Question

I have this simple code :

public class A
{
  int _private=3;
  public   A (B b)
    {
        b._private=5;
    }

}

public class B:A
{
}

this code compiles

• Via OOP – the b should not provide access to _private.

• Via private and A , A knows _private , but still the access is made through b !

what is going on here ?

Answer 1

The private access modifier gives access to a member within body of the type in which it is declared. It does not matter that b is another instance. It is an A so methods declared inside of A can access _private. I will also note that this example is more or less straight out of the C# 4.0 spec, section 3.5.2 about accessibility domains:

class A
{
   int x;

   static void F(B b) {
      b.x = 1;    // Ok
   }
}

class B: A
{
   static void F(B b) {
      b.x = 1;    // Error, x not accessible
   }
}

I think of the OOP recommendations for information hiding being about specifying the contract for client code. Public is something that is supported. Private is an implementation detail. Here, we could modify the implementation of A by removing _private and client code would be unaffected.