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C# and .NET OOP (Object oriented  programming) interview questions - Abstract classes and interfaces.

I have yet to remember a .NET interviewer who never asked about abstract classes ,  interfaces and object oriented interview questions . I am putting forward questions which comes around abstract classes and interfaces again and again...Hope every one benefits.

Normally the C# interviewer starts with.....

What is abstract class ?

Abstract class is a base class or a parent class. Abstract classes can  have empty abstract methods or it can have implemented methods which can be overridden by child classes.

The next question i expected was on interfaces and yes there it comes.

What are interfaces?

Interface is a contract class with empty methods , properties and functions. Any class which implements the interface has to compulsory implement all the empty methods , functions and properties of the interface.

Now the 1000% sure question was bound to come difference between them...

What's the difference between abstract class and interface?

There are many differences, below are some key two differences :-

• Abstract class are base class or parent class while interfaces are contracts.
• Abstract class can have some implemented methods and functions while interfaces methods and functions are completely empty.
• Abstract classes are inherited while interfaces are implemented.
• Abstract classes are used when we want to increase reusability in inheritance while interfaces are used to force a contract.

Can we create a object of abstract class or interface?

No we can not.

Now the practical question..

In what scenarios will you use a abstract class and in what scenarios will you use a  interface?

If you want to increase reusability in inheritance then abstract classes are good. If you want implement or force some methods across classes must be for uniformity you can use a interface. So to increase  reusability via inheritance use abstract class as it is nothing but a base class and to force methods use interfaces.

Really friends having C#  experience is one thing and cracking  Dot net  and C# interviews is a different ball game all together .