尝试使用基类对象访问派生类中的受保护成员

Question

此行给出错误：无法通过BaseClass 类型的限定符访问受保护的成员BaseClass.number，限定符必须为DerivedClass 类型（或派生自它）有人可以帮我解决这个问题。看下面的代码 提前感谢

using System;
public class BaseClass
{
    protected int number = 10;
}
public class DerivedClass: BaseClass
{   
    public void Print()
    {
        BaseClass obj = new BaseClass();
        //Console.WriteLine(obj.number); 
    // we get error if we try to print why?
    }
}
class Program
{
    public static void Main()
    {
        DerivedClass obj2 = new DerivedClass();
        obj2.Print();
    }
}

但是为什么编译器会出错。 为什么这种调用方法是错误的

Answer 1

您的问题似乎反映了文档中的this example。

只有通过派生类类型进行访问时，才能在派生类中访问基类的受保护成员。

public class DerivedClass : BaseClass
{   
    public void Print()
    {
        BaseClass baseClass = new BaseClass();
        DerivedClass derived = new DerivedClass();

        // This is ok
        Console.WriteLine(number); 

        // So is this
        Console.WriteLine(derived.number);

        // But you can't do this
        Console.WriteLine(baseClass.number); 
    }
}

如果没有此限制，可能会出现以下情况，这显然是一个安全漏洞：

public class AnotherDerivedClass : BaseClass
{
    public bool IsValid => number == 10;
}

public class DerivedClass : BaseClass
{   
    public void Print()
    {
        BaseClass anotherAsBase = new AnotherDerivedClass();

        anotherAsBase.number = 0;
    }
}

在上面的例子中，DerivedClass 的一个实例干扰了AnotherDerivedClass 的实现细节。由于DerivedClass 与AnotherDerivedClass 是完全不同的类型，所以它不应该这样做。

Answer 2

using System;
public class BaseClass
{
    protected int number = 10;
}
public class DerivedClass : BaseClass
{
    public void Print()
    {
        this.number = 20;  // you access it as a private filed 

    }
}
class Program
{
    public static void Main()
    {
        DerivedClass obj2 = new DerivedClass();
        obj2.Print();
    }
}