All 16 Dependency Injection questions
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.NET › Dependency Injection

What are keyed services (introduced in .NET 8), and what problem do they solve compared to multiple unnamed registrations?

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Answer

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

public class CheckoutService {
    public CheckoutService([FromKeyedServices("stripe")] IPaymentProcessor processor) {
        _processor = 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 (stripe, paypal, applepay) 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;RESTful Web APIs & Controllers
All 16 Dependency Injection questions