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Generic variance

C# generic interfaces can be covariant (out T) or contravariant (in T). Awaiten honors that when it resolves. A handler registered for a base event type can satisfy a request for a derived one, the same way the compiler would allow the assignment.

Contravariant handlers​

A handler declared with in T accepts more derived types. Register one for DomainEvent and it satisfies a request for IHandler<OrderPlaced>.

public interface IHandler<in T>;
public sealed class AuditHandler : IHandler<DomainEvent>;

[Container]
[Transient<AuditHandler, IHandler<DomainEvent>>]
[Transient<OrderConsumer>]      // needs IHandler<OrderPlaced>
public static partial class CoffeeShop;

IHandler<OrderPlaced> handler = shop.Resolve<IHandler<OrderPlaced>>();

Covariant factories​

A factory declared with out T produces more derived types. Register one for OrderPlaced and it satisfies a request for IFactory<DomainEvent>.

public interface IFactory<out T> where T : DomainEvent;

The rules​

  • An exact registration always wins over a variance match.
  • The nearest closure wins, mirroring how the compiler resolves overloads.
  • Invariant interfaces never match a different closure.
  • Keyed registrations are never variance candidates.

Variance applies to single resolution, to collections (exact matches lead, variance matches are unioned in), to awaited collections, and to Func<T> relationships.

Where to go next​