Top 30 Most Common C# Programming Interview Questions You Should Prepare For

Landing a job as a C# developer requires more than just coding skills; you also need to effectively communicate your knowledge during the interview process. C# programming interview questions can range from fundamental concepts to complex architectural patterns and problem-solving scenarios. Preparing thoroughly for these common C# programming interview questions is key to demonstrating your proficiency and confidence. This guide covers 30 frequently asked C# programming interview questions, providing concise, answer-ready responses to help you ace your next technical discussion.

What Are C# Programming Interview Questions?

C# programming interview questions are inquiries posed by hiring managers and technical interviewers to assess a candidate's understanding of the C# language, the .NET framework, and related technologies. These questions cover a broad spectrum, including object-oriented principles, data structures, algorithms, error handling, asynchronous programming, web development concepts (like ASP.NET Core and Web APIs), database interaction (Entity Framework), and general software design principles. The difficulty and depth of these C# programming interview questions often depend on the seniority level of the role. Junior roles might focus on fundamentals, while senior roles delve into performance optimization, design patterns, and system architecture. Mastering common C# programming interview questions is essential for showcasing your technical foundation.

Why Do Interviewers Ask C# Programming Interview Questions?

Interviewers ask C# programming interview questions for several critical reasons. Firstly, they need to validate your claimed skills and experience with the C# language and the .NET ecosystem. The responses to C# programming interview questions reveal your depth of knowledge, understanding of best practices, and ability to think critically about coding problems. Secondly, these questions help assess your problem-solving capabilities and how you approach technical challenges. Discussing different approaches to a problem or explaining complex concepts through C# programming interview questions shows your analytical thinking. Lastly, technical questions, including specific C# programming interview questions, gauge your communication skills, your passion for technology, and how well you might fit into the team's technical culture. Your ability to articulate technical concepts clearly is as important as knowing the answer itself.

Preview List

1. What is C#?

2. How is C# different from C?

3. What is Inheritance in C#?

4. What is the Difference Between Array and List?

5. What is the Purpose of a Web API?

6. How Do You Secure a C# Web API?

7. What is the Difference Between GET and POST in API Calls?

8. What is the Role of the Finally Block in Exception Handling?

9. How Do You Create a Custom Exception in C#?

10. What is Lazy Loading in Entity Framework?

11. What is the Difference Between EF Core and EF6?

12. How Do You Implement Migrations in Entity Framework?

13. What is an Abstract Class in C#?

14. How Does an Abstract Class Differ from an Interface?

15. Can an Abstract Class Have Constructors?

16. What is the Use of a HashSet in C#?

17. What is the Time Complexity of Accessing Elements in a List?

18. What are Partial Classes in C#?

19. What is the Difference Between String and StringBuilder?

20. What is Method Overloading in C#?

21. What is a Sealed Class in C#?

22. What is the Accessibility Modifier "Protected Internal"?

23. Can a Private Virtual Method Be Overridden?

24. What is Multithreading in C#?

25. How Do You Implement Multithreading in C#?

26. What is the Difference Between Array and ArrayList?

27. How Do You Return Custom Responses from a Controller?

28. Write a Program to Find the Second-Largest Number in an Array.

29. Write a Program to Merge Two Arrays.

30. Write a Program to Rotate an Array.

1. What is C#?

Why you might get asked this:

Tests your basic understanding of what C# is, its origins, and its fundamental nature within the .NET ecosystem, a common starting point for C# programming interview questions.

How to answer:

Define C# as an object-oriented language by Microsoft for .NET, highlighting its similarities to C/C++/Java and managed execution.

Example answer:

C# is a modern, object-oriented programming language developed by Microsoft. It runs on the .NET Framework or .NET and is primarily used for developing Windows applications, web applications, and services. It draws inspiration from C++, Java, and other languages.

2. How is C# different from C?

Why you might get asked this:

Checks if you understand the fundamental shift from procedural/low-level programming to object-oriented and managed languages, a classic C# programming interview questions type.

How to answer:

Highlight C#'s high-level nature, OOP features, garbage collection, type safety, and reliance on a runtime environment (.NET), contrasting it with C's manual memory management and lower-level focus.

Example answer:

C# is a high-level, object-oriented language with automatic garbage collection, type safety, and exception handling built-in. C is a lower-level, procedural language requiring manual memory management, lacking direct OOP support and runtime safety checks.

3. What is Inheritance in C#?

Why you might get asked this:

Evaluates your grasp of core OOP principles, a standard topic in C# programming interview questions.

