The question is “what is the difference between covariance and contravariance?” Covariance and contravariance are properties of a mapping function that associates one member of a set with another. More specifically, a mapping can be covariant or contravariant with respect to a relation on that set. Consider the following two subsets of the set of … Read more
// Contravariance interface IGobbler<in T> { void gobble(T t); } // Since a QuadrupedGobbler can gobble any four-footed // creature, it is OK to treat it as a donkey gobbler. IGobbler<Donkey> dg = new QuadrupedGobbler(); dg.gobble(MyDonkey()); // Covariance interface ISpewer<out T> { T spew(); } // A MouseSpewer obviously spews rodents (all mice are // … Read more
Via wikipedia: Early versions of Java and C# did not include generics (a.k.a. parametric polymorphism). In such a setting, making arrays invariant rules out useful polymorphic programs. For example, consider writing a function to shuffle an array, or a function that tests two arrays for equality using the Object.equals method on the elements. The implementation … Read more
The way to make this work is to iterate over the list and cast the elements. This can be done using ConvertAll: List<A> listOfA = new List<C>().ConvertAll(x => (A)x); You could also use Linq: List<A> listOfA = new List<C>().Cast<A>().ToList();
The out keyword in generics is used to denote that the type T in the interface is covariant. See Covariance and contravariance for details. The classic example is IEnumerable<out T>. Since IEnumerable<out T> is covariant, you’re allowed to do the following: IEnumerable<string> strings = new List<string>(); IEnumerable<object> objects = strings; The second line above would … Read more