c#-4.0 - 在子类中访问父类的 protected 字段

Question

我试图在子类中以不同的方式访问 protected 成员变量。我发现我不能通过使用子类对父类对象的对象引用来做到这一点。这里我指的是下面程序中的“int Number6”。

但是我可以访问公开的“int Number7”。我想知道这背后的原因。

public class Customer
{
    #region Fields



    protected int Number2; 
    protected int Number3;
    protected int Number4;
    protected int Number5;
    protected int Number6;

    public    int Number7;

    #endregion


}

public class CorporateCustomer : Customer
{
    public void PrintID()
    {
        CorporateCustomer CC = new CorporateCustomer();
        CC.Number2 = 101;

        base.Number3 = 103;

        this.Number4 = 104;            

        Customer C2 = new CorporateCustomer();
        C2.Number6 = 106; //->  Not Possible to access protected Number6 by this way                        

        C2.Number7  = 105; //-> However, can access public field                 

    }
}

Answer 1

有趣的问题 - the msdn states that this won't work :

A protected member of a base class is accessible in a derived class only if the access occurs through the derived class type.

由于您使用的是基类型而不是派生类型，因此这不起作用。

但是为什么呢？我可以想象这与客户也可以由 CorporateCustomer 之外的另一个类派生的问题有关。在这种情况下，您分配给 Customer 的实例不一定是 CorporateCustomer，因此 protected 属性正确地禁止访问 Number6 属性，因为这会破坏可访问性限制。

    public class PrivateCustomer : Customer
    {
    }

    public class CorporateCustomer : Customer
    {
        public void PrintID()
        {

            Customer C = new PrivateCustomer();
            C.Number6 = 106; //->  Not Possible to access protected Number6 by this way which is alright, as this is not a Corporate Customer                       

            C.Number7 = 105; //-> However, can access public field                 

        }
    }

C# 语言规范准确地说明了此示例作为此不起作用的原因:

3.5.3 Protected access for instance members When a protected instance member is accessed outside the program text of the class in which it is declared, and when a protected internal instance member is accessed outside the program text of the program in which it is declared, the access must take place within a class declaration that derives from the class in which it is declared. Furthermore, the access is required to take place through an instance of that derived class type or a class type constructed from it. This restriction prevents one derived class from accessing protected members of other derived classes, even when the members are inherited from the same base class.