Question

说我有几个课程;

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;
    }
}

如果我想从课程Base派生，我可以写这个;

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子类，如本示例所示？

public class Derived : Base {
    public B someobject;

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

Answer 1

不，这是不可能的 - 这样做会很危险。请注意：您当前正在使用字段，而不是属性，它们根本不允许您覆盖它们（尽管可以隐藏基地）。但是，即使使用属性，在覆盖时仍无法更改属性的返回类型。

请考虑以下事项：

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 { }

然而，你可以使用泛型来实现你想要的东西：

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

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

是否可以导出派生类的字段？

1 个答案: