In the interests of making code as usable as possible, interfaces and delegates with generic parameters should use the ``++out++`` and ``++in++`` modifiers when possible to make the interfaces and delegates covariant and contravariant, respectively.
The ``++out++`` keyword can be used when the type parameter is used only as a return type in the interface or delegate. Doing so makes the parameter covariant, and allows interface and delegate instances created with a sub-type to be used as instances created with a base type. The most notable example of this is ``++IEnumerable<out T>++``, which allows the assignment of an ``++IEnumerable<string>++`` instance to an ``++IEnumerable<object>++`` variable, for instance.
The ``++in++`` keyword can be used when the type parameter is used only as a method parameter in the interface or a parameter in the delegate. Doing so makes the parameter contravariant, and allows interface and delegate instances created with a base type to be used as instances created with a sub-type. I.e. this is the inversion of covariance. The most notable example of this is the ``++Action<in T>++`` delegate, which allows the assignment of an ``++Action<object>++`` instance to a ``++Action<string>++`` variable, for instance.
