c# - 如何制作仅包含一组自身类型或子类型的泛型类作为子类？

Question

abstract class Animal { }

class Mammal : Animal { }

class Dog : Mammal { }

class Reptile : Animal { }

class AnimalWrapper<T> where T : Animal
{
    public ISet<AnimalWrapper<T>> Children { get; set; }
}

class Program
{
    public static void Main(string[] args)
    {
        var foo = new AnimalWrapper<Mammal>();
        foo.Children = new HashSet<AnimalWrapper<Mammal>>();

        var child = new AnimalWrapper<Dog>();
        foo.Children.Add(child);
    }
}

由于 foo.Children.Add(child);，这显然无法编译

我不确定上面的代码是否是演示我想做的事情的最清晰方式，所以我会尽量用通俗易懂的英语解释:

我希望能够拥有一个类，其子对象位于 ISet 中属于相同的通用类型。因此，如果我也有 var child = new AnimalWrapper<Reptile>();它会在编译时失败 foo.Children.Add(child);因为Reptile不是也不继承自 Mammal .但是，很显然，即使是派生出来的，如上图，也是不行的。

最终，能够说 ISet<AnimalWrapper<Animal>> baz = new HashSet<AnimalWrapper<Animal>>(); 会很好然后添加 new AnimalWrapper<Mammal>()到那个集合，和new AnimalWrapper<Reptile>()到同一套。他们的 child 将拥有属性(property)Children那是一个ISet<AnimalWrapper<T>>在某种程度上，它是自己的类型，如上所述。

有什么办法还是我对 C# 的期望太高了？哎呀，我把自己搞糊涂了。 :)

编辑: 好的，所以我几乎想通了，没有 AnimalWrapper , 但以 IAnimal 为基数接口(interface)，它几乎可以工作:

interface IAnimal { }

abstract class Animal<T> : IAnimal where T : Animal<T>
{
    public ISet<T> Children { get; set; }
}

class Mammal : Animal<Mammal> { }

class Dog : Mammal { }

class Reptile : Animal<Reptile> { }

class Frog : Reptile { }

class Program
{
    public static void Main(string[] args)
    {
        var animals = new HashSet<IAnimal>(); // any animal can be in this
        var mammal = new Mammal();
        animals.Add(mammal);
        mammal.Children = new HashSet<Mammal>();
        var dog = new Dog();
        mammal.Children.Add(dog); // ok! a dog is a mammal
        dog.Children = new HashSet<Dog>(); // in theory, OK, but compile time error
        // because Dog : Mammal, and Mammal defines Animal<Mammal>, therefore Dog's
        // Children is actually ISet<Mammal>, rather than ISet<Dog> (which is what
        // I want, recursively apply the T in Animal.
        Mammal mammal2 = new Mammal();
        dog.Children.Add(mammal2); // should be verboten, but is allowed for the
        // same reason above.
    }
}

Answer 1

主要问题是在协方差向上转换(以及与 ISet 的逆变)中有点过于简单化

这样试试……

abstract class Animal { }
class Mammal : Animal { }
class Dog : Mammal { }
class Reptile : Animal { }

interface INode<out T> where T : Animal
{
    T MySelf { get; }
    IEnumerable<INode<T>> Children { get; }
}

class Node<T> : INode<T>
    where T : Animal
{
    public Node() { this.Children = new HashSet<INode<T>>(); }
    public T MySelf { get; set; }
    public ISet<INode<T>> Children { get; set; }
    IEnumerable<INode<T>> INode<T>.Children { get { return this.Children; } }
}

class Program
{
    static void Main(string[] args)
    {
        // this is a 'typical' setup - to test compiler 'denial' for the Reptile type...

        Node<Mammal> tree = new Node<Mammal>();
        tree.MySelf = new Mammal();

        var node1 = new Node<Mammal>();
        tree.Children.Add(node1);

        var node2 = new Node<Dog>();
        tree.Children.Add(node2);

        var node3 = new Node<Reptile>();
        // tree.Children.Add(node3); // this fails to compile


        // ...and similar just more 'open' - if you 'collect' animals, all are welcome

        Node<Animal> animals = new Node<Animal>();
        animals.MySelf = new Mammal();

        INode<Mammal> mamals = new Node<Mammal>();
        animals.Children.Add(mamals);

        var dogs = new Node<Dog>();
        animals.Children.Add(dogs);

        INode<Animal> reptiles = new Node<Reptile>();
        animals.Children.Add(reptiles);
    }
}

(查看评论)

这并不意味着它可以在您的现实生活中使用 - 因为这需要一些“设计重构”以使其适用于更复杂的结构(如果可能)。

...只是快点，如果需要我稍后再解释