuration in tests.
user-invocable: false

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

Reference Files

• [advanced-patterns.md](advanced-patterns.md): Testing with DI extensions, Akka.NET actor scope management, conditional/factory/keyed registration patterns

---

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>();
// ... 150 more lines ...

Problems: hard to find related registrations, no clear boundaries, can't reuse in tests, merge conflicts.

---

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();

---

Extension Method Pattern

Basic Structure

namespace MyApp.Users;

public static class UserServiceCollectionExtensions
{
    public static IServiceCollection AddUserServices(this IServiceCollection services)
    {
        services.AddScoped<IUserRepository, UserRepository>();
        services.AddScoped<IUserReadStore, UserReadStore>();
        services.AddScoped<IUserWriteStore, UserWriteStore>();
        services.AddScoped<IUserService, UserService>();
        services.AddScoped<IUserValidationService, UserValidationService>();

        return services;
    }
}

With Configuration

namespace MyApp.Email;

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

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

        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
    UserServiceCollectionExtensions.cs   # AddUserServices()
  MyApp.Orders/
    OrderServiceCollectionExtensions.cs  # AddOrderServices()
  MyApp.Email/
    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) |

---

Testing Benefits

The `Add*` pattern lets you **reuse production configuration in tests** and only override what's different. Works with WebApplicationFactory, Akka.Hosting.TestKit, and standalone ServiceCollection.

See [advanced-patterns.md](advanced-patterns.md) for complete testing examples.

---

Layered Extensions

For larger applications, compose extensions hierarchically:

public static class AppServiceCollectionExtensions
{
    public static IServiceCollection AddAppServices(this IServiceCollection services)
    {
        return services
            .AddDomainServices()
            .AddInfrastructureServices()
            .AddApiServices();
    }
}

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

---

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);
            });
    }
}

// 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.

---

Anti-Patterns

Don't: Register Everything in Program.cs

// BAD: Massive Program.cs with 200+ lines of registrations

Don't: Create Overly Generic Extensions

// BAD: Too vague, doesn't communicate what's