继承，子类从 C# 中的父类更改受保护字段

Question

我目前正在学习 C#，在理解继承方面存在问题。 这是我的代码：

这是家长班：

class Member
    {
        protected int annualFee; // protected field =only accessible within the same class in which it is declared and any class derived from it
        private string name; // private field = can be accesed of the same class or struct
        private int memberId;
        private int memberSince;


        // writing ToString() method to display the values of the four fields
        public override string ToString()
        {
            return "\nName: " + name + "\nMember ID: " + memberId + "\nMember Since: " + memberSince + "\nTotal Annual Fee: " + annualFee;
        }

        // adding two constructors to the member class
        public Member()
        {
            Console.WriteLine("Parent Constructor with no parameter");
        }

        public Member(string pName, int pMemberID, int pMemberSince)
        {
            Console.WriteLine("Parent Constructor with 3 parameters");
            name = pName;
            memberId = pMemberID;
            memberSince = pMemberSince;

            Console.WriteLine("Parent Constructor:" + name + ", " + memberId + ", " + memberSince);
        }
    }

这是子班：

    class VIPMember : Member
    { 
    
        public VIPMember(string name, int memberId, int memberSince) : base (name, memberId, memberSince)
        {
            Console.WriteLine("Child constructor with 3 paramemters");
        }

        public void CalculateAnnualFee()
        {
            annualFee = 1200 * 2 + 12 * 30; //ans = 2760
        }

        public int annualFee = 120000; // field inherit from parent class, will overwrite annual fee in CalculateAnnualfee() method
    }

创建子类的实例并调用构造函数和方法。

class ClassDemo
    {
        static void Main(string[] args)
        {
            VIPMember myChildMember2 = new VIPMember("Andy", 2, 2011);
            myChildMember2.CalculateAnnualFee(); // method from child class VIPMember
            Console.WriteLine(myChildMember2.ToString()); // method inherit from parent class Member
            Console.WriteLine("Field from VIPMember (protected to public) class: " + myChildMember2.annualFee);

        }
    }

答案：

Parent Constructor with 3 parameters
Parent Constructor:Andy, 2, 2011
Child constructor with 3 paramemters

Name: Andy
Member ID: 2
Member Since: 2011
Total Annual Fee: 0
Field from VIPMember (protected to public) class: 2760

我特意添加了 public int AnnualFee = 120000 字段来尝试一下。然而，输出并不是我所期望的。我很期待：

Total Annual Fee: 2760
Field from VIPMember (protected to public) class: 12000

有人可以解释一下背后的逻辑吗？

谢谢你。

Answer 1

当您在

public int annualFee

中定义

VIPMember

时，您会 遮蔽 中的

protected int annualFee

。您有效地创建了一个具有相同名称的全新字段，并使您只能从定义它的类中看到

Member

。编译器应该向您发出警告，表明您正在这样做。试试这个：

annualFee

你得到：

void Main()
{
    B b = new B();
    Console.WriteLine(b.x);
    b.x = 3;
    Console.WriteLine(b.x);
    A a = b;
    Console.WriteLine(a.x);
    a.x = 4;
    Console.WriteLine(a.x);
}

public class A
{
    public int x = 1;
}

public class B : A
{
    public int x = 2;
}

但是如果我没有被跟踪

2
3
1
4

那么我会这样编写代码：

x

请注意，我需要在 

void Main()
{
    B b = new B();
    Console.WriteLine(b.x);
    b.x = 3;
    Console.WriteLine(b.x);
    A a = b;
    Console.WriteLine(a.x);
    a.x = 4;
    Console.WriteLine(a.x);
    Console.WriteLine(b.x);
}

public class A
{
    public int x = 1;
}

public class B : A
{
    public B()
    {
        this.x = 2;
    }
}

的构造函数中更新

x

。要重写你的代码，你应该这样做：

B

然后它就如你所期望的那样工作了。