C# (pronounced C-Sharp) is a powerful, high-level, object-oriented programming language developed by Microsoft. Since its inception as part of the .NET framework, C# has become a cornerstone for building a wide range of applications—from enterprise-grade backend systems and cloud-native microservices to desktop applications using Windows Forms or WPF, and even immersive 3D games with Unity.
Its versatility, rich library ecosystem, strong type safety, and support for modern programming paradigms such as LINQ, asynchronous programming, and dependency injection make C# a top choice for developers and organizations alike.
For experienced software professionals, especially those targeting roles in enterprise .NET development, backend architecture, or game engineering, it’s not enough to simply understand the syntax of C#. You’re expected to demonstrate a deep understanding of language features, runtime behavior, architectural decisions, and best practices in real-world scenarios. Hiring managers increasingly look for developers who can write scalable, testable, and maintainable code while understanding things like memory management, multi-threading, and asynchronous workflows.
In this blog, we will break down the Top 10 most frequently asked C# interview questions specifically tailored for experienced developers. These are not just surface-level questions; they are designed to evaluate how well you apply your knowledge in solving complex technical challenges. Each question includes concise, practical explanations along with code examples to help you brush up on key concepts and prepare confidently for technical interviews.
Whether you’re aiming to land your next senior C# role, preparing for .NET system design interviews, or brushing up on fundamentals before stepping into a new project, this guide will help you build both competence and confidence in the most critical areas of the C# ecosystem.
1. What is the difference between IEnumerable and IQueryable?
IEnumerable<T> is used for in-memory collections, while IQueryable<T> is for querying out-of-memory data sources like databases.
Code Example:
csharp
// IEnumerable: operates on in-memory data
List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(n => n % 2 == 0); // In-memory filtering
// IQueryable: operates on database (like Entity Framework)
IQueryable<Product> products = dbContext.Products;
var expensiveProducts = products.Where(p => p.Price > 1000); // Translates to SQLUse IQueryable for deferred execution on external data sources and IEnumerable when working within memory.
2. Explain the use of async and await in C#.
These keywords simplify asynchronous programming by allowing non-blocking code execution.
Code Example:
csharp
public async Task<string> GetDataAsync()
{
HttpClient client = new HttpClient();
string result = await client.GetStringAsync("https://api.example.com/data");
return result;
}The method doesn’t block the main thread while waiting for data, improving performance in I/O-bound operations.
3. What is Dependency Injection in C#?
Dependency Injection (DI) is a design pattern that allows you to inject dependencies into a class rather than hardcoding them.
Code Example:
csharp
// Service
public interface IMessageService
{
void Send(string message);
}
public class EmailService : IMessageService
{
public void Send(string message) => Console.WriteLine($"Email: {message}");
}
// Consumer
public class Notification
{
private readonly IMessageService _service;
public Notification(IMessageService service)
{
_service = service;
}
public void Alert(string msg) => _service.Send(msg);
}DI promotes loose coupling, testability, and maintainability.
4. What are ref, out, and in parameters?
- ref allows both passing and modifying the value.
- out requires initialization inside the method.
- in makes the parameter read-only.
Code Example:
csharp
void Calculate(int a, int b, out int sum)
{
sum = a + b;
}5. Explain Garbage Collection in C#.
Garbage Collection (GC) in C# is an automatic memory management system. It frees unused objects in the heap to prevent memory leaks.
Key Concepts:
- Generations (0, 1, 2)
- GC.Collect() – Forces collection (use cautiously)
- Finalizers (destructors): ~MyClass() { }
GC improves performance but should be understood to optimize app behavior.
6. What is the difference between abstract and interface?
- An abstract class can have method definitions and implementations.
- An interface only contains definitions (until C# 8.0 added default implementations).
Code Example:
csharp
public interface ILogger
{
void Log(string message);
}
public abstract class FileLogger
{
public abstract void Log(string message);
public void SaveToFile() => Console.WriteLine("Saving log to file.");
}Use abstract classes when you need shared behavior; interfaces when defining contracts.
7. How does LINQ work internally?
LINQ (Language Integrated Query) translates query expressions into method calls using extension methods and lambda expressions.
Code Example:
csharp
var result = employees
.Where(e => e.Salary > 50000)
.Select(e => e.Name)
.ToList();Under the hood, LINQ converts the lambda into an expression tree (for IQueryable) or delegates (for IEnumerable).
8. What is the difference between Task and Thread?
- Thread creates a new thread (OS-level).
- Task is a lightweight object from the Task Parallel Library (TPL), and managed by the .NET runtime.
Code Example:
csharp
// Task
await Task.Run(() => Console.WriteLine("Running async"));
// Thread
Thread t = new Thread(() => Console.WriteLine("Running thread"));
t.Start();Prefer Task for better resource utilization.
9. What are extension methods in C#?
Extension methods let you add functionality to existing types without modifying them.
Code Example:
csharp
public static class StringExtensions
{
public static bool IsEmail(this string input)
{
return input.Contains("@");
}
}
string email = "test@example.com";
bool valid = email.IsEmail(); // trueUsed heavily in LINQ and utility libraries.
10. What is the role of yield keyword?
yield enables lazy evaluation, generating values on-demand.
Code Example:
csharp
public IEnumerable<int> GetEvenNumbers()
{
for (int i = 0; i < 10; i++)
{
if (i % 2 == 0)
yield return i;
}
}This helps improve performance for large datasets by avoiding unnecessary memory consumption.
Looking for premium job opportunities as a C# or .NET developer?
Conclusion
C# is a versatile, mature language that underpins countless enterprise applications. Interviewers today expect experienced professionals to go beyond theory—they want to see practical coding skills, architectural reasoning, and familiarity with real-world challenges.
By mastering these C# interview questions and backing them with code, you’ll be better prepared to discuss complex technical scenarios confidently. Practice consistently, contribute to open-source, and build production-grade applications using C#, and you’ll be positioned not only to succeed in interviews—but to lead high-impact development teams.
Whether you’re preparing for a .NET-focused role or aiming to level up your backend expertise, deep diving into topics like async programming, memory management, and testability will set you apart.
Talent500 is your go-to platform to land the best global jobs at top Global Capability Centers (GCCs) in India. Whether you’re a backend expert, a fullstack engineer, or specialize in cloud-native .NET applications, Talent500 connects you with Fortune 500 companies and fast-growing startups through vetted roles that offer scale, innovation, and career growth.
