Covariance and Contravariance

Program.cs

using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
    public void Main(string[] args)
    {
        Apple apple = new Apple();
        Orange orange = new Orange();
        Fruit fruit = new Apple();

        // covariance (out)
        // VALID - data only comes "out" of IEnumerable<out T>
        {
            IEnumerable<Fruit> fruitEnumerable = new List<Apple> { apple };
            Fruit first = fruitEnumerable.First(); // fine if it's an apple!
        }

        // contravariance (in)
        // VALID - data only goes "in" to the parameter of Func<in T, out TResult>
        {
            Func<Apple, object> appleFunction = (appleParam) => appleParam.MakeAppleJuice() ?? appleParam.Eat();
            Func<Fruit, object> fruitFunction = (fruitParam) => fruitParam.Eat();

            appleFunction = fruitFunction;
            appleFunction(apple); // ok because fruitFunction works on apples
        }

        // covariance (out)
        // COMPILE ERROR - not logical! Data can go in and come out of IList<T> (.Add and .First).
        {
            IList<Fruit> fruitList = new List<Apple>();
            fruitList.Add(new Orange()); // adding an orange, when the underlying collection only suppots apples!
        }

        // contravariance (in)
        // COMPILE ERROR - not logical! Data can come out of the Func<in T, out TResult> as return values.
        {
            Func<object, Apple> appleFunction = (_) => apple;
            Func<object, Fruit> fruitFunction = (_) => orange;

            appleFunction(apple);
            fruitFunction(fruit);

            // BOOM
            appleFunction = fruitFunction;
            Apple appleReturn = appleFunction(null); // oh no an Orange!
        }
    }

    public abstract class Fruit
    {
        public object Eat() => null;
    }
    public class Apple : Fruit
    {
        public object MakeAppleJuice() => null;
    }
    public class Orange : Fruit
    {
        public object MakeOrangeJuice() => null;
    }
}