Abstract Classes vs Interfaces

In this post I will review when Abstract Classes can be used over Interfaces and vice versa. To begin I start with a definition for Abstract Class and Interface. Then I outline the features and then finish out this post with the pressing question: When should Abstract Classes (or vice versa) be used over Interfaces? In a later post I will provide an example of Interfaces used in Booky.

Before we explore Abstract classes and interfaces let’s define them using Microsoft’s MSDN definitions:

Abstract Classes 

They are classes that cannot be instantiated, and are frequently either partially implemented, or not at all implemented. See http://msdn.microsoft.com/en-us/library/k535acbf(v=VS.71).aspx

An abstract class provides a common definition of a base class that multiple derived classes can share.

Interfaces

An interface defines a contract. A class or struct that implements an interface must adhere to its contract. See http://msdn.microsoft.com/en-us/library/87d83y5b(v=vs.71).aspx An interface contains definitions for a group of related functionalities that a class or a struct can implement.

Let’s explore the features of Abstract Classes and Interfaces before we determine the best or optimal time to use one over the other.

Interface Features

• Members are automatically public
• May contain properties, methods, events and indexers.
• An interface can inherit from one or more base interfaces
• Interfaces can be implemented by classes and structs.

Abstract Class Features

• Abstract class cannot be instantiated.
• Members have access modifiers (i.e. public, private)
• May contain fields, properties, constructors, destructors, methods, events and indexers.
• It is not possible to modify an abstract class with the sealed modifier, which means that the class cannot be inherited.
• A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.

When should Abstract Classes (or vice versa) be used over Interfaces? 

A comparison between the two (fix with full name)

Comparison

Abstract Classes	Interfaces
- They may contain implementation code	- They not contain implementation code
- They may only inherit from a single base class	- They may implement any number of interfaces

Use Abstract Classes when you have common shared implementation code. For example if your component requirement has shared functionality among objects then abstract classes may be a candidate for consideration when designing the system. 

Use Interfaces when the functionality you are creating will be used across a wide range of objects. When create small pieces of functionality then interfaces are a good choice.