How to answer:

Explain inheritance as allowing a new class (derived) to inherit members from an existing class (base), mentioning C#'s single inheritance for classes but support for multiple interface inheritance.

Example answer:

Inheritance is an OOP mechanism where a new class derives properties and behaviors from an existing class. This promotes code reusability. C# supports single inheritance for classes, meaning a class can inherit from only one base class, but it can implement multiple interfaces.

4. What is the Difference Between Array and List?

Why you might get asked this:

Assesses your knowledge of fundamental collection types and their characteristics, a practical question among C# programming interview questions.

How to answer:

State that Arrays are fixed-size, value-type elements potentially, while Lists are dynamic, reference-type elements, mentioning List's flexibility in size.

Example answer:

An Array is a fixed-size collection allocated contiguously in memory. A List is a dynamic collection provided by the System.Collections.Generic namespace that can grow or shrink in size, offering more flexibility for adding or removing elements.

5. What is the Purpose of a Web API?

Why you might get asked this:

Tests your understanding of modern web service architecture, crucial for many C# development roles and relevant C# programming interview questions.

How to answer:

Describe Web APIs as frameworks for building HTTP services that can be consumed by various clients (browsers, mobile apps, other servers) using standard web protocols.

Example answer:

A Web API is used to create HTTP services that expose data and functionality over the internet. It's primarily used to build RESTful services that allow different applications or systems to communicate with each other using standard request methods like GET, POST, PUT, DELETE.

6. How Do You Secure a C# Web API?

Why you might get asked this:

Evaluates your awareness of security best practices in web development, a critical aspect often covered in C# programming interview questions.

How to answer:

Mention authentication (e.g., JWT, OAuth), authorization (role-based access), data encryption (HTTPS), input validation, and protecting against common web vulnerabilities.

Example answer:

Securing a Web API involves authentication (verifying the user's identity), authorization (controlling what they can access), using HTTPS for encrypted communication, input validation to prevent injection attacks, and potentially rate limiting and logging. JWT and OAuth are common authentication methods.

7. What is the Difference Between GET and POST in API Calls?

Why you might get asked this:

Tests your understanding of fundamental HTTP methods, a common point in C# programming interview questions related to web development.

How to answer:

Explain GET is for retrieving data and should be idempotent and safe (no side effects), while POST is for sending data to create/update resources and can have side effects.

Example answer:

GET requests are used to retrieve data from a server and should not change the server's state; they are idempotent and safe. POST requests are used to send data to a server, typically to create or update a resource; they are not idempotent.

8. What is the Role of the Finally Block in Exception Handling?

Why you might get asked this:

Assesses your knowledge of robust error handling, a fundamental concept in C# programming interview questions.

How to answer:

State that the finally block guarantees execution regardless of whether an exception occurred in the try or catch block, typically used for cleanup like closing resources.

Example answer:

The finally block is used in exception handling (try-catch-finally) to contain code that must execute regardless of whether an exception was thrown or caught. It's commonly used for releasing resources, like closing file streams or database connections.

9. How Do You Create a Custom Exception in C#?

Why you might get asked this:

Tests your ability to extend the framework and create domain-specific error types, showing a deeper understanding of C# programming interview questions on exception handling.

How to answer:

Explain that you define a new class that inherits from System.Exception or a more specific exception type, often including standard constructors.

Example answer:

You create a custom exception by defining a class that inherits from System.Exception or one of its derived classes (like ApplicationException). It's good practice to include the standard constructors that match the base Exception class for compatibility.

10. What is Lazy Loading in Entity Framework?

Why you might get asked this:

Evaluates your understanding of ORM performance features, a relevant topic in C# programming interview questions for data-driven applications.

How to answer:

Describe lazy loading as delaying the loading of related data until that data is actually accessed, often enabled by default for navigation properties.

Example answer:

Lazy loading in Entity Framework is a technique where related data (like a list of orders for a customer) is not retrieved from the database immediately when the parent entity is loaded. Instead, it's loaded automatically the first time the navigation property is accessed.

11. What is the Difference Between EF Core and EF6?

Why you might get asked this:

Tests your knowledge of the evolution of Microsoft's ORM and its cross-platform capabilities, important for modern C# programming interview questions.

How to answer:

Explain EF Core is a complete rewrite, cross-platform, lightweight, and more extensible, while EF6 is Windows-only and part of the .NET Framework, though it can be used with .NET Core with some limitations.

