C# Coding Interview Questions (with Answers)

C# has been a cornerstone of software development for decades, and it remains one of the most in-demand languages for technical interviews today. Companies rely on it for enterprise applications, APIs, cloud services, and even game development with Unity. Because of this, interviewers frequently use C# coding interview questions to evaluate how well you can solve problems and apply concepts in real-world scenarios.

If you’re preparing for a coding interview, mastering C# coding interview questions will give you a strong edge. These questions aren’t just about syntax. They test your ability to work with object-oriented programming, understand the .NET framework, manage memory efficiently, and design applications that scale.

Each section in this guide includes detailed explanations, code examples, and full interview-style problems. By the end, you’ll have a complete toolkit for answering C# questions confidently and clearly.

Why C# Is a Popular Choice in Coding Interviews

C# has stood the test of time because it balances power, flexibility, and ease of use. It is the backbone of the Microsoft ecosystem, but thanks to .NET Core and .NET 6+, it has also become a cross-platform language that works on Windows, Linux, and macOS. This makes it a top choice for companies building enterprise systems, APIs, cloud-based services, and scalable web applications.

When it comes to preparing for coding interviews, employers use C# coding interview questions to test:

How well you apply modern language features like LINQ and async/await.
Your ability to design with object-oriented principles.
Whether you can write scalable, maintainable code.

You’ll face many C# coding interview questions because they measure how effectively you can bridge theoretical knowledge with practical development skills. Knowing C# inside and out will make you stand out in both junior and senior interviews.

Categories of C# Coding Interview Questions

To prepare effectively, you need to understand the categories of questions interviewers will ask. Each one focuses on a different aspect of the language and its ecosystem. Here are the main types of C# coding interview questions:

Basic syntax and data types – Variables, operators, value vs reference types.
Object-Oriented Programming (OOP) in C# – Classes, inheritance, polymorphism, abstraction, encapsulation.
Collections and Generics – Lists, dictionaries, sets, and custom generic classes.
Exception handling – Try/catch/finally, custom exceptions, best practices.
LINQ and functional-style programming – Queries, filtering, grouping, and aggregation.
Asynchronous programming (async/await, tasks) – Handling concurrency with clean, readable code.
Delegates, events, and lambdas – Building event-driven and functional-style applications.
Memory management and garbage collection – Understanding .NET’s memory model and IDisposable.
Design patterns in C# – Singleton, Factory, Observer, and Dependency Injection.
Advanced topics – Reflection, attributes, multithreading, and dependency injection in .NET Core.
System design problems in C# – Larger architectural problems using C# as the implementation language.

This roadmap ensures you’ll cover everything from fundamentals to advanced scenarios. By working through each section, you’ll be ready for the full range of C# coding interview questions you’re likely to face.

Basic C# Coding Interview Questions

Foundational questions are common in interviews because they test your understanding of the building blocks of the language. Many C# coding interview questions begin here.

1. What are value types and reference types in C#?

Value types: Store data directly. Examples: int, bool, struct.
Reference types: Store references to memory locations. Examples: class, string, object.

int x = 10;   // value type

string y = “Hello”; // reference type

Answer: Value types are stored on the stack, while reference types are stored on the heap.

2. Explain var, dynamic, and object

var: Type is inferred at compile time.
dynamic: Type is resolved at runtime.
object: Base type of all types in C#.

var name = “Alice”; // compile-time type: string

dynamic d = 5;      // can change type at runtime

object o = “Hello”; // requires casting for specific operations

3. Difference between const and readonly

const: Must be assigned at compile time.
readonly: Assigned at runtime (constructor or declaration).

const double PI = 3.14;

readonly int age;

public MyClass(int age) {

    this.age = age; // valid for readonly

}

4. What is the difference between == and Equals() in C#?

==: Compares references for objects unless overloaded.
Equals(): Can be overridden to check for value equality.

string a = “hello”;

string b = new string(“hello”.ToCharArray());

Console.WriteLine(a == b);       // True

Console.WriteLine(a.Equals(b));  // True

5. Explain boxing and unboxing

Boxing: Converting a value type to an object.
Unboxing: Extracting a value type from an object.

int x = 10;

object obj = x; // boxing

int y = (int)obj; // unboxing

Takeaway: These basic concepts are the foundation of C#. Many C# coding interview questions test your fluency here before diving into advanced topics like LINQ or async programming.

