C# Inheritance question

Question

I have a class that has private fields... (cars)

I then inherit from this class... (Audi)

In the (Audi) class, when I type this. in the constructor...

the private fields are not available...

Do I need to do anything special to expose this private fields in (cars) class so that they are accessible via this. in (Audi class)?

Answer 1

You should declare them as "protected" instead of private

Answer 2

One (bad) option is to make the fields protected - but don't do this; it still breaks proper encapsulation. Two good options:

• make the setter protected
• provide a constructor that accepts the values

examples:

public string Name { get; protected set; }

(C# 2.0)

private string name;
public string Name {
    get { return name; }
    protected set { name = value; }
}

or:

class BaseType {
  private string name;
  public BaseType(string name) {
    this.name = name;
  }
}
class DerivedType : BaseType {
  public DerivedType() : base("Foo") {}
}

Answer 3

Philippe's suggestion to declare the fields as protected instead of private will indeed work - but I suggest you don't do it anyway.

Why should a derived class care about an implementation detail of how the data is stored? I suggest you expose protected properties which are (currently) backed by those fields, instead of exposing the fields themselves.

I treat the API you expose to derived classes as very similar to the API you expose to other types - it should be a higher level of abstraction than implementation details which you may want to change later.

Answer 4

You are probably looking for a concept called constructor inheritance. You can forward arguments to the base classes constructor - see this example, where the Audi has a flag indicating whether it's an S-Line edition or not:

namespace ConstructorInheritance
{
    abstract class Car
    {
     private int horsePower;
     private int maximumSpeed;

     public Car(int horsePower, int maximumSpeed)
     {
      this.horsePower = horsePower;
      this.maximumSpeed = maximumSpeed;
     }
    }

    class Audi : Car
    {
     private bool isSLineEdition = false;

     // note, how the base constructor is called _and_ the S-Line variable is set in Audi's constructor!
     public Audi(bool isSLineEdition, int horsePower, int maximumSpeed)
      : base(horsePower, maximumSpeed)
     {
      this.isSLineEdition = isSLineEdition;
     }
    }

    class Program
{
 static void Main(string[] args)
 {
  Car car = new Audi(true, 210, 255);
  // break here and watch the car instance in the debugger...
 }
}    }