Inheritance and Composition

Favor composition over inheritance

  • Inheritance represents an "Is-A" relationship between types
    • From a general type to a more specific type
    • Useful when there is a common base for various types
    • Use a base class when you have a common implementation you want shared
    • Helpful when building off of a library (Open-Closed principle)
    • A Car is a PassengerVehicle
  • Composition represents a "Has-A" relationship between types
    • Complex types are more typically made up of other types
    • A Car has an Engine

Inheritance Example (Base Class)

public abstract class PassengerVehicle
{
    public int MaxOccupancy { get; protected set; }  // Passengers only
    public int CurrentOccupancy { get; protected set; } // Passengers only
    public void AddPassengers(int count)
    {
        if (CurrentOccupancy + count > MaxOccupancy)
          throw new Exception("Not enough room in vehicle");
        CurrentOccupancy += count;
    }
    public int UnloadPassengers()
    {
        int count = CurrentOccupancy;
        CurrentOccupancy = 0;
        return count;
    }
}

Inheritance Example (Base Class - cont.)

public abstract class PassengerVehicle
{
    // ...
    public abstract void Start();
    public abstract void Stop();
}

Inheritance Example (Derived Class)

public class Car : PassengerVehicle
{
    public Motor Engine { get; private set; }
    public override void Start()
    {
        // ...
    }
    public override void Stop()
    {
        // ...
    }
}

public class Motor
{
    // ...
}

Inheritance Example (Derived Class)

public class Rickshaw : PassengerVehicle
{
    public override void Start()
    {
        // ...
    }
    public override void Stop()
    {
        // ...
    }
}

Inheritance Example (Instantiation)

PassengerVehicle taxi = new Car();
taxi.AddPassengers(2);

// ... time to switch vehicles

PassengerVehicle touristCart = new Rickshaw();
touristCart.AddPassengers(taxi.UnloadPassengers());

Inheritance - A Practical Example

  • The Entity Framework Core library has a class called DbContext
    • It provides base functionality for working with databases
    • You inherit from DbContext and provide specifics of your database in your class
public class NorthwindTradersContext : DbContext
{
    // ...
}

The End

- [ ] Introduce inheritance and composition ("favor composition over inheritance") - [ ] Constructor chaining - [ ] Override .ToString() - polymorphism - [ ] abstract classes and abstract methods