Assignment compatibility asks whether an expression of type X may be assigned to a variable of type Y. In the type hierarchy, that relation acts like an arrow from X to Y: a Giraffe can be assigned to Animal or Object, but not to Tiger. section 7.2
Covariant generic interfaces
A generic interface does not automatically inherit the assignment relationships of its type argument. An IE<Giraffe> cannot initially be assigned to IE<Animal>. Declaring the type parameter with out makes the interface covariant, because T appears only as an output: IE<Giraffe> can then be assigned to IE<Animal>. section 7.2
Contravariant generic interfaces
Input-only type parameters reverse the direction. With IC<in T>, an IC<Animal> can be assigned to an IC<Giraffe>: a comparer capable of handling every Animal can also handle Giraffe values. The generic construction preserves assignment relationships while reversing their arrows. section 7.3
This perspective connects ordinary C# assignments with category theory. Types serve as objects, assignment compatibility serves as a morphism, and generic interfaces can behave as covariant or contravariant endofunctors. The resulting type relationships form a partial order rather than a single hierarchy: Giraffe and Tiger are incomparable even though both relate to Animal. section 7.2 section 7.4
