Covariance and Contraviarance

VarianceExample.Program.cs

namespace VarianceExample
{
    class Program
    {
        static void Main(string[] args)
        {
            ISkinned<Fruit> skinnedFruit = null;
            ISkinned<Banana> banana = null;

            ICovariantSkinned<Fruit> covariantSkinnedFruit = null;
            ICovariantSkinned<Banana> covariantBanana = null;

            IContravariantSkinned<Fruit> contravariantSkinnedFruit = null;
            IContravariantSkinned<Banana> contravariantBanana = null;

            Peel(skinnedFruit);
            // Peel(bananaFruit); // cannot convert from 'ISkinned<Banana>' to 'ISkinned<Fruit>'

            // PeelBanana(skinnedFruit); // cannot convert from 'ISkinned<Fruit>' to 'ISkinned<Banana>'
            PeelBanana(banana);

            PeelCovariant(covariantSkinnedFruit);
            PeelCovariant(covariantBanana); // ok, since 'in T' generic type specified

            PeelBananaContravariant(contravariantSkinnedFruit); // ok, since 'out T' generic type specified
            PeelBananaContravariant(contravariantBanana);
        }

        static void Peel(ISkinned<Fruit> param) { }
        static void PeelBanana(ISkinned<Banana> param) { }
        static void PeelCovariant(ICovariantSkinned<Fruit> param) { }
        static void PeelBananaContravariant(IContravariantSkinned<Banana> param) { }
    }

    class Fruit { }
    class Banana : Fruit { }
    interface ISkinned<T> { }
    interface ICovariantSkinned<out T>
    {
        T Get();
        // void Set(T outParam); // Invalid variance: The type parameter 'T' must be contravariantly valid on 'ICovariantSkinned<T>.Set(T)'. 'T' is covariant.
    }
    interface IContravariantSkinned<in T>
    {
        // T GetT(); // Invalid variance: The type parameter 'T' must be covariantly valid on 'IContravariantSkinned<T>.GetT()'. 'T' is contravariant.
        void Set(T inParam);
    }
}

// Based on https://stackoverflow.com/a/20660947/5540176