转换 collections 中使用的 generics？

Question

Let's say I want to create diet plans for pet owners to keep their pets healthy and I have the following classes:

// Pets
public class Pet { ... }
public class Dog : Pet { ... }
public class Cat : Pet { ... }

// Meals and diets
public class Meal<T> : ICollection<T> where T : Pet
{
    ...
    private List<T> allowedPets { get; set; } = new List<T>();
    ...
}
public class Diet<T> : ICollection<Meal<T>> where T : Pet
{
    ...
    private List<Meal<T>> dietMeals { get; set; } = new List<Meal<T>>();
    ...
}

While I could have a list of pets that can follow a certain diet, rather than a list of pets that are allowed to have certain meals, this is just an example. Either way, I could make some arbitrary diets like these to use them in some sort of application.

// Dogs and their diets
Dog labrador = new Dog() { ... };
Dog dalmatian = new Dog() { ... };
Meal<Dog> mediumDogBreakfast = new Meal<Dog>(...) { labrador, dalmatian };
Meal<Dog> mediumDogLunch = new Meal<Dog>(...) { labrador, dalmatian };
Meal<Dog> mediumDogDinner = new Meal<Dog>(...) { labrador, dalmatian };
Diet<Dog> mediumDogDiet = new Diet<Dog>()
{
    mediumDogBreakfast,
    mediumDogLunch,
    mediumDogDinner
};

// Cats and their diets
Cat siamese = new Cat() { ... };
Cat sphynx = new Cat() { ... };
Meal<Cat> orientalCatBreakfast = new Meal<Cat>(...) { siamese, sphynx };
Meal<Cat> orientalCatLunch = new Meal<Cat>(...) { siamese, sphynx };
Meal<Cat> orientalCatDinner = new Meal<Cat>(...) { siamese, sphynx };
Diet<Cat> orientalCatDiet = new Diet<Cat>()
{
    orientalCatBreakfast,
    orientalCatLunch,
    orientalCatDinner
};

But if I then want to put these diets into a single list, I get a conversion error:

// This isn't allowed!
List<Diet<Pet>> = new List<Diet<Pet>>()
{
    mediumDogDiet,
    orientalCatDiet
};

I was under the impression that the list would allow Cat and Dog objects because the Pet costraint is more general, but apparently there's something going wrong here. How can I alter my Pet class or the collection implementation to allow a list that can hold both dog and cat diets (or any other future pet derivative for that matter)?

Answer 1

You need co-varience:

Take note that this means that you have to create a collection of the interface IDiet

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace ConsoleApp8
{

    public class Pet {  }
    public class Dog : Pet {  }
    public class Cat : Pet { }

    // Meals and diets

    public interface IMeal<out T> : IReadOnlyCollection<T> where T : Pet
    {
    }

    public interface IDiet<out T> : IReadOnlyCollection<IMeal<T>> where T : Pet
    {
    }

    public class Meal<T> : Collection<T>, IMeal<T> where T : Pet
    {
        public List<T> AllowedPets { get; set; } = new List<T>();
    }
    public class Diet<T> : Collection<IMeal<T>>, IDiet<T> where T : Pet
    {
        public List<Meal<T>> DietMeals { get; set; } = new List<Meal<T>>();
    }

    class Program
    {
        static void Main(string[] args)
        {
            Dog labrador = new Dog() { };
            Dog dalmatian = new Dog() { };
            Meal<Dog> mediumDogBreakfast = new Meal<Dog>{ labrador, dalmatian };
            Meal<Dog> mediumDogLunch = new Meal<Dog>{ labrador, dalmatian };
            Meal<Dog> mediumDogDinner = new Meal<Dog>{ labrador, dalmatian };
            Diet<Dog> mediumDogDiet = new Diet<Dog>()
            {
                mediumDogBreakfast,
                mediumDogLunch,
                mediumDogDinner
            };

            // Cats and their diets
            Cat siamese = new Cat() { };
            Cat sphynx = new Cat() { };
            Meal<Cat> orientalCatBreakfast = new Meal<Cat>(){ siamese, sphynx };
            Meal<Cat> orientalCatLunch = new Meal<Cat>(){ siamese, sphynx };
            Meal<Cat> orientalCatDinner = new Meal<Cat>(){ siamese, sphynx };
            Diet<Cat> orientalCatDiet = new Diet<Cat>()
            {
                orientalCatBreakfast,
                orientalCatLunch,
                orientalCatDinner
            };

            List<IDiet<Pet>> diets = new List<IDiet<Pet>>
            {
                mediumDogDiet,
                orientalCatDiet
            };
        }
    }
}