Class Structures - Three Tier Class Hierarchy

Inheritance has two main functions: 1) to support substitution of any one of a set of derived class instances, depending on the application context, and 2) to provide in one place code and resources that are shared across all derived instances.

Any base class guarantees that code which uses a reference to the base will compile and operate with a reference to any class that derives from the base. It is important that you design the hierarchy so clients don't need to know the implementation details that distinguish each subtype. This substitutability is the most powerful aspect of inheritance.

A base class that provides non-virtual member functions intends to provide exactly that code to each derived instance without need to define the common operations in more than one place, e.g., just in the base. Similarly, if it declares a reference of some type, it intends to provide copies of the reference to each derived instance if initialized in the base, or, if not initialized, it intends to share that type among all derived objects.

You find three kinds of class structures within inheritance hierarchies:

Interface, Abstract Class, and Concrete Derived Classes

• Interface
  Is a C# construct with all abstract functions and no data members. Its purpose is to establish a contract for services that can be implemented by any concrete derived class.
• Abstract class
  Has at least one abstract function which prevents clients from creating instances. Abstract classes provide common code and sometimes common data, shared by every concrete derived class. Often an abstract class derives from an interface and defines some non-virtual functions to be shared. It may create instances of a common data type, shown as SharedResource in the Figure, above. If that type is initialized in the base, the instance is shared with all concrete derived classes. If not, then each concrete derived class gets a reference of the same type.

  Any abstract functions in its base must be defined by a concrete derived class. Otherwise the inherited function remains abstract and the derived class is also abstract.

• Concrete class
  Must have definitions for all functions. It either inherits the definitions from a base or defines itself. The inherited definitions may be provided by any base class, e.g., the abstract base, shown in the diagram.

Conclusions for Distinguish between Interface, Abstract Class, &Concrete Class: