Abstract Class & Interface: Two Villains of Every Interview - Part 2

This article is the second part of the series “Abstract Class & Interface: Two Villains of Every Interview” and explains the important key points of interfaces. 

Introduction

In my first article, I received a lot of feedback, some positive, some negative and the neutral but one question everyone asked is why I implied they are villains. I just want to clarify that I did not intend to portray them as villains. These are the most important concepts in programming and without them we cannot get better code efficiency.

Mostly in every interview, the first question is, "What is the difference between an Abstract Class and an Interface" and your entire interview depends on the answer to this question. Remember, “The first impression is the last impression”. So, these concepts are not villains but their role in an interview surely is. Enough about the critics, let's move to the next part of the series: interfaces.

An interface is the most important element of a software project and without it, a project is incomplete. Many wouldn't agree with me and debate that. Yes, it is a debatable question whether or not to use an interface. If yes, then how? In this article, I'm going to explain the important key aspects of interfaces. Like my previous article about abstract classes, I used the same approach of “What, Why and How” in this article and tried to explain it in the simplest possible way.

Roadmap

If you're reading this article for the first time, I request you to go for the Part 1 of this series and then return later to this article. Here is the road map of the series of articles under the title “Abstract Class & Interface: Two Villains of Every Interview”:

Interface
What an interface is

In the real world, an interface means a medium to interact with something. To be precise, it's a point where two systems, subjects, organizations and so on meet and interact. There are a few rules for the interaction to be done. Suppose you are going for an interview of Programmer Profile. The interview is only possible if the interviewer and you speak the same language. Moreover, you and the interviewer have the same skill set of programming languages to discuss upon.

Similarly, in the programming world, an interface means a contract to interact with multiple code modules. If a class wants to communicate with an interface, it must implement it and define its members. Consider it like the interviewer's question and you need to answer it correctly, if you want the job.

The MSDN Library defines the interface like a pure abstract class. An interface contains only the signatures of methods, properties, events, or indexers. It has no implementation of its own and can only be implemented by a class or a struct. Any of the two that implement the interface must provide the definitions to members specified in the interface. It is like a contract for all the derived classes to follow.

An interface is declared using the interface keyword. interface members are implicitly public and abstract, so we cannot prefix any access modifiers to it. An interface cannot contain fields, constant members, constructors, destructors and static members.

Why we need an interface

An interface is not a class. It contains only method signatures. It has no implementation on its own and cannot be instantiated. Its implementation logic is provided by the classes that derived from it. An interface is mostly considered to be a pure abstract class. However, there is the advantage of using an interface over an abstract class; that is "Multiple Inheritance Support". In C#, two classes (either abstract or concrete) cannot be inherited by the same derived class. It causes ambiguity in the derived class if both have the same method signature. We can do multiple inheritance in C# using interfaces.

An interface plays a vital role in the Service Oriented Architecture (SOA). In WCF, we use interfaces to define Service Contracts. A single class can implement any number of Service Contract Interfaces. It is generally accepted as the best practice to do so. However, we can also use classes for Service Contracts.

Most of the Design Patterns and Principles are based on interfaces rather than class inheritance. Some of the examples are Builder Design Pattern, Factory Pattern, Interface Segregation Principle and so on.

How to define an interface

Suppose we need to define a class for a Smart Phone. The class can have members like OS, AppStore and Call. The Smartphone can be either Android based or iOS based and cannot be both. There is no common functionality between an Android and iOS Smartphone, so we don't need to provide any default functionality. One approach is to make the SmartPhone class abstract and also all its members abstract. This approach works fine and several concrete classes like Samsung, Apple, HTC can inherit from it.

Now, after a few days Apple wants to add a Touch ID feature to its Smartphone. You can add TouchID as an abstract method in your abstract base class SmartPhone. But what if HTC doesn't want that feature and neither does Samsung? So, the TouchID method cannot be placed inside the abstract class SmartPhone. An alternative is to define another abstract class Features and add the TouchID method to it. This is also a bad idea since C# doesn't support inheritance of multiple classes (abstract or concrete) into a derived class.

