dotnet-csharp-dependency-injection

Advanced Microsoft.Extensions.DependencyInjection patterns for .NET applications. Covers service lifetimes, keyed services (net8.0+), decoration, factory delegates, scope validation, and hosted service registration.

Cross-references: [skill:dotnet-csharp-async-patterns] for BackgroundService async patterns, [skill:dotnet-csharp-configuration] for IOptions<T> binding.

Service Lifetimes

Lifetime	Registration	When to Use
Transient	`AddTransient<T>()`	Lightweight, stateless services. New instance per injection.
Scoped	`AddScoped<T>()`	Per-request state (EF Core `DbContext`, unit of work).
Singleton	`AddSingleton<T>()`	Thread-safe, stateless, or shared state (caches, config).

Lifetime Mismatches (Captive Dependencies)

Never inject a shorter-lived service into a longer-lived one: