使用父方法派生类

Question

Why can't i access the age method from either class A or B? I thought because it's a protected method, derived class instances should be able to use it?

class Program
{
    public static void Main(string[] args)
    {



    }

    public static void Test(A test)
    {
        A a = new A();
        B b = new B();

        Console.WriteLine(a.Age());
        Console.WriteLine(b.Age());
    }
}

public class A
{
    public virtual string Name { get { return "TestA"; } }
    protected string Age() { return "25";}
}

public class B : A
{

    public override string Name { get { return "TestB"; }  }
    public string Address { get; set; }
}

---As suggested by Jon Skeet--

 public class B : A
 {

    public override string Name { get { return "TestB"; }  }
    public string Address { get; set; }

    public void Testing()
    {
        B a = new B();
        a.Age();
    }
}

Answer 1

Protected means it can be used from code in the derived classes - it doesn't mean it can be used "from the outside" when working with derived classes.

The protected modifier can be somewhat tricky, because even a derived class can only access the protected member through instances of its own class (or further derived classes).

So within the code of B, you could write:

A a = new A();
Console.WriteLine(a.Age()); // Invalid - not an instance of B
B b = new B();
Console.WriteLine(b.Age()); // Valid - an instance of B
A ba = b;
Console.WriteLine(ba.Age()); // Invalid

The last of these is invalid because even though at execution time it's accessing the member on an instance of B , the compiler only knows of ba as being of type A .

Here's the start of section 3.5.3 of the C# 5 specification, which may clarify things:

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.