In this situation, an interface is useful and plays a vital role in solving the problem. An interface provides only the method definitions, just like an abstract class, but can be useful in multiple inheritances. You can make the Features class an interface and add the TouchID method to it. It provides only the method signature and whichever class inherits it can implement it in its own way. It is also completely valid for a class to inherit more than one interface in C#. Also, we can make the SmartPhone class an interface instead of an abstract class. It is better instead of making a pure abstract class, we can use interfaces. 

Note: The example is not a best one, but I think it gets the point across. It is just for the sake of understanding interfaces.

Let us consider the example discussed above and create a Console Application for it. Open Visual Studio and add a new console project as "InterfaceDemo". 

By default, it gives a class named Program with a Main method in it for code execution. Let's create an abstract classSmartPhone and define OS and AppStore abstract methods in it. We can create an abstract class by putting the keyword "abstract" before a class definition. If you're not familiar with abstract classes, please go to the Part 1 of this series. 

1. using System;   
2.    
3. namespace InterfaceDemo    
4. {   
5.      //Abstract Class SmartPhone with only abstract methods in it    
6.      abstract class SmartPhone    
7.      {    
8.          public abstract void OS();    
9.          public abstract void AppStore();    
10.      }    
11.    
12.      class Program    
13.      {    
14.         static void Main(string[] args)    
15.         {    
16.         }    
17.      }    
18. }   

Now define the concrete classes Apple and Samsung that inherits from Smartphone and provides the definitions to the abstract methods OS and AppStore.

1. using System;   
2.    
3. namespace InterfaceDemo    
4. {    
5.       //Abstract Class SmartPhone with only abstract methods in it    
6.       abstract class SmartPhone    
7.       {   
8.          public abstract void OS();   
9.          public abstract void AppStore();    
10.       }   
11.    
12.       class Apple : SmartPhone    
13.       {   
14.           public override void OS()    
15.           {    
16.             //Some Implementation Here    
17.           }   
18.    
19.           public override void AppStore()    
20.           {   
21.              //Some Implementation Here    
22.           }    
23.        }    
24.    
25.        class Samsung: SmartPhone    
26.        {   
27.           public override void OS()    
28.           {    
29.               //Some Implementation Here    
30.           }   
31.    
32.           public override void AppStore()    
33.           {   
34.               //Some Implementation Here    
35.           }   
36.        }   
37.        
38.        class Program    
39.        {    
40.           static void Main(string[] args)    
41.           {    
42.           }    
43.        }    
44. }   

If we compile the code now, it works fine. Our SmartPhone class is implemented by two different concrete classes Apple and Samsung and defined depending on them. Now, let us suppose Apple wants to provide TouchID features to its Smartphone. We can add another abstract method TouchID in the SmartPhone class and let Apple inherit it and implement it. 

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {    
5.     //Abstract Class SmartPhone   
6.     abstract class SmartPhone    
7.     {    
8.        public abstract void OS();    
9.        public abstract void AppStore();    
10.    
11.        //TouchID method meant only for Apple Class   
12.        public abstract void TouchID();    
13.     }    
14.    
15.     class Apple : SmartPhone    
16.     {    
17.          public override void OS()    
18.          {   
19.             //Some Implementation Here    
20.          }   
21.    
22.          public override void AppStore()    
23.          {    
24.              //Some Implementation Here    
25.          }   
26.    
27.          //Implementing the TouchID feature    
28.         public override void TouchID()    
29.         {    
30.             //Some Implementation Here    
31.         }    
32.     }    
33.    
34.     class Samsung: SmartPhone    
35.     {    
36.         public override void OS()    
37.         {    
38.              //Some Implementation Here    
39.         }   
40.    
41.         public override void AppStore()    
42.         {    
43.               //Some Implementation Here    
44.         }    
45.     }   
46.    
47.     class Program    
48.     {    
49.        static void Main(string[] args) { }    
50.     }   
51. }   

The Apple class inherits the TouchID method and provides a definition to it. Let's compile the code now and see what happens. 



It throws an error saying that the Samsung class doesn't implement the TouchID method. By the definition of abstract class, any class implements it must provide definitions to all its abstract members. The TouchID method is meant only for the Apple class and the Samsung class doesn't want to implement it. It clearly seems that our approach is wrong since the TouchID method cannot be placed in the SmartPhone abstract class. 

An alternative approach is to define another abstract class Features and define the TouchID method to it. This approach seems fine since whatever class inherits Features can implement the TouchID method. 

