10 Must Ask C# Interview Questions For Hiring Managers

Posted on March 27 2024 by Interview Zen Team

Introduction

C# is a versatile, object-oriented programming language developed by Microsoft as part of its .NET initiative. Known for its strong typing, automatic memory management, and extensive library support, C# has become a cornerstone of enterprise application development and modern software engineering.

According to the Stack Overflow 2024 Developer Survey, C# ranks among the top 10 most popular programming languages, used by over 27% of professional developers worldwide. Its integration with the Microsoft ecosystem and cross-platform capabilities make C# skills highly valuable in today’s job market.

This comprehensive guide provides hiring managers with essential C# interview questions designed to evaluate candidates’ understanding of language fundamentals, object-oriented principles, and practical application in .NET development.

What is C#?

C# (pronounced “C-sharp”) is a modern, object-oriented programming language developed by Microsoft in 2000. It runs on the .NET framework and combines the power of C++ with the simplicity of Visual Basic. C# is designed for building a wide variety of applications that run on .NET.

Key characteristics of C# include:

  • Type safety: Strong typing prevents many runtime errors
  • Automatic memory management: Garbage collection handles memory allocation/deallocation
  • Platform independence: Runs on Windows, Linux, and macOS via .NET Core/.NET 5+
  • Rich ecosystem: Extensive base class library and third-party packages
  • Versatile: Supports desktop, web, mobile, cloud, and game development

Top 10 Must-Ask C# Interview Questions

1. What is the difference between value types and reference types?

This fundamental concept affects memory management and performance.

Example Answer: “Value types store data directly and are allocated on the stack (int, bool, struct). Reference types store references to data allocated on the heap (class, string, array).

int a = 10;        // Value type - stored on stack
int b = a;         // b gets copy of a's value
b = 20;            // a is still 10

Person p1 = new Person("John");  // Reference type - stored on heap
Person p2 = p1;                  // p2 references same object as p1
p2.Name = "Jane";                // p1.Name is also "Jane"
```"

### 2. Explain the four pillars of Object-Oriented Programming in C#.

OOP principles are essential for C# development.

**Example Answer**: 
- "**Encapsulation**: Bundling data and methods, controlling access via access modifiers
```csharp
public class BankAccount {
    private decimal balance;  // Private field
    public decimal GetBalance() => balance;  // Public method
}
  • Inheritance: Classes inherit from base classes
  • Polymorphism: Same interface, different implementations (method overriding)
  • Abstraction: Hiding complex implementation details (abstract classes, interfaces)”

3. What is the difference between abstract classes and interfaces?

Understanding these concepts is crucial for design patterns.

Example Answer:

  • “Abstract classes can have both abstract and concrete methods, fields, constructors. Support single inheritance.
  • Interfaces define contracts with only method signatures (C# 8+ allows default implementations). Support multiple inheritance. ```csharp public abstract class Animal { protected string name; public abstract void MakeSound(); public void Sleep() => Console.WriteLine(“Sleeping”); }

public interface IFlyable { void Fly(); }

public class Bird : Animal, IFlyable { public override void MakeSound() => Console.WriteLine(“Chirp”); public void Fly() => Console.WriteLine(“Flying”); }


### 4. Explain async/await and when to use it.

Asynchronous programming is essential for modern C# applications.

**Example Answer**: "async/await enables non-blocking asynchronous operations, improving application responsiveness:
```csharp
public async Task<string> GetDataAsync() {
    using var client = new HttpClient();
    // Doesn't block calling thread
    string response = await client.GetStringAsync("https://api.example.com/data");
    return response;
}

// Calling code
public async Task ProcessData() {
    string data = await GetDataAsync();  // Waits without blocking UI
    Console.WriteLine(data);
}

Use for I/O operations, HTTP calls, database queries, file operations.”

5. What are generics and what are their benefits?

Generics provide type safety and performance benefits.

Example Answer: “Generics allow writing type-safe code that works with any data type:

public class Repository<T> where T : class {
    private List<T> items = new List<T>();
    
    public void Add(T item) => items.Add(item);
    public T GetById(int id) => items[id];
    public IEnumerable<T> GetAll() => items;
}

var userRepo = new Repository<User>();
var productRepo = new Repository<Product>();

Benefits: Type safety at compile time, no boxing/unboxing overhead, code reusability.”

6. Explain exception handling in C#.

Proper error handling is critical for robust applications.

Example Answer: “Use try-catch-finally blocks for exception handling:

try {
    int result = Divide(10, 0);
} 
catch (DivideByZeroException ex) {
    Console.WriteLine($"Division error: {ex.Message}");
    // Log exception details
} 
catch (Exception ex) {
    Console.WriteLine($"Unexpected error: {ex.Message}");
    // Handle general exceptions
} 
finally {
    // Cleanup code that always executes
    CleanupResources();
}

// Custom exceptions
public class InvalidUserException : Exception {
    public InvalidUserException(string message) : base(message) { }
}
```"

### 7. What is the difference between `String` and `StringBuilder`?

String manipulation performance is often critical.

