是否可以使派生 class 的字段也被派生？

Question

假设我有一对 classes;

public class A {
    public int value;

    public A(int value) {
        this.value = value;
    }
}

public class B : A {
    public B(int value) : base(value) { }
}

public class Base {
    public A someobject;

    public Base(A someobject)
    {
        this.someobject = someobject;
    }
}

如果我想派生自class Base，我可以这样写；

public class Derived : Base {
    public Derived(A someobject) : base(someobject) { }
}

但是是否可以将 someobject 字段的数据类型更改为派生的子字段class，就像本例中那样？

public class Derived : Base {
    public B someobject;

    public Derived(B someobject) : base(someobject) { }
}

Answer 1

不，这不可能 - 这样做很危险。请注意：您当前使用的是 fields，而不是 properties，它们根本不允许您覆盖它们（尽管您可以隐藏基域）。但是，即使有属性，您仍然无法在覆盖时更改属性的 return 类型。

考虑以下因素：

void Main()
{
    Base myThing = new Derived();
    //We view it as a Base object, so we can assign any Animal to MyAnimal
    myThing.MyAnimal = new Cat();

    //Now let's cast it back to Derived
    Derived myCastThing = (Derived)myThing;
    myCastThing.MyAnimal; //We expect a Dog, but we get a Cat?
}

public class Base
{
    public virtual Animal MyAnimal { get; set; }
}
public class Derived : Base
{
    public override Dog MyAnimal { get; set; }
}
public class Animal { }
public class Dog : Animal { }
public class Cat : Animal { }

您可以使用泛型实现您想要的，但是：

public class Base<TAnimalType> where TAnimalType : Animal
{
    public virtual TAnimalType MyAnimal { get; set; }
}

public class Derived : Base<Dog>
{
    public override Dog MyAnimal { get; set; }
}

是否可以使派生 class 的字段也被派生？

Is it possible to make the fields of a derived class also be derived?

c#

oop

inheritance

field