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {    
5.       //Abstract Class SmartPhone    
6.       abstract class SmartPhone    
7.       {   
8.          public abstract void OS();   
9.          public abstract void AppStore();    
10.       }   
11.    
12.       //Abstract Class Features for TouchID method   
13.       abstract class Features    
14.       {   
15.          public abstract void TouchID();    
16.       }    
17.    
18.      //Apple Class inherits both SmartPhone and Features   
19.       class Apple : SmartPhone, Features    
20.       {   
21.          public override void OS()   
22.          {    
23.             //Some Implementation Here    
24.          }   
25.            
26.          public override void AppStore()    
27.          {   
28.              //Some Implementation Here    
29.          }   
30.    
31.          //Implementation of TouchID method in Apple Class    
32.          public override void TouchID()    
33.          {   
34.              //Some Implementation Here    
35.          }   
36.       }   
37.    
38.       class Samsung : SmartPhone    
39.       {   
40.          public override void OS()    
41.          {    
42.            //Some Implementation Here    
43.          }    
44.    
45.          public override void AppStore()   
46.          {   
47.             //Some Implementation Here    
48.          }    
49.        }   
50.    
51.        class Program    
52.       {   
53.           static void Main(string[] args)    
54.           {   
55.           }    
56.       }    
57. }   

Let's compile the code and see what happens. 

It again throws an error saying we cannot have multiple base classes in a derived class. This is called Multiple Inheritance of classes and is not allowed in C#. So, our second approach also fails to implement the TouchID method. This is where an interface is useful and helps to solve the "Multiple Inheritance" issue in C#. We can define both the SmartPhone and Features as interfaces and let the classes implement them as they need to. We can also have more than one interface in a class. This is the only way to do multiple inheritance in C#. 

Let's re-create the same project using interfaces. We can create an interface using the keyword interface. It is considered a good practice to prefix "I" before the interface name, however the point is arguable and the choice is yours. 

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {   
5.      interface ISmartPhone       //Definition of Interface    
6.      {    
7.          public void OS();    
8.          public void AppStore();    
9.      }    
10.    
11.      class Program    
12.      {     
13.         static void Main(string[] args)    
14.         {    
15.         }    
16.      }    
17. }   

We have defined the interface ISmartPhone with the method signatures OS and AppStore in it. If we compile the code now, it throws an error straightaway. 

It says we cannot prefix public modifiers with method signatures. In fact, no access modifier is allowed with interface methods. Interface methods are implicitly public in C# because an interface is a contract meant to be used by other classes. Moreover, we must declare these methods as public in derived classes, when we provide implementations to these methods. Also, we cannot declare these methods as static.

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {   
5.      interface ISmartPhone //Definition of Interface   
6.      {    
7.          static void OS();    
8.          static void AppStore();    
9.      }    
10.    
11.      class Program    
12.      {    
13.         static void Main(string[] args)    
14.         {    
15.         }    
16.      }    
17. }   

If we compile the code, it again gives us an error.



Let's define the interface methods without any access modifier and create a concrete class Apple that inherits the ISmartPhone interface and provides definitions to its members.  

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {    
5.     interface ISmartPhone    
6.     {    
7.          void OS();    
8.          void AppStore();    
9.     }    
10.    
11.     class Apple: ISmartPhone    
12.     {    
13.         //OS Method Implementation    
14.         public void OS()    
15.         {    
16.                Console.WriteLine("OS Method: The OS of this Smartphone is iOS8");    
17.         }    
18.    
19.         //AppStore Method Implementation    
20.         public void AppStore()    
21.         {    
22.                Console.WriteLine("AppStore Method: The Application Store of this Smartphone is iTunes");    
23.         }    
24.      }    
25.         
26.      class Program    
27.      {    
28.        static void Main(string[] args)    
29.        {    
30.        }    
31.      }    
32. }  

An important point that should be noted here is that whenever we implement interface members in derived classes, the access modifier must always be public otherwise it throws an error. If we write a protected modifier instead of public to the OS method, the compiler throws an error.

