C# Interview Questions & Answers

Basics of C#:

1. What is C#?

   • C# (pronounced C-sharp) is a modern, object-oriented programming language developed by Microsoft. It is designed for building Windows applications, web applications, and services.

2. Explain the difference between value types and reference types in C#.

   • Value types store their data directly, while reference types store a reference to the location in memory where the data is stored. Examples of value types include primitive types like int and float, while objects are reference types.

3. What is the Common Language Runtime (CLR) in C#?

   • The CLR is a component of the .NET framework that manages the execution of C# programs. It provides services such as memory management, exception handling, and security.

4. What is the Global Assembly Cache (GAC) in C#?

   • The GAC is a repository for shared assemblies in the .NET framework. It allows multiple applications to share the same version of an assembly.

5. Explain the concept of garbage collection in C#.

   • Garbage collection is the automatic process of releasing memory that is no longer in use. The CLR's garbage collector identifies and frees up memory occupied by objects that are no longer reachable.

Object-Oriented Programming (OOP) in C#:

6. What is an object in C#?

   • An object is an instance of a class in C#. It is a runtime entity that combines data (fields) and behavior (methods).

7. Explain the difference between a class and an object in C#.

   • A class is a blueprint or template for creating objects. An object, on the other hand, is an instance of a class.

8. What is inheritance in C#?

   • Inheritance is a mechanism that allows a class to inherit properties and behaviors from another class. It promotes code reusability and establishes a "is-a" relationship between classes.

9. What is polymorphism in C#?

   • Polymorphism allows objects of different types to be treated as objects of a common base type. It includes method overloading and method overriding.

10. Explain the concepts of encapsulation and abstraction in C#.

    • Encapsulation is the bundling of data and methods that operate on the data into a single unit called a class. Abstraction is the process of hiding the implementation details and exposing only what is necessary.

C# Language Features:

11. What is the var keyword in C#?

    • The var keyword is used for implicitly typed local variables. The compiler determines the type based on the assigned value.

12. What is the purpose of the using statement in C#?

    • The using statement is used for automatic resource management. It ensures that the IDisposable object is properly disposed of when it goes out of scope.

13. Explain the difference between String and StringBuilder in C#.

    • String is immutable, meaning its value cannot be changed once it's assigned. StringBuilder is mutable and is more efficient for concatenating strings in scenarios with frequent modifications.

14. What is the significance of the async and await keywords in C#?

    • The async and await keywords are used for asynchronous programming. They allow the execution of asynchronous code without blocking the calling thread.

15. What is the purpose of the yield keyword in C#?

    • The yield keyword is used in iterator methods to simplify the implementation of enumerators. It returns each element of the collection one at a time.

Exception Handling in C#:

16. What is exception handling in C#?

    • Exception handling is a mechanism to handle runtime errors gracefully. It involves catching and handling exceptions to prevent application crashes.

17. Explain the difference between try, catch, and finally blocks in C#.

    • The try block contains the code that might throw an exception. The catch block handles the exception, and the finally block contains code that will be executed regardless of whether an exception is thrown.

18. What is the purpose of the throw statement in C#?

    • The throw statement is used to explicitly throw an exception. It is often used in conjunction with custom exception classes.

19. What is the role of the finally block in exception handling?

    • The finally block contains code that is executed whether an exception is thrown or not. It is commonly used for cleanup operations.

20. Explain the concept of custom exceptions in C#.

    • Custom exceptions are user-defined exception classes that inherit from the Exception class. They allow developers to create specific exception types for their applications.

.NET Framework and ASP.NET:

21. What is the .NET Framework?
    • The .NET Framework is a software framework developed by

Microsoft that provides a runtime environment for running applications. It includes a large class library known as the Base Class Library (BCL).

22. What is ASP.NET?

    • ASP.NET is a web application framework developed by Microsoft for building dynamic web applications and services. It is part of the .NET framework.

23. Explain the concept of ViewState in ASP.NET.

    • ViewState is a client-side state management technique used in ASP.NET to persist state information across postbacks. It stores the state of server-side controls on the client.

24. What is the purpose of the Global.asax file in ASP.NET?

    • The Global.asax file is used to handle application-level events in ASP.NET, such as application start, end, and error events.

25. Explain the difference between ASP.NET Web Forms and ASP.NET MVC.

    • ASP.NET Web Forms follows a stateful, event-driven programming model, while ASP.NET MVC follows a stateless, model-view-controller pattern. MVC is more suitable for complex applications and promotes separation of concerns.

LINQ and Entity Framework:

26. What is LINQ (Language Integrated Query) in C#?

    • LINQ is a set of features in C# that enables developers to write queries directly within the C# language. It supports querying arrays, collections, databases, XML, and more.