Example answer:

EF Core is the newer, lightweight, extensible, and cross-platform version of Entity Framework. EF6 is the older version, primarily for the .NET Framework (Windows-only), though limited support exists for .NET Core. EF Core offers better performance in many scenarios and a redesigned architecture.

12. How Do You Implement Migrations in Entity Framework?

Why you might get asked this:

Assesses your practical experience with database schema evolution using EF, a common task addressed by C# programming interview questions about data access.

How to answer:

Describe the process using the dotnet ef or Add-Migration/Update-Database commands in the command line or Package Manager Console to scaffold changes and apply them.

Example answer:

You use the EF Core command-line tools or Package Manager Console commands. First, run dotnet ef migrations add [MigrationName] (or Add-Migration [MigrationName]) to scaffold the migration code based on model changes. Then, run dotnet ef database update (or Update-Database) to apply the changes to the database.

13. What is an Abstract Class in C#?

Why you might get asked this:

Tests your understanding of abstraction and class design, fundamental OOP concepts found in C# programming interview questions.

How to answer:

Define an abstract class as a base class that cannot be instantiated on its own, intended for inheritance, and capable of having both abstract (no implementation) and concrete methods.

Example answer:

An abstract class is a class that cannot be directly instantiated. It serves as a base class for other classes and can contain abstract members (declared but not implemented) which derived classes must override, as well as concrete members with implementation.

14. How Does an Abstract Class Differ from an Interface?

Why you might get asked this:

Evaluates your ability to distinguish between key abstraction mechanisms, a common and important C# programming interview questions topic.

How to answer:

Contrast that abstract classes can have concrete methods, state (fields), and constructors, while interfaces only define a contract of members without implementation (though default interface methods exist since C# 8). A class can inherit one abstract class but implement multiple interfaces.

Example answer:

An abstract class can have fields, properties, abstract methods, and concrete methods with implementation. An interface can only declare members (methods, properties, events, indexers) and since C# 8, default method implementations. A class can inherit one abstract class but many interfaces.

15. Can an Abstract Class Have Constructors?

Why you might get asked this:

A nuanced question testing a specific detail about abstract classes, often included in more in-depth C# programming interview questions.

How to answer:

Yes, abstract classes can have constructors. Explain that these constructors are called by the constructors of the derived classes using the base keyword, as the abstract class cannot be instantiated directly.

Example answer:

Yes, an abstract class can have constructors. While you cannot create an instance of an abstract class directly, its constructors can be called by the constructors of concrete derived classes using the base keyword to initialize members defined in the abstract class.

16. What is the Use of a HashSet in C#?

Why you might get asked this:

Assesses your knowledge of different collection types and their use cases, a common theme in practical C# programming interview questions.

How to answer:

Explain that HashSet stores unique elements and is optimized for very fast lookups, additions, and removals, sacrificing element ordering.

Example answer:

A HashSet is a collection that stores unique elements without any specific order. Its primary benefit is highly efficient performance (close to O(1) on average) for operations like adding, removing, and checking for the existence of an element. It's ideal for tasks like removing duplicates or checking membership quickly.

17. What is the Time Complexity of Accessing Elements in a List?

Why you might get asked this:

Tests your understanding of collection performance characteristics, important for writing efficient code and answering C# programming interview questions about data structures.

How to answer:

State that accessing an element by its index in a List is an O(1) operation, meaning it takes constant time regardless of the list's size.

Example answer:

Accessing an element in a List by its index (e.g., myList[index]) has a time complexity of O(1). This means the time taken to access an element is constant, regardless of how many elements are in the list, because the list is internally based on an array.

18. What are Partial Classes in C#?

Why you might get asked this:

Evaluates your knowledge of language features that aid code organization, particularly relevant in large projects or with generated code, a common topic in C# programming interview questions.

How to answer:

Describe partial classes as a way to split the definition of a single class, struct, or interface across multiple source files using the partial keyword.

Example answer:

Partial classes allow you to split the definition of a class, struct, or interface into two or more source files. When the application is compiled, all parts are combined into a single class. This is often used when working with automatically generated code (like in WinForms or LINQ to SQL) alongside manual code.

19. What is the Difference Between String and StringBuilder?

Why you might get asked this:

Tests your understanding of immutability and performance optimization for string manipulation, a classic question among C# programming interview questions.

How to answer:

Explain that string objects are immutable (cannot be changed after creation), leading to new objects on modification, while StringBuilder is mutable and efficient for frequent string modifications.

Example answer:

System.String is immutable; every modification operation creates a new string object. System.Text.StringBuilder is mutable; it allows efficient modification of a string without creating new objects repeatedly, making it suitable for operations involving many concatenations or changes.