**Example Answer**: "String is immutable - each modification creates a new string object. StringBuilder is mutable and efficient for multiple string operations:
```csharp
// Inefficient - creates multiple string objects
string result = "";
for (int i = 0; i < 1000; i++) {
    result += i.ToString();  // Creates new string each time
}

// Efficient - single buffer, resized as needed
var sb = new StringBuilder();
for (int i = 0; i < 1000; i++) {
    sb.Append(i);  // Modifies existing buffer
}
string result = sb.ToString();

Use StringBuilder for multiple concatenations, String for simple operations.”

8. Explain LINQ and provide examples.

LINQ is a powerful feature for data querying and manipulation.

Example Answer: “LINQ (Language Integrated Query) provides unified syntax for querying collections:

var users = new List<User> {
    new User { Name = "John", Age = 30, City = "NYC" },
    new User { Name = "Jane", Age = 25, City = "LA" },
    new User { Name = "Bob", Age = 35, City = "NYC" }
};

// Method syntax
var adults = users.Where(u => u.Age >= 18)
                  .OrderBy(u => u.Name)
                  .Select(u => new { u.Name, u.Age });

// Query syntax
var nycUsers = from u in users
               where u.City == "NYC"
               orderby u.Age
               select u;

Works with objects, databases (Entity Framework), XML, and more.”

9. What are delegates and events?

Delegates enable functional programming patterns and event-driven architecture.

Example Answer: “Delegates are type-safe function pointers that can hold references to methods:

public delegate void ProcessData(string data);

public class DataProcessor {
    public event ProcessData OnDataProcessed;  // Event based on delegate
    
    public void Process(string data) {
        // Process data
        OnDataProcessed?.Invoke(data);  // Notify subscribers
    }
}

// Usage
var processor = new DataProcessor();
processor.OnDataProcessed += data => Console.WriteLine($"Processed: {data}");
processor.OnDataProcessed += data => LogData(data);
processor.Process("Hello World");

Events provide publisher-subscriber pattern for loose coupling.”

10. How does garbage collection work in C#?

Memory management understanding is crucial for performance optimization.

Example Answer: “C# uses automatic garbage collection to manage memory:

  • Generations: Objects organized into Gen 0 (short-lived), Gen 1 (medium), Gen 2 (long-lived)
  • Mark and Sweep: GC marks reachable objects, sweeps unreachable ones
  • Finalization: Objects with finalizers get additional cleanup phase 
    public class ResourceManager : IDisposable {
  private bool disposed = false;
    
  public void Dispose() {
      Dispose(true);
      GC.SuppressFinalize(this);  // Skip finalization
  }
    
  protected virtual void Dispose(bool disposing) {
      if (!disposed) {
          if (disposing) {
              // Dispose managed resources
          }
          // Dispose unmanaged resources
          disposed = true;
      }
  }
}

    Use using statements or IDisposable for deterministic resource cleanup.”

Modern C# Features

For advanced positions, explore these recent additions:

C# 9.0+ Features

  • Records: Immutable reference types
  • Pattern matching: Enhanced switch expressions
  • Init-only properties: public string Name { get; init; }
  • Top-level programs: Simplified Program.cs

C# 10+ Features

  • Global using directives: global using System;
  • File-scoped namespaces: namespace MyApp.Services;
  • Constant interpolated strings: const string Template = $"Hello {World}";

Performance Features

  • **Span** and **Memory**: Zero-allocation array operations
  • ref structs: Stack-only types for performance
  • ValueTask: Reduced allocations for async operations

Practical Assessment Tips

When interviewing C# candidates:

  1. Code review: Present C# code for explanation and improvement suggestions
  2. Design patterns: Assess knowledge of Singleton, Factory, Observer patterns
  3. Framework knowledge: Test ASP.NET, Entity Framework, or relevant framework experience
  4. Debugging skills: Present buggy code for identification and fixes
  5. Best practices: Discuss SOLID principles, unit testing, and code organization

Conclusion

C# continues to evolve as a powerful, versatile programming language suitable for a wide range of applications. These interview questions help evaluate both fundamental understanding and practical application skills, enabling you to identify candidates capable of building robust, scalable applications.

The best C# developers combine technical proficiency with understanding of software architecture, design patterns, and modern development practices including unit testing, dependency injection, and continuous integration.

Consider using Interview Zen’s technical interview platform to create comprehensive C# assessments and observe candidates’ problem-solving approaches in real-time during .NET development interviews.