Transient, Scoped, Singleton, and the BackgroundService trap.

DI in .NET 9 — Lifetimes, Keyed Services, Scope Pattern

The .NET built-in DI container registers services with three lifetimes — Transient (new per request), Scoped (new per HTTP request / scope), Singleton (one for app lifetime). .NET 8 added Keyed Services (IKeyedServiceProvider) for named service variants. Captive dependency bugs (Singleton capturing Scoped) are caught at startup. BackgroundServices must use IServiceScopeFactory, not inject Scoped services directly.

Interactive Pipeline Flow

FoundationCore LogicProduction

📝 Register — builder.Services.Add...

Register services at startup: services.AddTransient<T>(), AddScoped<T>(), AddSingleton<T>(), or AddSingleton<IFoo, FooImpl>().

Architecture Diagrams

Named service variants with Keyed Services in .NET 8+.

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Key Design Decisions

🚨Enable ValidateScopes in development

services.Configure<ServiceProviderOptions>(o => { o.ValidateScopes = true; o.ValidateOnBuild = true; }) catches captive dep bugs at startup.

🔑Keyed services replace factory patterns

Instead of injecting a factory that returns ICache by name, use keyed services and [FromKeyedServices]. Less code, fully DI managed.

🏭IServiceScopeFactory for BackgroundService

Never inject Scoped services into Singleton BackgroundService constructors. Always create a scope per work unit with IServiceScopeFactory.

Discussion