Object-Oriented Programming in C#

Object-Oriented Programming (OOP) is the backbone of C#. Many C# coding interview questions revolve around your understanding of OOP principles because they show how well you can design maintainable and scalable applications.

1. Explain the four pillars of OOP

Encapsulation: Bundling data and behavior together, restricting direct access.
Inheritance: Reusing code by deriving new classes from existing ones.
Abstraction: Hiding implementation details and exposing only functionality.
Polymorphism: One interface, many implementations.

class Animal {

    public virtual void Speak() {

        Console.WriteLine(“Animal sound”);

    }

}

class Dog : Animal {

    public override void Speak() {

        Console.WriteLine(“Bark”);

    }

}

Here, inheritance allows Dog to extend Animal, abstraction hides how “Speak” works, and polymorphism lets different animals provide different behaviors.

2. What are abstract classes vs interfaces?

Abstract class: Can have both abstract (unimplemented) and concrete methods. Supports fields and constructors.
Interface: Only defines contracts (before C# 8.0). Multiple interfaces can be implemented.

abstract class Shape {

    public abstract double Area();

}

interface IPrintable {

    void Print();

}

Abstract classes are ideal when you share common base logic, while interfaces define capabilities.

3. Method overriding vs method overloading

Overriding: Changing behavior of a base class method using override.
Overloading: Defining multiple methods with the same name but different parameters.

class Calculator {

    public int Add(int a, int b) => a + b;             // Overload 1

    public double Add(double a, double b) => a + b;    // Overload 2

}

4. Difference between virtual, override, and new keywords

virtual: Marks a method in the base class as overridable.
override: Changes the implementation of a virtual method in a subclass.
new: Hides the base class method instead of overriding it.

class Base {

    public virtual void Show() => Console.WriteLine(“Base”);

}

class Derived : Base {

    public override void Show() => Console.WriteLine(“Derived”);

}

5. Real-world example of inheritance in C#

class Vehicle {

    public string Brand { get; set; }

    public void Start() => Console.WriteLine(“Starting vehicle”);

}

class Car : Vehicle {

    public int Doors { get; set; }

}

Here, Car inherits from Vehicle, reusing the Start() method and adding its own properties.

Takeaway: Expect plenty of C# coding interview questions on OOP. Interviewers want to see if you can use encapsulation, inheritance, abstraction, and polymorphism effectively in real-world applications.

Collections and Generics

Collections and generics are essential for handling data. Many C# coding interview questions test your ability to choose the right collection for the job and implement generic types.

1. Difference between Array, List, and Dictionary

Array: Fixed size, fast indexed access.
List: Resizable array.
Dictionary: Key-value storage.

int[] numbers = {1, 2, 3};

List<int> list = new List<int> {1, 2, 3};

Dictionary<int, string> dict = new() { {1, “One”}, {2, “Two”} };

2. What are generics and why are they useful?

Generics allow you to define type-safe classes and methods without committing to specific types. They improve reusability and performance.

List<string> names = new List<string>();

names.Add(“Alice”);

3. Example: Write a generic class for a stack

class Stack<T> {

    private List<T> _items = new();

    public void Push(T item) => _items.Add(item);

    public T Pop() {

        var last = _items[^1];

        _items.RemoveAt(_items.Count – 1);

        return last;

    }

}

4. Difference between IEnumerable, ICollection, and IList

IEnumerable: Supports iteration.
ICollection: Adds count, add, remove operations.
IList: Adds indexed access.

