Top 100 OOPs Interview Questions and Answers PDF Download

1. What is Object-Oriented Programming (OOP)?

   Object-Oriented Programming (OOP) is a programming paradigm that organizes data and behavior into objects. It focuses on creating reusable and modular code by defining classes and objects.

2. What are the four main principles of OOP?

   The four main principles of OOP are:

   • Encapsulation: Binding data and methods that operate on the data within a single unit.
   • Abstraction: Hiding the implementation details of an object and exposing only relevant information.
   • Inheritance: Allowing a class (subclass) to inherit properties and behaviors from another class (superclass).
   • Polymorphism: Providing a single interface to entities of different types, allowing flexibility and extensibility.

3. What is a Class in OOP?

   A class is a blueprint or a template for creating objects. It defines the properties and behaviors that the objects of the class will possess.

4. What is an Object in OOP?

   An object is an instance of a class. It represents a real-world entity and encapsulates its data and behavior.

5. What is Encapsulation in OOP?

   Encapsulation is the bundling of data and methods that operate on the data within a single unit (class). It ensures that data is accessed and modified only through the defined methods, maintaining data integrity and security.

6. What is Abstraction in OOP?

   Abstraction is the process of hiding the implementation details of an object and exposing only relevant information. It allows us to focus on what an object does rather than how it does it.

7. What is Inheritance in OOP?

   Inheritance is a mechanism by which a class (subclass) acquires the properties and behaviors of another class (superclass). It promotes code reusability and allows creating a hierarchy of classes.

8. What are the types of inheritance in OOP?

   The types of inheritance in OOP are:

   • Single Inheritance: A class inherits from only one superclass.
   • Multiple Inheritance: A class inherits from multiple superclasses. Not supported in all programming languages.
   • Multilevel Inheritance: A class inherits from another class, which, in turn, inherits from another class.

9. What is Polymorphism in OOP?

   Polymorphism is the ability of objects of different classes to be treated as objects of a common superclass. It allows flexibility in handling different object types through a single interface.

10. What are the two types of polymorphism in OOP?

    The two types of polymorphism in OOP are:

    • Compile-time Polymorphism: Occurs at compile time and is achieved through method overloading and operator overloading.
    • Runtime Polymorphism: Occurs at runtime and is achieved through method overriding.

11. What is a Virtual Method in OOP?

    A virtual method is a method declared in a base class that can be overridden by derived classes using the "override" keyword.

12. What is an Abstract Method in OOP?

    An abstract method is a method declared in an abstract class or interface that does not have any implementation in the base class.

13. Can we instantiate an Abstract Class?

    No, we cannot instantiate an abstract class directly. It must be inherited by a concrete (non-abstract) class, which provides implementations for its abstract methods.

14. What is the "sealed" keyword in OOP?

    The "sealed" keyword is used to prevent a class from being inherited. It is used to create a non-inheritable class.

15. What is a Delegate in C#?

    A delegate is a type that represents references to methods with a specific signature. It is used to implement events and callback functions.

16. What is a Multicast Delegate in C#?

    A multicast delegate is a delegate that holds references to multiple methods. When called, all the referenced methods are executed in the order they were added.

17. What is a Lambda Expression in C#?

    A lambda expression is a concise way to represent an anonymous function. It is used to create delegates or expression tree types.

18. What are Events in C#?

    Events are a way of providing notifications to clients when something of interest happens in an object. They are based on delegates and are commonly used in GUI programming.

19. What is the "async" and "await" keywords in C#?

    The "async" and "await" keywords are used to implement asynchronous programming in C#. They allow non-blocking execution of methods.

20. What are Indexers in C#?

    Indexers allow objects to be indexed in a manner similar to arrays. They are used to provide custom index-based access to the elements of an object.

21. What are Extension Methods in C#?

    Extension methods allow adding new methods to existing types without modifying the original type's code.

22. What is Boxing and Unboxing in C#?

    Boxing is the process of converting a value type to an object reference, while unboxing is the process of converting an object reference back to a value type.

23. What is the "using" directive in C#?

    The "using" directive is used to include namespaces in the current C# file. It helps avoid writing the fully qualified name of types.

24. What are Nullable Value Types in C#?

    Nullable value types are value types that can have a value or a special value of "null."

