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C# OOP interview questions and answers: - I do not want to implement all the interface methods?

This is a simple c# OOP interview question which was asked to one of our readers in an IT company. Let us try to understand the question.

Let’s say we have a simple interface called as “IDal”.

public interface IDal

{

        void Insert();

        void Update();

}

Let’s assume that this interface is implemented by a class called as “SQL” as shown in the below code snippet. Now in this “SQL” class we would like to implement “Insert” method but we are not interested in the “Update” method. So the .NET interview question is “How can we implement the interface and not implement some methods of that interface,  as well as ensure that the program compiles properly ?”.

public class SQL : IDal

    {

        publicvoid Insert()

        {

            SqlConnection obj = newSqlConnection();

            // Code for database insert goes here

        }

        publicvoid Update()

        {

            // Kept purposely empty

            // as we do not want to implement

            // the same.
             
thrownewNotImplementedException();

        }

    }
 

Now there are two ways of doing this first is a dirty way and second is the proper way ;-).  

Now one important point we need to remember about interface is that if we implement an interface we have to implement all the methods. So there is really no escape that if you are implementing interface and you can get away not implementing some methods.

So the dirty way is to leave the method empty or throw an exception not implemented as shown in the above code snippet.

Keeping a method empty is definitely not a good practice as developers can get confused when they call the method and nothing happens. So in case you are forced to implement the interface method you can always raise “NotImplementedException” error and let the consumer of your class know that this method has no implementation.

That was a bad way, the proper way is to apply ISP (Interface segregation principle).  In other words  create two  interfaces one which has only “Insert” method and the other which has “Insert” + “Update” as shown in the below code snippet.

  publicinterfaceIDalInsert

    {

        void Insert();     

    }

    publicinterfaceIDal : IDalInsert

    {   

        void Update();

    }

 

Now you can see the “SQL” class has just implemented “IDalInsert” interface and we just implement the “Insert” method – Clean right…

public class SQL : IDalInsert

    {

        publicvoid Insert()

        {

            SqlConnection obj = newSqlConnection();

            // Code for database insert goes here

        }     

    }


So the way you will answer this .NET interview question is, we can use ISP (Interface segregation principle) and divide the interface in to two parts and then implement the necessary interface as needed. In case interviewer does not agree then you can talk about the dirty method i.e. throwing a not implemented exception.

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Author:
Shivprasad koirala Koirala
I am a Microsoft MVP for ASP/ASP.NET and currently a CEO of a small E-learning company in India. We are very much active in making training videos , writing books and corporate trainings. Do visit my site http://www.questpond.com for .NET, C# , design pattern , WCF , Silverlight , LINQ , ASP.NET , ADO.NET , Sharepoint , UML , SQL Server training and Interview questions and answers
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