dependency-injection

---
name: dependency-injection
description: >
license: MIT
---

# Dependency Injection

## Core Principles

1. **Constructor injection is the default** — Inject dependencies through the constructor (primary constructors make this clean). No service locator, no property injection.
2. **Match lifetimes carefully** — A singleton must never depend on a scoped or transient service. This is the most common DI bug.
3. **Register interfaces, resolve interfaces** — Register `services.AddScoped<IOrderService, OrderService>()`, not the concrete type.
4. **Keyed services for strategy pattern** — .NET 8+ keyed services replace manual factory patterns for selecting between implementations.

## Patterns

### Keyed Services (.NET 8+)

Use keyed services to register and resolve multiple implementations of the same interface.

```csharp
// Registration
builder.Services.AddKeyedScoped<INotificationService, EmailNotificationService>("email");
builder.Services.AddKeyedScoped<INotificationService, SmsNotificationService>("sms");
builder.Services.AddKeyedScoped<INotificationService, PushNotificationService>("push");

// Resolution via attribute
public class OrderHandler([FromKeyedServices("email")] INotificationService notifier)
{
    public async Task Handle(CreateOrder.Command command, CancellationToken ct)
    {
        // ... create order
        await notifier.SendAsync(notification, ct);
    }
}

// Resolution via IServiceProvider
public class NotificationRouter(IServiceProvider provider)
{
    public INotificationService GetService(string channel)
    {
        return provider.GetRequiredKeyedService<INotificationService>(channel);
    }
}
```

### Decorator Pattern

```csharp
// Base service
public interface IOrderService
{
    Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct);
}

public class OrderService(AppDbContext db, TimeProvider clock) : IOrderService
{
    public async Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct)
    {
        var order = Order.Create(request, clock.GetUtcNow());
        db.Orders.Add(order);
        await db.SaveChangesAsync(ct);
        return Result.Success(order);
    }
}

// Decorator — adds logging
public class LoggingOrderService(IOrderService inner, ILogger<LoggingOrderService> logger) : IOrderService
{
    public async Task<Result<Order>> CreateAsync(CreateOrderRequest request, CancellationToken ct)
    {
        logger.LogInformation("Creating order for customer {CustomerId}", request.CustomerId);
        var result = await inner.CreateAsync(request, ct);
        if (result.IsSuccess)
            logger.LogInformation("Order {OrderId} created", result.Value.Id);
        return result;
    }
}

// Registration with Scrutor
builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.Decorate<IOrderService, LoggingOrderService>();
```

### Registration by Convention (Scrutor)

```csharp
// Auto-register all services matching a convention
builder.Services.Scan(scan => scan
    .FromAssemblyOf<Program>()
    .AddClasses(classes => classes.AssignableTo<ITransientService>())
    .AsImplementedInterfaces()
    .WithTransientLifetime()
    .AddClasses(classes => classes.AssignableTo<IScopedService>())
    .AsImplementedInterfaces()
    .WithScopedLifetime());
```

### Factory Pattern

When you need runtime logic to select an implementation.

```csharp
builder.Services.AddScoped<IPaymentProcessor>(sp =>
{
    var config = sp.GetRequiredService<IOptions<PaymentOptions>>().Value;
    return config.Provider switch
    {
        "stripe" => ActivatorUtilities.CreateInstance<StripeProcessor>(sp),
        "paypal" => ActivatorUtilities.CreateInstance<PayPalProcessor>(sp),
        _ => throw new InvalidOperationException($"Unknown payment provider: {config.Provider}")
    };
});
```

### Options Registration

```csharp
// Bind configuration section to a strongly-typed options class
builder.Services.AddOptions<JwtOptions>()
    .BindConfiguration("Jwt")
    .ValidateDataAnnotations()
    .ValidateOnStart();

// Inject as IOptions<T>
public class TokenService(IOptions<JwtOptions> options)
{
    private readonly JwtOptions _jwt = options.Value;
}
```

## Anti-patterns

### Don't Capture Scoped Services in Singletons

```csharp
// BAD — DbContext is scoped, captured by singleton = memory leak + stale data
builder.Services.AddSingleton<OrderCache>(); // depends on AppDbContext

// GOOD — use IServiceScopeFactory in singleton
public class OrderCache(IServiceScopeFactory scopeFactory)
{
    public async Task<Order?> GetAsync(Guid id)
    {
        await using var scope = scopeFactory.CreateAsyncScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        return await db.Orders.FindAsync(id);
    }
}
```

### Don't Register Everything as Singleton

```csharp
// BAD — making a service singleton when it holds mutable state
builder.Services.AddSingleton<OrderService>(); // has DbContext dependency

// GOOD — match the lifetime to the service's needs
builder.Services.AddScoped<OrderService>();
```

## Decision Guide

| Scenario | Recommendation |
|----------|---------------|
| Stateless service | Scoped (default) or Transient |
| Configuration / cache | Singleton |
| DbContext | Scoped (registered by `AddDbContext`) |
| Multiple implementations | Keyed services (strategy pattern) |
| Cross-cutting behavior | Decorator pattern |
| Convention-based registration | Scrutor |
| Runtime implementation selection | Factory delegate |
| Audit existing registrations | `get_di_registrations` MCP tool — lifetimes, duplicates, captive-dependency risks in one call |
| Strongly-typed config | `AddOptions<T>().BindConfiguration()` |