When to use Covariance and Contravariance?

Question

C++ / C#

I still do not understand where Covariance and Contravariance can be applied. Can someone explain? In which cases ?

Answer 1

Return the most specific type from the method, and take the most general type in the arguments.

Answer 2

https://en.wikipedia.org/wiki/Covariance_and_contr...

For example, in C#, if Cat is a subtype of Animal, then:

• IEnumerable<Cat>is a subtype of IEnumerable<Animal>. The subtyping is preserved because IEnumerable<T>is covariant on T.
  
• Action<Animal>is a subtype of Action<Cat>. The subtyping is reversed because Action<T>is contravariant on T.
  

I don't even know what to add.