Question

Suppose I have BaseClass with public methods A and B, and I create DerivedClass through inheritance.

e.g.

public DerivedClass : BaseClass {} 

Now I want to develop a method C in DerivedClass that uses A and B. Is there a way I can override methods A and B to be private in DerivedClass so that only method C is exposed to someone who wants to use my DerivedClass?

Answer 1

It’s not possible, why?

In C#, it is forced upon you that if you inherit public methods, you must make them public. Otherwise they expect you not to derive from the class in the first place.

Instead of using the is-a relationship, you would have to use the has-a relationship.

The language designers don’t allow this on purpose so that you use inheritance more properly.

For example one might accidentally confuse a class Car to derive from a class Engine to get it’s functionality. But an Engine is functionality that is used by the car. So you would want to use the has-a relationship. The user of the Car does not want to have access to the interface of the Engine. And the Car itself should not confuse the Engine’s methods with it’s own. Nor Car’s future derivations.

So they don’t allow it to protect you from bad inheritance hierarchies.

What should you do instead?

Instead you should implement interfaces. This leaves you free to have functionality using the has-a relationship.

Other languages:

In C++ you simply specify a modifier before the base class of private, public or protected. This makes all members of the base that were public to that specified access level. It seems silly to me that you can’t do the same in C#.

The restructured code:

interface I {     void C(); }  class BaseClass {     public void A() { MessageBox.Show('A'); }     public void B() { MessageBox.Show('B'); } }  class Derived : I {     public void C()     {         b.A();         b.B();     }      private BaseClass b; } 

I understand the names of the above classes are a little moot 🙂

Other suggestions:

Others have suggested to make A() and B() public and throw exceptions. But this doesn’t make a friendly class for people to use and it doesn’t really make sense.