OOPS (Object-oriented programming) Interview Questions

What is Polymorphisms?

Polymorphism means one interface and many forms. Polymorphism is a characteristics of being able to assign a different meaning or usage to something in different contexts specifically to allow an entity such as a variable, a function or an object to have more than one form.

There are two types of Polymorphism.
Compile time: function or operator overloading
Runtime: Inheritence & virtual functions

What is Abstract method?

Abstract method doesn't provide the implementation & forces the derived class to override the method.

What is Virtual method?

Virtual Method has implementation & provide the derived class with the option to override it.

Can Struct be inherited?

No, Struct can't be inherited as this is implicitly sealed.

What is Object?

Object is anything that is identifiable as a single material item.

What is Class?

A Class is the generic definition of what an object is a template.

The keyword class in C# indicates that we are going to define a new class (type of object)

What is Static field?

To indicate that a field should only be stored once no matter how many instance of the class we create.

What is Static Method?

It is possible to declare a method as Static provided that they don't attempt to access any instance data or other instance methods.

What is Inheritance?

It provides a convenient way to reuse existing fully tested code in different context thereby saving lot of coding.

Inheritance of classes in C# is always implementation Inheritance.

What is Virtual keyword?

This keyword indicates that a member can be overridden in a child class. It can be applied to methods, properties, indexes and events.

What is New modifiers?

The new modifiers hides a member of the base class. C# supports only hide by signature.

What is Abstract Class?

Abstract class is a class that can not be instantiated, it exists extensively for inheritance and it must be inherited. There are scenarios in which it is useful to define classes that is not intended to instantiate; because such classes normally are used as base-classes in inheritance hierarchies, we call such classes abstract classes.

Abstract classes cannot be used to instantiate objects; because abstract classes are incomplete, it may contain only definition of the properties or methods and derived classes that inherit this implements it's properties or methods.

Static, Value Types & interface doesn't support abstract modifiers. Static members cannot be abstract. Classes with abstract member must also be abstract.

What is Sealed modifiers?

Sealed types cannot be inherited & are concrete.
Sealed modifiers can also be applied to instance methods, properties, events & indexes. It can't be applied to static members.

Sealed members are allowed in sealed and non-sealed classes.

What is an Interface?

An interface is a contract & defines the requisite behavior of generalization of types.

An interface mandates a set of behavior, but not the implementation. Interface must be inherited. We can't create an instance of an interface.

An interface is an array of related function that must be implemented in derived type. Members of an interface are implicitly public & abstract.

An interface can inherit from another interface.

When to use Interface over abstract class?

Abstract Classes: Classes which cannot be instantiated. This means one cannot make a object of this class or in other way cannot create object by saying ClassAbs abs = new ClassAbs(); where ClassAbs is abstract class.
Abstract classes contains have one or more abstarct methods, ie method body only no implementation.
Interfaces: These are same as abstract classes only difference is we can only define method definition and no implementation.
When to use wot depends on various reasons. One being design choice.
One reason for using abstarct classes is we can code common
functionality and force our developer to use it. I can have a complete
class but I can still mark the class as abstract.
Developing by interface helps in object based communication.

What is pure virtual function?

When you define only function prototype in a base class without and do the complete implementation in derived class. This base class is called abstract class and client won’t able to instantiate an object using this base class.

A pure virtual function is a function that must be overridden in a derived class and need not be defined. A virtual function is declared to be "pure" using the curious "=0"
syntax:
class Base {
public:
void f1(); // not virtual
virtual void f2(); // virtual, not pure
virtual void f3() = 0; // pure virtual
};