microsoft-extensions-dependency-injection

What this skill does

# 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:

```csharp
// 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:

```csharp
// 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

```csharp
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

```csharp
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

```csharp
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) |

```csharp
// 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

```csharp
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

```csharp
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

```csharp
public class UserServiceTests
{
    private readonly ServiceProvider _provider;

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

        // Reuse production registrations
        services.AddUserServices();

        // Add test infrastructure
        services.AddSin