Covariance and contravariance are terms that refer to the ability to use a less derived (less specific) or more derived type (more specific) than originally specified. Generic type parameters support covariance and contravariance to provide greater flexibility in assigning and using generic types. When you are referring to a type system, covariance, contravariance, and invariance have the following definitions. The examples assume a base class named Base and a derived class named Derived
Covariance
Enables you to use a more derived type than originally specified.
You can assign an instance of IEnumerable<Derived> (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable<Base>.
Contravariance
Enables you to use a more generic (less derived) type than originally specified.
You can assign an instance of IEnumerable<Base> (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable<Derived>.
Invariance
Means that you can use only the type originally specified; so an invariant generic type parameter is neither covariant nor contravariant.
You cannot assign an instance of IEnumerable<Base> (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable<Derived> or vice versa.
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Covariance and Contravariance in Generics
What is covariance and contravariance in c# ?
