• Download source files - 8.75 KB

Introduction

In this article along with the demo project I will discuss Interfaces versus Abstract classes. The concept of Abstract classes and Interfaces is a bit confusing for beginners of Object Oriented programming. Therefore, I am trying to discuss the theoretical aspects of both the concepts and compare their usage. And finally I will demonstrate how to use them with C#.

Background

An Abstract class without any implementation just looks like an Interface; however there are lot of differences than similarities between an Abstract class and an Interface. Let's explain both concepts and compare their similarities and differences.

What is an Abstract Class?

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

What is an Interface?

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn�t support multiple inheritance, interfaces are used to implement multiple inheritance.

Both Together

When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.

When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.

There are some similarities and differences between an interface and an abstract class that I have arranged in a table for easier comparison:

Using the Code

Let me explain the code to make it a bit easier. There is an Employee abstract class and an IEmployee interface. Within the Abstract class and the Interface entity I am commenting on the differences between the artifacts.

I am testing both the Abstract class and the Interface by implementing objects from them. From the Employee abstract class, we have inherited one object: Emp_Fulltime. Similarly from IEmployee we have inherited one object: Emp_Fulltime2.

In the test code under the GUI, I am creating instances of both Emp_Fulltime and Emp_Fulltime2 and then setting their attributes and finally calling the calculateWage method of the objects.

Abstract Class Employee

Collapse| Copy Code

 System; AbstractsANDInterfaces
{   Employee{  id;  lname;  fname;   ID{;;}   FirstName{;;}   LastName{;;}  Update(){  + id +  + lname +  + fname + ;}  Add(){  + id +  + lname +  + fname + ;}  Delete(){  + id +  + lname +  + fname + ;}  Search(){  + id +  + lname +  + fname + ;}   CalculateWage();}
}

Interface Employee

Collapse| Copy Code

 System; AbstractsANDInterfaces
{  IEmployee{ ID{;;} FirstName{;;} LastName{;;} Update(); Add(); Delete(); Search(); CalculateWage();}
}

Inherited Objects

Emp_Fulltime:

Collapse| Copy Code

 System; AbstractsANDInterfaces
{  Emp_Fulltime : Employee{ Emp_Fulltime(){}   ID{{ id;}{id = value;}}   FirstName{{ fname;}{fname = value;}}   LastName{{ lname;}{lname = value;}}   Add(){ .Add();}   Delete(){ .Delete();}   Search(){ .Search();}   Update(){ .Update();}   CalculateWage(){  + .fname +  + ;}}
}

Emp_Fulltime2:

Collapse| Copy Code

 System; AbstractsANDInterfaces
{  Emp_fulltime2 : IEmployee{  id;  lname;  fname; Emp_fulltime2(){}  ID{{ id;}{id = value;}}  FirstName{{ fname;}{fname = value;}}  LastName{{ lname;}{lname = value;}}  Add(){  + fname + ;}  Delete(){  + fname + ;}  Search(){  + fname + ;}  Update(){  + fname + ;}  CalculateWage(){  + fname +  + ;}}
}

Code for Testing

Collapse| Copy Code

  InterfaceExample_Click( sender, System.EventArgs e)
{{IEmployee emp;Emp_fulltime2 emp1 =  Emp_fulltime2();emp =  emp1;emp.ID = ;emp.FirstName=  ;emp.LastName =  ;MessageBox.Show(emp.Add().ToString());MessageBox.Show(emp.CalculateWage().ToString());}(Exception ex){MessageBox.Show(ex.Message);}}  cmdAbstractExample_Click( sender, System.EventArgs e)
{Employee emp;emp =  Emp_Fulltime();emp.ID = ;emp.FirstName=  ;emp.LastName =  ;MessageBox.Show(emp.Add().ToString());MessageBox.Show(emp.CalculateWage().ToString());}

Conclusion

In the above examples, I have explained the differences between an abstract class and an interface. I have also implemented a demo project which uses both abstract class and interface and shows the differences in their implementation.

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)