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.