5. When should you use HashSet vs Dictionary?

HashSet: Store unique values.
Dictionary: Store key-value pairs.

HashSet<int> uniqueNumbers = new() {1,2,2,3}; // {1,2,3}

Dictionary<int, string> phoneBook = new() { {1,”John”} };

Takeaway: Collections are heavily tested in interviews. Knowing when to use arrays, lists, dictionaries, and generics is key to solving C# coding interview questions.

Exception Handling in C#

Handling errors gracefully is critical in production code. Many C# coding interview questions include scenarios around exception handling.

1. Purpose of try-catch-finally

try: Code that may throw an exception.
catch: Handles exceptions.
finally: Executes regardless of success/failure.

try {

    int x = 10/0;

} catch (DivideByZeroException ex) {

    Console.WriteLine(“Cannot divide by zero”);

} finally {

    Console.WriteLine(“Cleanup done”);

}

2. Checked vs unchecked exceptions

In C#, exceptions are unchecked (not enforced at compile time). Java differs here, so some interviewers ask this to check your understanding.

3. Custom exceptions with examples

class InvalidAgeException : Exception {

    public InvalidAgeException(string msg) : base(msg) {}

}

Use custom exceptions when the built-in ones don’t express the problem clearly.

4. Throw vs throw ex

throw: Preserves the original stack trace.
throw ex: Resets the stack trace (less preferred).

Takeaway: Always use structured exception handling. Expect C# coding interview questions about best practices, such as when to create custom exceptions and how to avoid swallowing errors.

LINQ in C# Coding Interview Questions

LINQ (Language Integrated Query) is one of the most popular C# features. Many C# coding interview questions focus on LINQ because it simplifies querying collections.

1. What is LINQ and why is it useful?

LINQ provides a consistent query syntax for objects, databases, XML, and more.

var nums = new List<int> {1,2,3,4,5};

var even = nums.Where(n => n % 2 == 0);

2. Deferred execution vs immediate execution

Deferred: Query runs when enumerated.
Immediate: Runs right away with methods like ToList(), Count().

var query = nums.Where(n => n > 2); // deferred

var list = query.ToList();          // executed now

3. LINQ query to group employees by department

var grouped = employees.GroupBy(e => e.Department);

foreach(var g in grouped)

    Console.WriteLine($”{g.Key}: {g.Count()}”);

4. Join two collections using LINQ

var result = from emp in employees

             join dept in departments

             on emp.DeptId equals dept.Id

             select new { emp.Name, dept.Name };

5. Explain Select, SelectMany, Where, and Aggregate

// Select

var names = employees.Select(e => e.Name);

// SelectMany (flattening)

var allSkills = employees.SelectMany(e => e.Skills);

// Where

var highSalary = employees.Where(e => e.Salary > 50000);

// Aggregate

int sum = nums.Aggregate((a,b) => a+b);

Takeaway: Mastering LINQ is crucial for interviews. Many C# coding interview questions require transforming, filtering, and aggregating data efficiently with LINQ.

Asynchronous Programming

Modern applications demand responsiveness. That’s why C# coding interview questions often test async programming.

1. Synchronous vs asynchronous programming

Synchronous: Tasks execute sequentially, blocking threads.
Asynchronous: Tasks execute concurrently without blocking.

2. How do async and await work in C#?

async marks a method as asynchronous. await pauses execution until a task completes, without blocking the thread.

async Task FetchData() {

    var client = new HttpClient();

    string data = await client.GetStringAsync(“https://example.com”);

    Console.WriteLine(data);

}

3. Task Parallel Library (TPL)

TPL provides higher-level abstractions for parallelism with the Task class.

Task t1 = Task.Run(() => Console.WriteLine(“Task 1”));

Task t2 = Task.Run(() => Console.WriteLine(“Task 2”));

Task.WaitAll(t1, t2);

4. Common pitfalls in async/await

Deadlocks when calling .Result or .Wait().
Forgetting to use await inside async methods.
Blocking UI threads.