25. What is the "lock" keyword in C# used for?

    The "lock" keyword is used to create a critical section, ensuring that only one thread can enter it at a time, preventing race conditions in multithreaded applications.

26. What is the difference between "readonly" and "const" in C#?

    "readonly" fields can be assigned a value either at the time of declaration or in the constructor, while "const" fields must have a constant value at the time of declaration and cannot be changed.

27. What is the "checked" and "unchecked" keywords in C#?

    The "checked" keyword is used to explicitly enable arithmetic overflow checking for integral-type arithmetic operations. The "unchecked" keyword disables such checking.

28. What is the difference between "ref" and "out" parameters in C#?

    "ref" parameters must be initialized before being passed to a method, while "out" parameters do not require initialization and are used to return values from methods.

29. What is the difference between "is" and "as" operators in C#?

    The "is" operator is used to check if an object is of a specific type, while the "as" operator is used to perform safe type casting.

30. What is a Tuple in C#?

    A tuple is a data structure that allows storing multiple elements of different types in a single object.

31. What are Anonymous Types in C#?

    Anonymous types are a way to create objects with read-only properties in a concise manner without explicitly defining a class.

32. What is the "nameof" operator in C#?

    The "nameof" operator is used to obtain the name of a variable, type, or member as a string at compile-time.

33. What is Polymorphism in OOP?

    Polymorphism is the ability of objects of different classes to be treated as objects of a common superclass. It allows flexibility in handling different object types through a single interface.

34. What are Abstract Classes in C#?

    Abstract classes are classes that cannot be instantiated and may contain abstract methods. They serve as base classes for other classes to inherit from.

35. What is Method Overloading in C#?

    Method overloading is the ability to define multiple methods with the same name but different parameter lists within the same class.

36. What is Method Overriding in C#?

    Method overriding is the ability of a subclass to provide a specific implementation of a method that is already defined in its superclass.

37. What is a Constructor in C#?

    A constructor is a special method that is automatically called when an object of a class is created. It is used to initialize the object's state.

38. What are Static Members in C#?

    Static members are members (fields, methods, properties) that belong to the class itself rather than to any specific instance of the class.

39. What is the "base" keyword in C#?

    The "base" keyword is used to access members of the base class from within a derived class.

40. What are Access Modifiers in C#?

    Access modifiers define the visibility and accessibility of types and members in C#. The common access modifiers are "public," "private," "protected," "internal," and "protected internal."

41. What is the "params" keyword in C# used for?

    The "params" keyword allows a method to accept a variable number of arguments of the same type.

42. What is the Difference between a Struct and a Class in C#?

    • Structs are value types and stored on the stack, while classes are reference types and stored on the heap.
    • Structs do not support inheritance, but classes do.
    • Structs have a default parameterless constructor, whereas classes do not unless explicitly defined.

43. What is the Difference between a Shallow Copy and a Deep Copy in C#?

    A shallow copy creates a new object but copies only the references of the original object's members, while a deep copy creates separate copies of the members themselves.

44. What is the "async" keyword in C# used for?

    The "async" keyword is used to indicate that a method is asynchronous and may perform non-blocking operations.

45. What is Asynchronous Programming in C#?

    Asynchronous Programming allows non-blocking execution of methods, improving the responsiveness of applications.

46. What is an Interface in C#?

    An interface is a contract that defines a set of methods and properties that a class must implement. It enables multiple inheritance in C#.

47. What is Multiple Inheritance in C#?

    Multiple Inheritance is the ability of a class to inherit properties and behaviors from multiple classes.

48. What is the "using" statement in C#?

    The "using" statement is used to ensure that an object is disposed of (cleaned up) after it is no longer needed, primarily used with objects that implement IDisposable.

49. What is the "this" keyword in C# used for?

    The "this" keyword refers to the current instance of the class and is used to differentiate between class members and method parameters with the same name.

50. What is the "readonly" keyword in C# used for?

    The "readonly" keyword is used to specify that a field can only be assigned a value during object construction or within the constructor.

51. What is the "const" keyword in C# used for?

    The "const" keyword is used to define constants that cannot be changed after they are assigned a value.

52. What is a Destructor in C#?

    A destructor is a special method used to free up resources and perform cleanup tasks before an object is destroyed.

53. What is the Difference between an Abstract Class and an Interface in C#?

    • Abstract classes can have both abstract and concrete methods, while interfaces can only have abstract methods.
    • A class can implement multiple interfaces, but it can inherit from only one abstract class.

