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 .