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: 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

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 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);
            });
    }<