Top 57 OOP Interview Questions And Answers

Question 1

What is _Object-Oriented Programming_ (OOP)?

Accepted Answer

OOP is a technique to develop logical modules, such as classes that contain properties, methods, fields, and events. An object is created in the program to represent a class. Therefore, an object encapsulates all the features, such as data and behavior that are associated to a class. OOP allows developers to develop modular programs and assemble them as software. Objects are used to access data and behaviors of different software modules, such as classes, namespaces, and sharable assemblies. .NET Framework supports only OOP languages, such as Visual Basic .NET, Visual C#, and Visual C++.

Question 2

What is _Inheritance_?

Accepted Answer

**Inheritance** allows us to define a class in terms of another class, which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and speeds up implementation time.

When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the base class, and the new class is referred to as the derived class.

The idea of inheritance implements the IS-A relationship. For example, mammal IS A animal, dog IS-A mammal hence dog IS-A animal as well, and so on.

Question 3

What is _Polymorphism_?

Accepted Answer

The word polymorphism means having many forms. In object-oriented programming paradigm, polymorphism is often expressed as _one interface, multiple functions_.

Question 4

What is the difference between _procedural_ and _object-oriented_ programming?

Accepted Answer

Procedural programming is based upon the modular approach in which the larger programs are broken into procedures. Each procedure is a set of instructions that are executed one after another. On the other hand, OOP is based upon objects. An object consists of various elements, such as methods and variables.  
  
Access modifiers are not used in procedural programming, which implies that the entire data can be accessed freely anywhere in the program. In OOP, you can specify the scope of a particular data by using access modifiers - _public_, _private_, _internal_, _protected_, and _protected_ internal.

Question 5

Why is the `virtual` keyword used in code?

Accepted Answer

The `virtual` keyword is used while defining a class to specify that the methods and the properties of that class can be overridden in derived classes.

Question 6

Explain the concept of _Constructor_

Accepted Answer

**Constructor** is a special method of a class, which is called automatically when the instance of a class is created. It is created with the same name as the class and initializes all class members, whenever you access the class. The main features of a constructor are as follows:
* Constructors do not have any return type.
* Constructors can be overloaded.
* It is not mandatory to declare a constructor; it is invoked automatically by .NET Framework.

Question 7

What is an `object`?

Accepted Answer

Objeects are instance of classes. It is a basic unit of a system. An object is an entity that has attributes, behavior, and identity. Attributes and behavior of an object are defined by the class definition.

Question 8

What is the difference between a `class` and a `structure`?

Accepted Answer

**Class**:
* A class is a _reference type_
* While instantiating a class, CLR allocates memory for its instance in _heap_
* Classes support inheritance
* Variables of a class can be assigned as `null`
* Class can contain constructor/destructor

**Structure**:
* A structure is a _value type_
* In structure, memory is allocated on _stack_
* Structures do not support inheritance
* Structure members cannot have `null` values
* Structure does not require constructor/destructor and members can be initialiazed automatically

Question 9

What is _Encapsulation_?

Accepted Answer

**Encapsulation** is defined _as the process of enclosing one or more items within a physical or logical package_. Encapsulation, in object oriented programming methodology, prevents access to implementation details.

Question 10

Can you _inherit_ `private` members of a class?

Accepted Answer

No, you cannot inherit `private` members of a class because `private` members are accessible only to that class and not outside that class.

Top 57 OOP Interview Questions

What is Inheritance?

What is Object-Oriented Programming (OOP)?

What is the difference between a class and a structure?

Why is the virtual keyword used in code?

Can you inherit private members of a class?

Explain the concept of Constructor

What is the difference between procedural and object-oriented programming?

What is Encapsulation?

What is Polymorphism?

What is an object?

What is a class?

What is the relationship between a class and an object?

Explain the basic features of OOPs

Can you specify the accessibility modifier for methods inside the interface?

Is it possible for a class to inherit the constructor of its base class?

How could you define Abstraction in OOP?

State the features of an Interface

What do you mean by Data Encapsulation?

What are similarities between a class and a structure?

Interface or an Abstract class: which one to use?

How is method overriding different from method overloading?

What is Unit Of Work?

What are the different ways a method can be Overloaded?

What's the difference between a method and a function in OOP context?

How can you prevent your class to be inherited further?

What is the difference between Virtual method and Abstract method?

What is the difference between Interface and Abstract Class?

What is Polymorphism, what is it for, and how is it used?

What are abstract classes? What are the distinct characteristics of an abstract class?

How can you prevent a class from overriding in C#?

When should I use a struct instead of a class?

Explain the concept of Destructor

How to solve Circular Reference?

Could you explain some benefits of Repository Pattern?

What is Coupling in OOP?

What exactly is the difference between an Interface and abstract class?

Explain different types of Inheritance

What's the advantage of using getters and setters - that only get and set - instead of simply using public fields for those variables?

Differentiate between an abstract class and an interface

What is a static constructor?

What is the difference between an abstract function and a virtual function?

What is the difference between Cohesion and Coupling?

When should I use an Interface and when should I use a Base Class?

What is Cohesion in OOP?

Can you declare an overridden method to be static if the original method is not static?

Does .NET support Multiple Inheritance?

Why prefer Composition over Inheritance? What trade-offs are there for each approach? When should you choose Inheritance over Composition?

Could you elaborate Polymorphism vs Overriding vs Overloading?

What is LSP (Liskov Substitution Principle) and what are some examples of its use (good and bad)?

You have defined a destructor in a class that you have developed by using the C#, but the destructor never executed. Why?

Can you declare a private class in a namespace?

In terms that an OOP programmer would understand (without any functional programming background), what is a monad?

What is the difference between Association, Aggregation and Composition?

What is the difference between a Mixin and Inheritance?

What does it mean to Program to an Interface?

Can you provide a simple explanation of methods vs. functions in OOP context?

Why doesn't C# allow static methods to implement an interface?

What is the difference between a `class` and a `structure`?

Why is the `virtual` keyword used in code?

Can you inherit `private` members of a class?

What is an `object`?

What is a `class`?

What is the relationship between a `class` and an `object`?

What are similarities between a `class` and a `structure`?

How is method `overriding` different from method `overloading`?

What's the difference between a `method` and a `function` in OOP context?

What are `abstract` classes? What are the distinct characteristics of an `abstract` class?

When should I use a `struct` instead of a `class`?

What exactly is the difference between an Interface and `abstract` class?

What's the advantage of using getters and setters - that only `get` and `set` - instead of simply using public fields for those variables?

Differentiate between an `abstract` class and an `interface`

What is a `static` constructor?

What is the difference between an `abstract` function and a `virtual` function?

Can you declare an overridden method to be `static` if the original method is not static?

Can you declare a `private` class in a namespace?

In terms that an OOP programmer would understand (without any functional programming background), what is a `monad`?