Composition vs. Inheritance

Reusing existing code and avoiding code duplication lands itself to better, scalable and maintainable design and application architecture and is practiced by the Object-Oriented Programming (OOP) community. Inheritance is officially one of the three pillars of OOP, although composition isn't. The difference between the two is, inheritance depicts a "Is-A" relationship where composition is "Has-A" relationship.

As an example, let's take the object "book". I have a special type of book, called "technical book" which derives from the object "book". Certainly a technical book is a special type of book, but it is a book. Composition on the other hand is the having one object containing another object, hopefully related, but not necessarily. Check out the object of "Cover". The "book" has a "cover". Here is a C# code snippet.

 
 
   1:  public class TechnicalBook : Book
   2:  {
   3:    // every method & property of Book class (protected + public) 
   4:    // other methods and properties of TechnicalBook
   5:  }
   6:   
   7:  public class TechnicalBook
   8:  {
   9:     private technicalBookCover = new Cover();
  10:     
  11:     public string Title()
  12:     {
  13:        technicalBookCover.Title = "Design Patterns";
  14:        return technicalBookCover.Title;
  15:     } 
  16:  }
  17:   
  18:  public class Cover()
  19:  {
  20:     private string title;   
  21:   
  22:     public string Title { get return title; set title = value; }
  23:  }

Why is this important? Well, if we want to create applications which will be maintained over time and be used and extended in the future, I believe it is critical we make this distinction and model our applications accordingly.

My past experience had been more towards inheritance, but I resented the times that I have painted myself to a corner, since the dependencies created via inheritance becomes a burden to carry forward. Inheritance always exposes every public member in the base class while composition gives you an option to selectively choose, if any