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dependency-injection

Dependency injection patterns for .NET 10. Covers service lifetimes, keyed services, the decorator pattern, factory pattern, and common DI pitfalls. Load this skill when registering services, resolving lifetime issues, designing service composition, or when the user mentions "DI", "dependency injection", "service registration", "AddScoped", "AddTransient", "AddSingleton", "keyed services", "decorator", "Scrutor", "IServiceCollection", or "captive dependency".

skills/dependency-injection/SKILL.md

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# 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.