1. using System;    
2.    
3. namespace InterfaceDemo    
4. {   
5.       interface ISmartPhone    
6.       {    
7.            void OS();    
8.            void AppStore();    
9.       }    
10.          
11.       class Apple: ISmartPhone    
12.       {    
13.            //OS Method kept as Protected   
14.            protected void OS()    
15.            {    
16.                Console.WriteLine("OS Method: The OS of this Smartphone is iOS8");    
17.            }    
18.    
19.            //AppStore Method Implementation    
20.            public void AppStore()    
21.            {    
22.                 Console.WriteLine("AppStore Method: The Application Store of this Smartphone is iTunes");    
23.            }    
24.        }    
25.    
26.        class Program    
27.        {    
28.           static void Main(string[] args) { }    
29.        }    
30. }   

In the code above, I replaced the access modifier of the OS method from public to protected. Let's compile the code and see what happens. 

Yes, it throws an error saying that the Apple class cannot implement the OS method because it is not public. So, always keep your method implementations public in derived class. We can define another concrete class Samsung that also implements the interface ISmartPhone and provides definitions to its members.

1. using System;   
2.    
3. namespace InterfaceDemo    
4. {    
5.      interface ISmartPhone     
6.      {    
7.          void OS();    
8.          void AppStore();    
9.      }    
10.    
11.      class Apple: ISmartPhone    
12.      {    
13.          //OS Method Implementation    
14.          public void OS()    
15.          {    
16.             Console.WriteLine("OS Method: The OS of this Smartphone is iOS8");    
17.          }    
18.    
19.          //AppStore Method Implementation    
20.          public void AppStore()    
21.          {    
22.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is iTunes");    
23.          }    
24.      }    
25.    
26.      class Samsung : ISmartPhone    
27.      {    
28.          //OS Method Implementation    
29.          public void OS()    
30.          {    
31.             Console.WriteLine("OS Method: The OS of this smartphone is Android");    
32.          }    
33.    
34.          //AppStore Method Implementation    
35.          public void AppStore()    
36.          {    
37.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is Google Play");    
38.          }    
39.       }    
40.    
41.       class Program    
42.       {    
43.           static void Main(string[] args) { }    
44.       }    
45. }   

This code works fine since various concrete classes implement the interface and provide definitions to its members in their own way. Now if the Apple class wants to implement TouchID features, it can easily be done by defining another interface IFeatures. The Apple class can simply inherit the interface and implement the TouchID functionality to its class. This is the case where an interface is useful instead of an abstract class.  

1. using System;   
2.    
3. namespace InterfaceDemo    
4. {    
5.      interface ISmartPhone     
6.      {    
7.          void OS();    
8.          void AppStore();    
9.      }    
10.    
11.      //New Interface meant only for Apple Class    
12.      interface IFeatures    
13.      {    
14.           void TouchID();   
15.      }   
16.    
17.    
18.      class Apple: ISmartPhone, IFeatures    
19.      {    
20.          //OS Method Implementation    
21.          public void OS()    
22.          {    
23.             Console.WriteLine("OS Method: The OS of this smartphone is iOS8");    
24.          }    
25.    
26.          //AppStore Method Implementation    
27.          public void AppStore()    
28.          {    
29.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is iTunes");    
30.          }   
31.    
32.    
33.          //TouchID Method Implementation    
34.          public void TouchID()    
35.          {    
36.             Console.WriteLine("TouchID Method: This method provides Touch/Gesture control features.");     
37.          }   
38.      }    
39.    
40.      class Samsung : ISmartPhone    
41.      {    
42.          //OS Method Implementation    
43.          public void OS()    
44.          {    
45.             Console.WriteLine("OS Method: The OS of this smartphone is Android");    
46.          }    
47.    
48.          //AppStore Method Implementation    
49.          public void AppStore()    
50.          {    
51.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is Google Play");    
52.          }    
53.      }    
54.    
55.      class Program    
56.      {    
57.          static void Main(string[] args) { }    
58.      }    
59. }   

So, this way we can get multiple inheritance in C#. Let's create the objects of the concrete classes Apple and Samsung and build the project.