54. What is the Difference between a Value Type and a Reference Type in C#?

    • Value types are stored on the stack, whereas reference types are stored on the heap.
    • Value types hold the actual data, while reference types hold references to the data.

55. What is the Difference between a Class and an Object in C#?

    • A class is a blueprint or template for creating objects, whereas an object is an instance of a class.
    • A class defines the properties and behaviors that objects of the class will possess.

56. What is the Difference between a Static Class and a Non-Static Class in C#?

    • A static class cannot be instantiated, and all its members must be static.
    • A non-static class can be instantiated, and it can have both static and instance members.

57. What is the Difference between a Constructor and a Method in C#?

    • A constructor is a special method used for initializing the object's state when it is created.
    • A method is a general-purpose function that performs a specific action.

58. What is the Difference between "==" and "Equals" in C#?

    • "==" is used for comparing value equality for value types and reference equality for reference types.
    • "Equals" is a method that can be overridden to define custom equality comparison for reference types.

59. What is the Difference between a Namespace and an Assembly in C#?

    • A namespace is a way to organize code and prevent naming conflicts. It can span multiple assemblies.
    • An assembly is a compiled code library that contains metadata and intermediate language code.

60. What is the Difference between a Stack and a Heap in C#?

    • The stack is used for storing value types and method call information, and it follows the Last In, First Out (LIFO) approach.
    • The heap is used for storing reference types, objects, and dynamic memory allocations.

61. What is the Difference between "StringBuilder" and "String" in C#?

    • "StringBuilder" is used for mutable strings and provides better performance for string concatenation operations.
    • "String" is used for immutable strings, and each concatenation creates a new string object.

62. What is the Difference between a Public, Private, Protected, and Internal Access Modifier in C#?

    • "public" members are accessible from any code.
    • "private" members are accessible only within the same class.
    • "protected" members are accessible within the same class and its derived classes.
    • "internal" members are accessible within the same assembly.

63. What is the Difference between a Property and a Field in C#?

    • A property is a member that provides a way to read, write, or compute the value of a private field.
    • A field is a data member that holds the value of an object.

64. What is the Difference between "new" and "override" in C#?

    • "new" is used to create a new member in a derived class that hides a member in the base class with the same name.
    • "override" is used to provide a specific implementation of a virtual method in a derived class.

65. What is the Difference between "sealed" and "abstract" in C#?

    • "sealed" is used to prevent a class from being inherited, while "abstract" is used to define a class that cannot be instantiated directly and must be inherited.

66. What is the Difference between "readonly" and "const" in C#?

    • "readonly" fields can be assigned a value either at the time of declaration or in the constructor, while "const" fields must have a constant value at the time of declaration and cannot be changed.

67. What is the Difference between a Destructor and a Finalizer in C#?

    • A destructor is used to free up resources and perform cleanup tasks before an object is destroyed.
    • A finalizer (also known as "destructor" in C#) is used to release unmanaged resources.

68. What is the Difference between "ref" and "out" parameters in C#?

    • "ref" parameters must be initialized before being passed to a method, while "out" parameters do not require initialization and are used to return values from methods.

69. What is the Difference between "is" and "as" operators in C#?

    • The "is" operator is used to check if an object is of a specific type, while the "as" operator is used to perform safe type casting.

70. What is the Difference between Early Binding and Late Binding in C#?

    • Early Binding occurs at compile-time and is resolved using static type information. It is faster and more efficient.
    • Late Binding occurs at runtime and is resolved using dynamic type information. It is less efficient but more flexible.

71. What is the Difference between "abstract class" and "interface" in C#?

    • An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods.
    • A class can implement multiple interfaces, but it can inherit from only one abstract class.

72. What are the Advantages of OOP over Procedural Programming?

    • Modularity: OOP allows breaking down a complex problem into smaller, manageable modules (objects).
    • Reusability: Objects can be reused in different parts of the program and in different programs.
    • Flexibility: OOP allows adding new features without modifying existing code.
    • Encapsulation: OOP promotes data hiding and prevents direct access to object internals.
    • Maintenance: OOP makes code easier to understand and maintain.

73. What is the Difference between a Private Constructor and a Static Constructor in C#?

    • A private constructor is used to prevent the instantiation of a class from outside the class, creating a singleton pattern.
    • A static constructor is used to initialize the class type itself and is called only once before any static members are accessed.