5. Simple program to fetch data asynchronously

async Task Main() {

    var client = new HttpClient();

    string response = await client.GetStringAsync(“https://jsonplaceholder.typicode.com/posts/1”);

    Console.WriteLine(response);

}

Takeaway: Understanding async/await is critical. Many C# coding interview questions test whether you can build responsive applications without blocking threads.

Delegates, Events, and Lambdas

C# embraces event-driven and functional programming through delegates, events, and lambdas. Many C# coding interview questions explore these topics because they show how well you understand decoupled, flexible code.

1. What are delegates in C#?

A delegate is a type-safe function pointer. It defines a reference to a method.

public delegate void Notify(string message);

class Program {

    static void ShowMessage(string msg) => Console.WriteLine(msg);

    static void Main() {

        Notify notifier = ShowMessage;

        notifier(“Hello from delegate!”);

    }

}

Delegates are powerful because they allow passing methods as parameters.

2. Difference between Action, Func, and Predicate delegates

Action: Encapsulates methods with no return type.
Func: Encapsulates methods that return a value.
Predicate: Specialized Func<T, bool>.

Action<string> log = Console.WriteLine;

Func<int, int, int> add = (a, b) => a + b;

Predicate<int> isEven = n => n % 2 == 0;

3. What are events and how do they differ from delegates?

Delegates: Define method references.
Events: Provide controlled access to delegates, ensuring only subscribers can add/remove handlers.

public class Publisher {

    public event Action<string> OnMessage;

    public void Send(string msg) => OnMessage?.Invoke(msg);

}

4. Example: Event-driven notification system

class Subscriber {

    public void OnNotify(string msg) => Console.WriteLine($”Received: {msg}”);

}

var publisher = new Publisher();

var subscriber = new Subscriber();

publisher.OnMessage += subscriber.OnNotify;

publisher.Send(“New notification!”);

5. Explain lambda expressions in C# with examples

Lambdas are inline functions. They make delegates and LINQ queries concise.

Func<int, int> square = x => x * x;

Console.WriteLine(square(5)); // 25

Takeaway: Delegates, events, and lambdas are at the heart of flexible design. Expect plenty of C# coding interview questions around these topics, especially for event-driven or LINQ-heavy roles.

Memory Management and Garbage Collection

Memory management is critical in C#. The .NET runtime includes garbage collection (GC), but you must still understand its mechanics.

1. How does garbage collection work in C#?

The CLR automatically reclaims memory for objects no longer in use. GC uses a generational model:

Gen 0: Short-lived objects.
Gen 1: Objects surviving Gen 0 collections.
Gen 2: Long-lived objects.

2. What are finalizers and destructors?

A finalizer (~ClassName) is called before GC reclaims memory. It’s slow and should be avoided unless necessary.

class Resource {

    ~Resource() {

        Console.WriteLine(“Finalizer called”);

    }

}

3. Difference between IDisposable and using statement

IDisposable: Provides Dispose() for manual cleanup.
using: Ensures disposal automatically.

using (var reader = new StreamReader(“file.txt”)) {

    Console.WriteLine(reader.ReadLine());

}

4. Memory leaks in C# and how to prevent them

Common causes:

Event handlers not unsubscribed.
Static references holding objects.
Unmanaged resources not disposed.

Solution: Always call Dispose() or use using.

Takeaway: Many C# coding interview questions test GC and IDisposable because they show whether you can build efficient, leak-free apps.

Design Patterns in C#

Design patterns provide reusable solutions to common problems. Interviewers love asking about them.

1. Singleton Pattern

class Singleton {

    private static readonly Lazy<Singleton> instance =

        new(() => new Singleton());

    private Singleton() {}

    public static Singleton Instance => instance.Value;

}

Ensures only one instance exists. Thread-safe with Lazy<T>.

