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Covariance: Assign Derived class to Base class. Enables you to use a more derived type than originally specified. You can assign an instance of IEnumerable<derived> to a variable of type IEnumerable<base />.

Contravariance: Assign Base class to Derived class. Enables you to use a more generic (less derived) type than originally specified. You can assign an instance of IEnumerable<base />to a variable of type IEnumerable<derived>.

Invariance: Supports only the type of class originally specified. So it is neither covariant nor contravariant. You cannot assign an instance of IEnumerable<base /> to a variable of type IEnumerable<derived> or vice versa.

Quote:

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.