20. What is Method Overloading in C#?

Why you might get asked this:

Evaluates your knowledge of polymorphism and function signature resolution, a core OOP concept in C# programming interview questions.

How to answer:

Define method overloading as having multiple methods in the same class with the same name but different parameter lists (different number, types, or order of parameters).

Example answer:

Method overloading is defining multiple methods in the same class with the same name but distinct parameter lists (signatures). The compiler determines which overload to call based on the number, types, and order of the arguments provided at the call site.

21. What is a Sealed Class in C#?

Why you might get asked this:

Tests your understanding of controlling inheritance and class design, a specific C# language feature often included in C# programming interview questions.

How to answer:

Explain that a sealed class cannot be inherited from, preventing further derivation and often used for security or versioning reasons.

Example answer:

A sealed class is a class that cannot be used as a base class by other classes. You apply the sealed modifier to a class declaration. This is often done to prevent unintended inheritance or to optimize performance in some scenarios.

22. What is the Accessibility Modifier "Protected Internal"?

Why you might get asked this:

Assesses your detailed knowledge of accessibility levels, a common point in C# programming interview questions regarding object visibility.

How to answer:

Describe protected internal as allowing access within the same assembly OR by derived classes in any other assembly. It's a combination of protected and internal.

Example answer:

The protected internal accessibility modifier allows access to a member within the same assembly (like internal) AND from derived classes in other assemblies (like protected). It's essentially the union of protected and internal access.

23. Can a Private Virtual Method Be Overridden?

Why you might get asked this:

A tricky question that probes a specific interaction between accessibility and polymorphism, designed to test your precise understanding in C# programming interview questions.

How to answer:

No. Explain that private members are only accessible within the declaring class, so a derived class cannot even see or access a private virtual method, let alone override it.

Example answer:

No, a private virtual method cannot be overridden. The private access modifier restricts visibility and access solely to the declaring class, making it impossible for a derived class to see or call the virtual method, thus preventing overriding.

24. What is Multithreading in C#?

Why you might get asked this:

Tests your understanding of concurrent programming, a crucial topic for performance and responsiveness, frequently appearing in C# programming interview questions.

How to answer:

Define multithreading as allowing multiple execution paths (threads) within a single program concurrently, enabling simultaneous task execution and improving application responsiveness.

Example answer:

Multithreading is the ability of a program to manage the execution of multiple threads simultaneously. Each thread represents a separate path of execution within the same process, allowing different parts of the program to run concurrently, which can improve performance and responsiveness.

25. How Do You Implement Multithreading in C#?

Why you might get asked this:

Assesses your practical experience with concurrency APIs, a common follow-up to the previous question among C# programming interview questions.

How to answer:

Mention using System.Threading.Thread, the ThreadPool, Task and Task from the Task Parallel Library (TPL), or async/await keywords for asynchronous operations which often leverage threads.

Example answer:

Multithreading can be implemented using several approaches: the System.Threading.Thread class for explicit thread creation, the ThreadPool for managing a pool of worker threads, or more commonly, using the Task Parallel Library (TPL) with Task objects and the async/await keywords for easier asynchronous programming.

26. What is the Difference Between Array and ArrayList?

Why you might get asked this:

Evaluates your awareness of older vs. newer collection types and the benefits of generics, a comparison often made in C# programming interview questions.

How to answer:

Explain that Array is fixed-size and strongly typed, while ArrayList (from System.Collections) is dynamic and non-generic (stores object), requiring boxing/unboxing and offering less type safety than List.

Example answer:

An Array is fixed in size and strictly typed (int[], string[]). ArrayList is dynamically sized but stores elements as object, requiring casting (boxing/unboxing) and lacking type safety at compile time. List is the preferred modern alternative to ArrayList.

27. How Do You Return Custom Responses from a Controller?

Why you might get asked this:

Tests your practical knowledge of building Web APIs or MVC applications, a common scenario addressed by C# programming interview questions.

How to answer:

Explain using built-in ActionResult types like Ok(), NotFound(), BadRequest(), StatusCode(), or returning custom objects which the framework serializes (usually to JSON).

Example answer:

In ASP.NET Core controllers, you can return ActionResult types like Ok(data), NotFound(), BadRequest("message"), or StatusCode(500). You can also return custom objects directly, and the framework will automatically serialize them, typically to JSON, with a default 200 OK status.