1. using System;   
2.    
3. namespace InterfaceDemo    
4. {    
5.      interface ISmartPhone     
6.      {    
7.          void OS();    
8.          void AppStore();    
9.      }    
10.    
11.      //New Interface meant only for Apple Class    
12.      interface IFeatures    
13.      {    
14.           void TouchID();   
15.      }   
16.    
17.    
18.      class Apple: ISmartPhone, IFeatures    
19.      {    
20.          //OS Method Implementation    
21.          public void OS()    
22.          {    
23.              Console.WriteLine("OS Method: The OS of this smartphone is iOS8");    
24.          }    
25.    
26.          //AppStore Method Implementation    
27.          public void AppStore()    
28.          {    
29.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is iTunes");    
30.          }   
31.    
32.          //TouchID Method Implementation    
33.          public void TouchID()    
34.          {   
35.             Console.WriteLine("TouchID Method: This method provides Touch/Gesture Control features.");    
36.          }   
37.      }    
38.    
39.      class Samsung : ISmartPhone    
40.      {    
41.          //OS Method Implementation    
42.          public void OS()    
43.          {    
44.             Console.WriteLine("OS Method: The OS of this smartphone is Android");    
45.          }    
46.    
47.          //AppStore Method Implementation    
48.          public void AppStore()    
49.          {    
50.             Console.WriteLine("AppStore Method: The Application Store of this smartphone is Google Play");    
51.          }    
52.       }    
53.    
54.       class Program    
55.       {    
56.           static void Main(string[] args)    
57.          {    
58.              Console.WriteLine("//////////////////// - Interface Demo - //////////////////// \n");                       
59.              Console.WriteLine("Apple SmartPhone:");    
60.              Apple apple = new Apple();    
61.              apple.OS();    
62.              apple.AppStore();    
63.              apple.TouchID();    
64.                 
65.             Console.WriteLine("\n\n");    
66.             Console.WriteLine("Samsung SmartPhone:");    
67.             Samsung samsung = new Samsung();    
68.             samsung.OS();    
69.             samsung.AppStore();    
70.             Console.ReadKey(); }   
71.       }    
72. }   

If we run the code now, it works perfectly.  

This is the simplest example of using interfaces. However, this is just a real-world analogy and the approach can be debatable. My intent in this demo is to let beginners understand how to work with interfaces. The following are the key points to be remembered when working with interfaces.  

Key Points

1. Interface Reference Variable
   An interface has no implementation and cannot be instantiated. However, it can be referenced to the class object that implements it. It may be noted that the object can only access the inherited members of the interface. Consider the following code:
   1. using System;    
   2.    
   3. namespace InterfaceDemo    
   4. {    
   5.     interface IDriveable    
   6.     {    
   7.         void Drive();    
   8.     }    
   9.        
   10.     class Car : IDriveable    
   11.     {    
   12.       public void Drive()    
   13.       {    
   14.           Console.WriteLine("Car Class: I can drive a Car.");    
   15.       }    
   16.     }    
   17.    
   18.     class Truck : IDriveable    
   19.     {    
   20.         public void Drive()    
   21.         {    
   22.            Console.WriteLine("Truck Class: I can drive a Truck.");    
   23.         }    
   24.     }    
   25.    
   26.     class Program    
   27.     {    
   28.        static void Main(string[] args)    
   29.        {    
   30.          Console.WriteLine("//////////////////// - Interface Demo - //////////////////// \n");                   
   31.          IDriveable DriveCar = new Car();    
   32.          IDriveable DriveTruck = new Truck();    
   33.    
   34.          DriveCar.Drive();         //Calls Car's Drive() method   
   35.          DriveTruck.Drive();       //Calls Truck's Drive() method   
   36.          Console.ReadKey();    
   37.         }    
   38.       }    
   39. }   
   The code shows the declaration of objects with the same interface Reference but with various functionalities. 
   
   
2. Explicit Interface Implementation
   When working with interfaces, there occurs a situation when a class implements two interfaces and both the interfaces contain a member with the same signature. When the class provides a definition to interface members, it gets confused about which member gets the definition since both have the same name. In that case, we'll an Explicit Interface Implementation.
   Suppose we have two interfaces ICreditCard and IDebitCard and both of these interfaces have the same method signature called CardNumber and a class Customer implements both of these interfaces.
   1. using System;    
   2.    
   3. namespace InterfaceDemo    
   4. {    
   5.       //First Interface IDebitCard   
   6.       interface IDebitCard    
   7.       {    
   8.          void CardNumber();    
   9.       }    
   10.    
   11.       //Second Interface ICreditCard    
   12.       interface ICreditCard    
   13.       {    
   14.          void CardNumber();    
   15.       }    
   16.    
   17.       //Customer Class implementing both the Interfaces    
   18.       class Customer: IDebitCard, ICreditCard    
   19.      {    
   20.      }    
   21.    
   22.      class Program    
   23.      {    
   24.        static void Main(string[] args) { }    
   25.      }    
   26. }   
   There are two ways to provide method definitions to interface members in a derived class. If you right-click on the interface name, Visual Studio gives you two options to implement them.  
   
