How can I create variant generic interfaces and delegates myself?

The out keyword marks a type parameter as covariant, and the in keyword marks it as contravariant. The two most important rules to remember:

• You can mark a generic type parameter as covariant if it is used only as a method return type and is not used as a type of formal method parameters.
• And vice versa, you can mark a type as contravariant if it is used only as a type of formal method parameters and not used as a method return type.