27. Explain the role of the select keyword in LINQ.

    • The select keyword in LINQ is used to project the result set. It specifies the data or transformation that should be returned in the query.

28. What is Entity Framework in C#?

    • Entity Framework (EF) is an Object-Relational Mapping (ORM) framework that enables developers to interact with databases using object-oriented principles. It provides a high-level abstraction over database operations.

29. What is Code-First in Entity Framework?

    • Code-First is an approach in Entity Framework where the database is created based on the entity classes and their relationships. Developers write the entity classes first, and the database schema is generated later.

30. Explain the concept of lazy loading in Entity Framework.

    • Lazy loading is a feature in Entity Framework that defers the loading of related entities until they are actually accessed. It helps improve performance by loading only the necessary data.

Web Services and RESTful APIs:

31. What is a Web Service in C#?

    • A web service is a software component that enables communication between different applications over a network. It can be accessed using standard protocols like HTTP.

32. Explain the difference between SOAP and REST.

    • SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information in web services. REST (Representational State Transfer) is an architectural style that uses standard HTTP methods for communication and is more lightweight.

33. What is WCF (Windows Communication Foundation) in C#?

    • WCF is a framework for building service-oriented applications in .NET. It enables the development of distributed and interoperable services using a variety of communication protocols.

34. What is the purpose of the DataContract attribute in WCF?

    • The DataContract attribute is used to mark a class as serializable by WCF. It specifies that the class should be included in data contracts for communication.

35. Explain the concept of RESTful APIs in C#.

    • RESTful APIs use HTTP methods (GET, POST, PUT, DELETE) to perform CRUD (Create, Read, Update, Delete) operations. They are designed to be stateless, scalable, and resource-oriented.

Unit Testing in C#:

36. What is unit testing?

    • Unit testing is a software testing method where individual units or components of a program are tested in isolation. It helps ensure that each unit of the software functions as expected.

37. Explain the purpose of NUnit and MSTest in C#.

    • NUnit and MSTest are popular unit testing frameworks in C#. They provide a structure for writing and executing unit tests, including features for assertions, test fixtures, and setup/teardown methods.

38. What is the AAA pattern in unit testing?

    • AAA stands for Arrange, Act, and Assert, which is a pattern followed in unit testing. It involves arranging the necessary preconditions, acting on the system under test, and asserting that the expected outcomes are met.

39. What is the Mock object in unit testing?

    • A mock object is a simulated object used in unit testing to mimic the behavior of real objects. It allows testing the interactions between different components of a system.

40. Explain the concept of test-driven development (TDD) in C#.

    • Test-driven development is a development approach where tests are written before the actual code. The developer writes a failing test, writes the minimum amount of code to make the test pass, and then refactors the code.

Asynchronous Programming in C#:

41. What is asynchronous programming in C#?

    • Asynchronous programming allows writing non-blocking code to improve application responsiveness. It is achieved using the async and await keywords.

42. Explain the purpose of the async and await keywords in C#.

    • The async keyword is used to define an asynchronous method, and the await keyword is used to asynchronously wait for the completion of a task without blocking the calling thread.

43. What is the Task Parallel Library (TPL) in C#?

    • The Task Parallel Library is a set of APIs in .NET for parallel and concurrent programming. It includes the Task class for asynchronous programming and parallelization.

44. How does the async modifier affect the return type of a method?

    • The async modifier changes the return type of a method to Task or Task<T>. It indicates that the method contains asynchronous operations that can be awaited.

45. What is the purpose of the Task.Run method in C#?

    • The Task.Run method is used to run a specified delegate asynchronously on a thread pool thread. It is often used to offload CPU-bound work from the main thread.

Dependency Injection and IoC Containers:

46. What is Dependency Injection (DI) in C#?

    • Dependency Injection is a design pattern in which the dependencies of a class are injected from the outside rather than created within the class. It promotes loose coupling and testability.

47. Explain the concept of an IoC (Inversion of Control) container.

    • An IoC container is a framework for managing and injecting dependencies in an application. It automates the process of creating and resolving objects, improving the maintainability and scalability of the code.

48. What is the purpose of the IServiceProvider interface in C#?

    • The IServiceProvider interface defines a mechanism for retrieving services or objects that are registered in an IoC container. It is commonly used in dependency injection scenarios.

49. What are some popular IoC containers in C#?

    • Popular IoC containers in C# include Unity, Autofac, Ninject, and Microsoft.Extensions.DependencyInjection.

50. Explain the difference between Constructor Injection and Property Injection in DI.

    • Constructor Injection involves passing dependencies through