2. Factory Method Pattern

abstract class Vehicle { public abstract void Drive(); }

class Car : Vehicle { public override void Drive() => Console.WriteLine(“Car driving”); }

class Bike : Vehicle { public override void Drive() => Console.WriteLine(“Bike riding”); }

class VehicleFactory {

    public static Vehicle Create(string type) =>

        type switch {

            “car” => new Car(),

            “bike” => new Bike(),

            _ => throw new ArgumentException(“Invalid type”)

        };

}

3. Dependency Injection vs Service Locator

Dependency Injection: Dependencies passed in via constructors.
Service Locator: Central registry provides dependencies.

DI is preferred for testability and decoupling.

4. Observer Pattern with Events

class Stock {

    public event Action<int> OnPriceChange;

    public void UpdatePrice(int price) => OnPriceChange?.Invoke(price);

}

Subscribers react to updates automatically.

Why is it important in interviews?

Patterns show you understand architecture, not just syntax. Many C# coding interview questions test them for senior roles.

Advanced C# Coding Interview Questions

These advanced C# coding interview questions are common for senior roles where deeper .NET knowledge is tested.

1. Explain reflection in C# and its use cases

Reflection allows inspecting metadata at runtime.

var type = typeof(string);

foreach (var method in type.GetMethods())

    Console.WriteLine(method.Name);

Use cases: serialization, dependency injection, dynamic loading.

2. What are attributes in C#?

Attributes add metadata to code.

[Obsolete(“Use NewMethod instead”)]

public void OldMethod() {}

3. Dependency injection in .NET Core

services.AddScoped<IService, MyService>();

.NET Core provides built-in DI containers for managing lifetimes (Scoped, Transient, Singleton).

4. Multithreading vs async programming

Multithreading: Multiple threads run concurrently.
Async/await: Cooperative multitasking using Tasks.

5. Real-world: Thread-safe logger

class Logger {

    private static readonly object _lock = new();

    public void Log(string msg) {

        lock(_lock) {

            File.AppendAllText(“log.txt”, msg + “\n”);

        }

    }

}

Takeaway: These advanced C# coding interview questions reveal whether you can architect enterprise-grade systems.

Practice Section: Mock C# Coding Interview Questions

Practice is the best preparation. Here are 5 sample problems:

1. Implement a custom generic linked list

class Node<T> {

    public T Data;

    public Node<T> Next;

    public Node(T data) => Data = data;

}

2. LINQ query: Top 3 highest-paid employees

var top = employees.OrderByDescending(e => e.Salary).Take(3);

3. Async method to call multiple APIs

var t1 = client.GetStringAsync(url1);

var t2 = client.GetStringAsync(url2);

await Task.WhenAll(t1, t2);

Takeaway: Working through mock problems like these gives you hands-on readiness for real C# coding interview questions.

Tips for Solving C# Coding Interview Questions

Here are strategies to shine in interviews:

Write clean, modular code: Keep methods small and classes focused.
Explain time and space complexity: Show you understand performance, not just correctness.
Compare approaches: Highlight traditional loops vs LINQ, manual threads vs async/await.
Avoid overengineering: Stick to the simplest solution that works.
Talk through your reasoning: Interviewers value clarity of thought.

These tips will make your answers to C# coding interview questions clear, structured, and professional.

Wrapping Up

C# remains one of the strongest languages for enterprise and cross-platform development. By now, you’ve seen how interviews test everything from OOP and collections to LINQ, async/await, memory management, and design patterns.

Mastering C# coding interview questions will give you the confidence to succeed in both junior and senior interviews. It’s about showing you can design scalable, maintainable, and efficient systems.

The best way to prepare is through consistent practice. Solve coding challenges, work on real projects, and revisit the fundamentals regularly. Every mock question you solve builds the muscle memory you’ll need in interviews.

Keep learning modern C# features, apply them in real-world projects, and push yourself with interview-style challenges. Doing so will ensure you’re always ready to impress.

C# Coding Interview Questions​ (with Answers)