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C# › Dependency Injection & IoC Principles

How would you implement the Factory pattern using dependency injection to resolve a specific implementation at runtime based on a parameter, when the standard container can't decide statically?

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Answer

Inject a factory delegate (Func<string, IService>) or a dedicated factory interface/class instead of the service directly — the factory itself is registered normally in DI, and its implementation resolves the correct concrete service (often via a keyed lookup among several registered implementations) at the moment it's actually needed, based on runtime information the container itself doesn't have.
public interface IPaymentProcessorFactory { IPaymentProcessor Create(string provider); }

public class PaymentProcessorFactory : IPaymentProcessorFactory {
  private readonly IEnumerable<IPaymentProcessor> _processors;
  public PaymentProcessorFactory(IEnumerable<IPaymentProcessor> processors) { _processors = processors; }
  public IPaymentProcessor Create(string provider) =>
    _processors.First(p => p.ProviderName == provider);
}
Real-world example Selecting between multiple registered payment gateway implementations (Stripe, PayPal, etc.) at runtime based on a user's chosen provider.

Common follow-ups: How does .NET 8's newer KEYED SERVICES feature (AddKeyedSingleton, etc.) simplify this exact pattern natively?

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All 10 Dependency Injection & IoC Principles questions