   
   
   If we implement the interface normally and provide a definition to the CardNumber method, it will cause both interfaces to use CardNumber as their implementation. We cannot provide different functionality to the interface members. 
   1. using System;    
   2.    
   3. namespace InterfaceDemo    
   4. {    
   5.       //First Interface IDebitCard    
   6.       interface IDebitCard    
   7.       {    
   8.         void CardNumber();    
   9.       }    
   10.    
   11.       //Second Interface ICreditCard    
   12.       interface ICreditCard    
   13.       {    
   14.            void CardNumber();    
   15.       }    
   16.    
   17.       //Customer Class implements both the interfaces    
   18.       class Customer: IDebitCard, ICreditCard    
   19.       {    
   20.           public void CardNumber()    
   21.           {    
   22.               Console.WriteLine("Card Number: My Card Number is 12345678901234567890");    
   23.           }    
   24.       }    
   25.    
   26.       class Program   
   27.       {    
   28.          static void Main(string[] args)    
   29.         { }    
   30. }   
   If we compile the program now, the output creates more confusion since we are unable to decide which interface method was implemented since both of the interfaces share CardNumber as their method.
   In this case, we need to tell the compiler which method is specific to which interface using Explicit Implementation. It can be done by prefixing the interface name with the method definitions in the derived class. It may be noted that explicit interface definitions are automatically public and hence no access modifier is allowed with the method definitions. We can still have the shared method definition in it.
   1. using System;    
   2.    
   3. namespace InterfaceDemo    
   4. {    
   5.      //First Interface IDebitCard    
   6.      interface IDebitCard    
   7.      {     
   8.        void CardNumber();    
   9.      }    
   10.    
   11.      //Second Interface ICreditCard    
   12.      interface ICreditCard    
   13.      {    
   14.        void CardNumber();    
   15.      }    
   16.    
   17.      //Customer Class implements both the interfaces    
   18.      class Customer: IDebitCard, ICreditCard    
   19.      {   
   20.     
   21.        void IDebitCard.CardNumber()    
   22.        {    
   23.          Console.WriteLine("Debit Card Number: My Card Number is 12345XXXXX");    
   24.        }    
   25.           
   26.        void ICreditCard.CardNumber()    
   27.        {    
   28.           Console.WriteLine("Credit Card Number: My Card Number is 98999XXXXX");    
   29.        }    
   30.           
   31.        public void CardNumber()    
   32.        {    
   33.           Console.WriteLine("Customer ID Number: My ID Number is 54545XXXXX");    
   34.        }    
   35.      }    
   36.    
   37.      class Program    
   38.      {    
   39.        static void Main(string[] args)    
   40.        {    
   41.           Console.WriteLine("////////////////////- Implicit and Expliction Implementation -//////////////////// \n\n");    
   42.           Customer customer = new Customer();    
   43.           IDebitCard DebitCard = new Customer();    
   44.           ICreditCard CreditCard = new Customer();    
   45.    
   46.           customer.CardNumber();    
   47.           DebitCard.CardNumber();    
   48.           CreditCard.CardNumber();    
   49.    
   50.           Console.ReadKey();    
   51.        }    
   52.      }    
   53. }   
   If we run the program now, we are able to differentiate members using an Explicit Interface. 
   
   
3. If you have some kind of default functionality to share across classes in the hierarchy, you can use an abstract class. But if you don't have any default implementation and just need to define contracts for derived classes to follow; interface is the most preferred choice.
4. It is a standard rule when using an interface, be sure you have done it right the first time. Once the interface is implemented by derived classes, it is difficult to update or modify the interface since everyone else's code breaks.

Conclusion 
I hope this article helps you to understand the various possibilities of interfaces. Your feedback and constructive criticism is always appreciated, keep it coming. Until then, try to put a ding in the Universe.

Source collected from Csharpcorner.com
http://www.c-sharpcorner.com/UploadFile/d0e913/abstract-class-interface-two-villains-of-every-interview756/