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ASP.NET Core › Dependency Injection

What are keyed services (introduced in .NET 8), and what problem do they solve compared to injecting IEnumerable<T> when you need one specific implementation?

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Answer

Keyed services let you register multiple implementations of the same interface under distinct string or object keys, resolved explicitly via [FromKeyedServices("key")] or GetRequiredKeyedService(key) -- solving the ambiguity of IEnumerable<T> injection (which gives you everything, requiring filtering) when you actually need one specific, named implementation deterministically, without a hand-rolled factory pattern or string-based service locator.
builder.Services.AddKeyedScoped<IPaymentProcessor, StripeProcessor>("stripe");
builder.Services.AddKeyedScoped<IPaymentProcessor, PayPalProcessor>("paypal");

public class CheckoutService([FromKeyedServices("stripe")] IPaymentProcessor processor) {
    // explicitly gets the Stripe implementation
}
Real-world example A multi-payment-provider e-commerce platform uses keyed services to explicitly wire different checkout flows to specific payment processors without needing a custom factory class or dictionary-based lookup that keyed services now handle natively.

Common follow-ups: How do keyed services interact with IEnumerable<T> injection -- can you still get all of them?;What's the migration path from a hand-rolled factory pattern to keyed services?

Configuration & Options Pattern;Controllers vs Minimal APIs
All 15 Dependency Injection questions