When defining a generic interface, have you received a hint from Resharper like “The type parameter T could be declared as covariant” (or “contravariant”)? If so, have you then blindly applied the proposed refactoring which decorates your generic parameter with the in or out keyword? Like so: I know I’ve done this a few times before deciding to dig deeper into what these terms actually mean and how they affect my type’s behavior. Type variance is one of the topics …