28. Write a Program to Find the Second-Largest Number in an Array.

Why you might get asked this:

A common coding challenge among C# programming interview questions to assess basic algorithm and array manipulation skills.

How to answer:

Provide a concise C# code snippet that sorts the array and accesses the second-to-last element, or an approach that iterates once or twice to find the largest and second-largest.

Example answer:

int[] nums = { 12, 45, 7, 23, 56, 89, 34 };
Array.Sort(nums);
// Handle edge cases like array size < 2
if (nums.Length >= 2) {
    int secondLargest = nums[nums.Length - 2];
    Console.WriteLine($"Second largest: {secondLargest}");
} else {
    Console.WriteLine("Array too small.");
}

29. Write a Program to Merge Two Arrays.

Why you might get asked this:

A basic data manipulation question in C# programming interview questions to check array handling and memory allocation understanding.

How to answer:

Provide a code snippet that creates a new array with a combined size and copies elements from both source arrays into it.

Example answer:

int[] arr1 = { 1, 2, 3 };
int[] arr2 = { 4, 5, 6 };
int[] merged = new int[arr1.Length + arr2.Length];
Array.Copy(arr1, 0, merged, 0, arr1.Length);
Array.Copy(arr2, 0, merged, arr1.Length, arr2.Length);
Console.WriteLine(string.Join(", ", merged)); // Output: 1, 2, 3, 4, 5, 6

30. Write a Program to Rotate an Array.

Why you might get asked this:

A common algorithmic question in C# programming interview questions testing array manipulation logic.

How to answer:

Provide a code snippet demonstrating array rotation by a given number of positions, likely involving temporary storage or multiple reversals.

Example answer:

int[] arr = { 1, 2, 3, 4, 5 };
int k = 2; // Rotate by 2 positions
int n = arr.Length;
k = k % n; // Handle rotations larger than array size

int[] rotated = new int[n];
Array.Copy(arr, n - k, rotated, 0, k);
Array.Copy(arr, 0, rotated, k, n - k);

Console.WriteLine(string.Join(", ", rotated)); // Output for k=2: 4, 5, 1, 2, 3

Other Tips to Prepare for a C# Programming Interview Questions

Beyond mastering these specific C# programming interview questions, holistic preparation is key. Practice writing code on a whiteboard or shared editor to simulate the environment. "Code is like humor. When you have to explain it, it’s bad," says a fictional programming guru, highlighting the importance of writing clear, self-explanatory code during live coding segments often accompanying C# programming interview questions. Review fundamental data structures and algorithms; while C# specific syntax is important, underlying computer science principles are crucial. Prepare questions to ask the interviewer; this shows engagement and genuine interest in the role and company. Use resources like Verve AI Interview Copilot to practice your responses to common C# programming interview questions and get feedback on your delivery and content. Verve AI Interview Copilot can help you refine your answers and gain confidence. Leverage Verve AI Interview Copilot at https://vervecopilot.com for mock interviews tailored to C# programming interview questions, helping you identify areas for improvement. Remember, interviewers are also assessing your cultural fit and enthusiasm. Be prepared to discuss past projects, challenges you've faced, and how you overcame them. "The only way to do great work is to love what you do," a sentiment attributed to Steve Jobs, resonates with demonstrating passion during your interview, especially when discussing complex C# programming interview questions or project experiences. Use tools like Verve AI Interview Copilot to rehearse explaining technical concepts clearly and concisely.

Frequently Asked Questions

Q1: How important is knowing .NET Core vs .NET Framework for C# programming interview questions?
A1: Very important. Modern roles often require .NET Core/.NET knowledge due to cross-platform capabilities, but knowing Framework shows foundational understanding.

Q2: Should I prepare for algorithm questions for C# programming interview questions?
A2: Yes, basic to intermediate algorithm and data structure questions are common to test problem-solving skills.

Q3: How do I practice coding problems for C# programming interview questions?
A3: Use platforms like LeetCode, HackerRank, or Advent of Code, implementing solutions in C#.

Q4: Is it okay to say "I don't know" to a C# programming interview question?
A4: Yes, it's better than guessing incorrectly. You can then offer to think through how you might find the answer or approach the problem.

Q5: How can Verve AI Interview Copilot help with C# programming interview questions?
A5: Verve AI Interview Copilot offers AI-powered mock interviews and feedback, helping you practice answering common C# programming interview questions effectively.

Q6: What non-technical aspects are assessed during C# programming interview questions?
A6: Communication skills, problem-solving approach, enthusiasm, cultural fit, and ability to explain technical concepts.