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Top 50 C# OOPS Interview Questions and Answers | Freshers and Experience

List of 50 C# Object-Oriented Programming (OOP) interview questions along with brief answers.

What is Object-Oriented Programming (OOP)?

Answer: OOP is a programming paradigm that uses objects and classes for organizing code. It revolves around the concepts of encapsulation, inheritance, and polymorphism.


Define encapsulation?

Answer: Encapsulation is the bundling of data and the methods that operate on that data into a single unit, known as a class.


What is a class in C#?

Answer: A class is a blueprint or a template for creating objects. It defines the data and behavior that the objects of the class will have.


Explain inheritance in C#.

Answer: Inheritance is a mechanism by which a class can inherit the properties and behaviors of another class. It promotes code reuse and establishes a relationship between the parent (base) class and the child (derived) class.

How is polymorphism achieved in C#?

Answer: Polymorphism is achieved through method overloading and method overriding. Method overloading involves having multiple methods with the same name but different parameters, while method overriding occurs when a child class provides a specific implementation for a method defined in its parent class.

What is an interface in C#?

Answer: An interface is a contract that defines a set of methods that a class must implement. It provides a way to achieve multiple inheritances and is used to define a common set of methods that classes can implement.


Differentiate between abstract classes and interfaces.

Answer: Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods. A class can implement multiple interfaces, but it can inherit from only one abstract class.


Explain the 'sealed' keyword in C#.

Answer: The 'sealed' keyword is used to prevent a class from being inherited. It is often applied to prevent further modification of a class's behavior.


What is the purpose of the 'using' statement in C#?

Answer: The 'using' statement is used for automatic resource management, ensuring that the resources are properly disposed of when they are no longer needed.


Define a delegate in C#.

Answer: A delegate is a type-safe function pointer that holds a reference to a method. It is used to define callback methods and implement events.


What is a lambda expression?

Answer: A lambda expression is an anonymous function that can have input parameters and a body. It is often used for a concise representation of functions.

Explain the 'var' keyword in C#.

Answer: The 'var' keyword is used for implicit typing. The compiler determines the type of the variable based on the assigned value.


What is the purpose of the 'readonly' keyword?

Answer: The 'readonly' keyword is used to create immutable fields. Once assigned a value, a readonly field cannot be modified.


Differentiate between 'const' and 'readonly' in C#.

Answer: 'const' is used for compile-time constants, while 'readonly' is used for runtime constants. 'const' values are determined at compile-time, and 'readonly' values are determined at runtime or during object initialization.


What is the difference between value types and reference types?

Answer: Value types store their data directly, while reference types store a reference to the data. Value types include simple types like int and float, and reference types include classes and interfaces.


Explain the purpose of the 'this' keyword.

Answer: The 'this' keyword refers to the current instance of the class and is used to differentiate between instance variables and parameters with the same name.


What is the significance of the 'base' keyword?

Answer: The 'base' keyword is used to refer to the immediate base class of the current derived class. It is often used to call methods or access members of the base class.


What is a constructor?

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


Differentiate between a shallow copy and a deep copy.

Answer: A shallow copy duplicates the object and references within the object, while a deep copy creates a new object and recursively copies all referenced objects.


Explain the concept of polymorphism with an example.

Answer: Polymorphism allows a single interface to be used for different types of objects. For example, a 'Shape' interface may have 'Draw' methods, and different classes like 'Circle' and 'Square' can implement this interface with their own specific 'Draw' implementations.


What is the purpose of the 'abstract' keyword in C#?

Answer: The 'abstract' keyword is used to declare abstract classes and methods. Abstract classes cannot be instantiated and are meant to be subclassed.


How does C# support multiple inheritances?

Answer: C# supports multiple inheritances through interfaces. A class can implement multiple interfaces, thereby achieving a form of multiple inheritance.


What is method overloading?

Answer: Method overloading is the ability to define multiple methods in the same class with the same name but different parameters.


Explain the 'is' and 'as' operators in C#.

Answer: The 'is' operator is used to check whether an object is of a certain type, while the 'as' operator is used for casting an object to a specific type, returning null if the cast is not possible.


How does exception handling work in C#?

Answer: Exception handling in C# is done using try, catch, and finally blocks. The 'try' block contains the code that may throw an exception, the 'catch' block handles the exception, and the 'finally' block contains code that will always be executed, whether an exception is thrown or not.


What is the purpose of the 'throw' keyword?

Answer: The 'throw' keyword is used to explicitly throw an exception. It can be used in methods to signal that an error condition has occurred.


Explain the concept of interface segregation.