74. What is the Difference between "has-a" and "is-a" relationships in OOP?

    • "Has-a" relationship is a composition where an object contains another object as part of its state.
    • "Is-a" relationship is an inheritance where an object derives properties and behaviors from another object.

75. What is a Dependency Injection in C#?

    Dependency Injection is a design pattern in which the dependencies of a class are passed as objects during its construction, promoting loose coupling between classes.

76. What are SOLID principles in OOP?

    SOLID is an acronym for:

    • Single Responsibility Principle (SRP): A class should have only one reason to change.
    • Open/Closed Principle (OCP): A class should be open for extension but closed for modification.
    • Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of its subclass without affecting the correctness of the program.
    • Interface Segregation Principle (ISP): A class should not be forced to implement interfaces it does not use.
    • Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules. Both should depend on abstractions.

77. What is the Law of Demeter in OOP?

    The Law of Demeter (LoD) states that a class should only communicate with its immediate dependencies and not with the dependencies of its dependencies.

78. What is a Composition in OOP?

    Composition is a design principle where a class contains objects of other classes as part of its state.

79. What is a Design Pattern?

    A design pattern is a general repeatable solution to a commonly occurring problem in software design. It provides a template for solving a specific design issue.

80. What is the Singleton Design Pattern?

    The Singleton Design Pattern ensures that a class has only one instance and provides a global point of access to that instance.

81. What is the Factory Design Pattern?

    The Factory Design Pattern provides an interface for creating objects but allows subclasses to alter the type of objects that will be created.

82. What is the Observer Design Pattern?

    The Observer Design Pattern defines a dependency between objects so that when one object changes state, all its dependents (observers) are notified and updated automatically.

83. What is the Decorator Design Pattern?

    The Decorator Design Pattern allows adding behavior to individual objects dynamically without affecting other objects of the same class.

84. What is the Adapter Design Pattern?

    The Adapter Design Pattern allows incompatible interfaces to work together by providing a wrapper that converts one interface to another.

85. What is the Template Method Design Pattern?

    The Template Method Design Pattern defines the structure of an algorithm in a method but allows its steps to be implemented by its subclasses.

86. What is the Prototype Design Pattern?

    The Prototype Design Pattern creates new objects by copying existing objects and customizing them as needed.

87. What is the Command Design Pattern?

    The Command Design Pattern encapsulates a request as an object, allowing parameterization of clients with different requests and supporting undoable operations.

88. What is the Strategy Design Pattern?

    The Strategy Design Pattern allows selecting an algorithm at runtime from a family of interchangeable algorithms.

89. What is the State Design Pattern?

    The State Design Pattern allows an object to change its behavior when its internal state changes.

90. What is the Composite Design Pattern?

    The Composite Design Pattern allows treating individual objects and compositions of objects uniformly.

91. What is the Facade Design Pattern?

    The Facade Design Pattern provides a unified interface to a set of interfaces in a subsystem, making it easier to use.

92. What is the Proxy Design Pattern?

    The Proxy Design Pattern provides a surrogate or placeholder for another object, allowing control over its access.

93. What is the Chain of Responsibility Design Pattern?

    The Chain of Responsibility Design Pattern allows multiple objects to handle a request without the sender needing to know which object will ultimately process it.

94. What is the Iterator Design Pattern?

    The Iterator Design Pattern provides a way to access elements of a collection sequentially without exposing its underlying representation.

95. What is the Memento Design Pattern?

    The Memento Design Pattern allows capturing the internal state of an object without exposing its internal structure and restoring it later.

96. What is the Flyweight Design Pattern?

    The Flyweight Design Pattern conserves memory by sharing objects that have similar intrinsic data.

97. What is the Mediator Design Pattern?

    The Mediator Design Pattern centralizes communication between objects, promoting loose coupling.

98. What is the Interpreter Design Pattern?

    The Interpreter Design Pattern defines a grammar for a language and provides an interpreter to interpret sentences in that language.

99. What is the Visitor Design Pattern?

    The Visitor Design Pattern allows adding new operations to existing classes without modifying their structure.

100. What is the Builder Design Pattern?

     The Builder Design Pattern separates the construction of a complex object from its representation, allowing the same construction process to create different representations.

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