# Abstract Vs Interface ## 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 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. | Differences | Interface | Abstract | | ----------- | --------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- | | 1 | A class may have inherit several interfaces. | A class may inherit only one abstract class. | | 2 | An interface cannot provide any code,just the signature.(After C#8.0 u can use interface like an abstract). | An abstract class can prodive complete,default code and/or just details that have to be overriden. | | 3 | An interface cannot have access modifiers for the subs,functions,properties etc everything is assumed as public. | An abstract class can contain access modifiers for the subs,functions,properties. | | 4 | Interfaces are used to define the peripheral abilities of a class.In other words both Human and Vehicle can inherit from a IMovable interface. | An abstract class defines the core identity of a class and there it is used for objects of the same type. | | 5 | If various implementations only share method signatures then it is better to use Interfaces. | If various implementations are of the same kind and use common behaviour or status then abstract class is better to use. | | 6 | In terms of speed; requires more time to find the actual method in the corresponding classes. | Fast | | 7 | If we add a new method to an Interface then we have to track down all the implementattions of the interface and define implementation for the new method. | If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly. | Now let's see this difference at the code behind;\ Firstly we create Abstract class; ``` public abstract class Worker; public string id; public string FirstName; public string LastName; //properties public abstract string ID { get; set; } public abstract string FirstName { get; set; } //completed methods public String Add() { return "Worker"+id+""+LastName+""+Firstname+"added"; } public String Delete() { return "Worker"+id+""+LastName+""+Firstname+"deleted"; } public String Update() { return "Worker"+id+""+LastName+""+Firstname+"updated"; } ``` Now we create Interface class; ``` public interface IWorker { //In this fields , just signature of the properties and methods. setting a rule or contract to be followed by implementations. String Id { get; set; } String Firstname { get; set; } String LastName { get; set; } //Can not have implementation cannot have modifiers public all are assumed public.Cannot have virtual String Update(); String Add(); String Delete(); ``` Now get an object and inherit from both of them; Firstly Abstract; ``` public class WorkerZone : Worker //uses all the properties of the abstract class therefore no properties or fields here! Public WorkerZone () { } public ovverride String ID { get { return id; } set { id=value; } } public override String FirstName { get { return FirstName; } set { FirstName=value } } ///These methods that are implemented in the abstract class public new String Add() { return base.Add(); } public new String Update() { return base.Update(); } public new String Delete(); { return base.Delete(); } } ``` then now by using interface; ``` public class IWorkerZone : IWorker// "I" is commonly used for interface. { //****All the properties and fields defined here!!!*** public String id; public String FirstName; public String LastName; public IWorkerZone() { //TODO:Add constructor logic here } public string id { get { return id; } set { id=value; } } public string FirstName { get { return Firstname; } set { Firstname=value; } } public string LastName { get { return LastName; } set { LastName=value; } } //all the manipulations including Add,Delete, //Search, Update, Calculate are done //within the object as there are not //implementation in the Interface entity. public String Add() { return "Fulltime Worker" + FirstName + " Added."; } public String Delete() { return "Fulltime Worker" + FirstName + " Deleted."; } public String Update() { return "Fulltime Worker" + FirstName + " Updated."; } ``` As a conclusion , interface doesn't let to change any properties on the subclasses.Because interface inherit the subclass static rule but in abstract you can override.\ \ Reference :CodeProject/ [Rahman Mahmoodi](https://www.codeproject.com/script/Membership/View.aspx?mid=444844)\ guru99/Anonymous\ Stackoverflow/\