Answer: Interface segregation is a principle that suggests that a class should not be forced to implement interfaces it does not use. It encourages creating multiple small, specific interfaces rather than a large, monolithic one.


What is the difference between the 'StringBuilder' and 'String' classes?

Answer: 'String' is immutable, meaning its value cannot be changed after creation. 'StringBuilder' is mutable and allows for efficient manipulation of strings, especially when concatenating multiple strings.


How does garbage collection work in C#?

Answer: Garbage collection in C# automatically reclaims memory occupied by objects that are no longer reachable. The runtime has a garbage collector that identifies and frees up memory used by objects that are no longer in use.


Explain the concept of the 'using' statement for IDisposable objects.

Answer: The 'using' statement is used for automatic resource management, especially with objects that implement the IDisposable interface. It ensures that the Dispose method is called when the object goes out of scope.


What is a namespace in C#?

Answer: A namespace is a way to organize code by grouping related types. It helps in avoiding naming conflicts and enhances code readability.


How can you achieve multiple catch blocks in C#?

Answer: Multiple catch blocks can be used to catch different types of exceptions. They are specified in the order from the most specific to the most general exception type.


Explain the concept of the 'using' directive.

Answer: The 'using' directive is used to include a namespace in the program, making its types accessible without fully qualifying their names.


What is a property in C#?

Answer: A property is a member of a class that provides a way to read or write the value of a private field. It is defined using get and set accessors.


What is the purpose of the 'get' and 'set' accessors in a property?

Answer: The 'get' accessor is used to retrieve the value of the property, while the 'set' accessor is used to set the value of the property.


Explain the difference between the 'ref' and 'out' keywords.

Answer: Both 'ref' and 'out' are used for passing parameters by reference. However, 'out' is used when the parameter is intended to be an output parameter, and it does not need to be initialized before being passed to the method.


What is the purpose of the 'params' keyword in C#?

Answer: The 'params' keyword allows a method to accept a variable number of parameters. It is used to simplify the calling syntax for methods that take a variable number of arguments.


Explain the 'static' keyword in C#.

Answer: The 'static' keyword is used to declare members (fields, methods, properties) that belong to the type itself rather than an instance of the type. Static members are shared among all instances of the class.


What is a delegate in C#?

Answer: A delegate is a type that represents references to methods. It is used for defining and invoking methods at runtime.


What is the purpose of the 'yield' keyword in C#?

Answer: The 'yield' keyword is used in an iterator block to indicate that the method should return an iterator, which allows the caller to iterate over a sequence of values.


Explain the concept of extension methods.

Answer: Extension methods allow adding new methods to existing types without modifying their source code. They are defined as static methods in a static class and are called as if they were instance methods of the extended type.


What is the 'async' and 'await' keywords used for?

Answer: The 'async' and 'await' keywords are used for asynchronous programming. 'async' is used to define an asynchronous method, and 'await' is used to asynchronously wait for a task to complete.


How does C# support events?

Answer: C# supports events through delegates. Events provide a way for a class to notify other classes or objects when something of interest happens.


What is a generic class in C#?

Answer: A generic class is a class that is parameterized with one or more type parameters. It allows the creation of classes that can work with any data type.


Explain the 'lock' keyword in C#.

Answer: The 'lock' keyword is used to ensure that only one thread can access a critical section of code at a time. It is used to prevent race conditions in multithreaded applications.


What is the purpose of the 'using' statement in the context of IDisposable?

Answer: The 'using' statement ensures that the Dispose method of an IDisposable object is called, providing a convenient way to release resources such as file handles or database connections.


Explain the concept of indexers in C#.

Answer: Indexers allow instances of a class to be indexed like arrays. They provide a way to access elements of a class using the indexing syntax.


What is the 'nameof' operator used for?

Answer: The 'nameof' operator returns the name of a variable, type, or member as a string. It is useful for avoiding hardcoding names and improving code maintainability.


What is the purpose of the 'override' keyword in C#?

Answer: The 'override' keyword is used to declare that a method in a derived class is intended to override a method in its base class.


How does C# support polymorphism through interfaces?

Answer: C# allows a class to implement multiple interfaces, enabling polymorphism through interface-based programming. Instances of different classes that implement the same interface can be treated uniformly based on the common interface they share.


Keep in mind that the depth of the answers can vary based on the interviewer's expectations and the level of the position. Feel free to ask for clarification on any specific question!

This list covers a wide range of C# OOP concepts, and I hope it helps you in preparing for your interview! If you have any specific questions or need further clarification on any topic, feel free to ask.

By Anil Singh | Rating of this article (*****)

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