Object Oriented Class Modeling C#

Question

I have two different classes namely Car and Radio. How I can make sure that class Radio can be only used by class Car? Or is there better implementation here?

I'm coding in c#.

class Car
{
    // car has a radio
}  


class Radio
{
    // some properties

        // some methods
        void TurnOn(bool onOff) { }        
}

Thanks in advance.

Answer 1

class Car {

  public Car() {
    myRadio = new Radio();
  }

  private class Radio {
    void TurnOn(bool onOff) {}
  }
}