C# Interview Questions Explained – A Practical Guide for Developers

Whether you are a fresher or an experienced .NET developer, C# interview questions test your depth of understanding — from language fundamentals to advanced concepts like async programming and memory management. This guide covers the most frequently asked C# questions with clear explanations and practical examples to help you ace your next interview.

Why C# Questions Matter in Interviews

Interviewers use C# questions to evaluate your understanding of memory management, knowledge of modern language features, ability to write efficient and maintainable code, and awareness of best practices in enterprise applications.

1. What is the difference between a class and a struct in C#?

A class is a reference type used to represent complex objects with identity and behavior. Instances are allocated on the heap, and multiple references can point to the same object. Classes support inheritance, polymorphism, and abstraction, making them ideal for domain models and business logic.

A struct is a value type designed for small, immutable data. Instances are typically allocated on the stack and are copied on assignment, which avoids heap allocation overhead.

Interview Tip: Saying “structs are stored on the stack and classes on the heap” is technically incomplete. Actual allocation depends on scope and usage — but value vs reference semantics is the key difference.

2. What is the difference between string and StringBuilder in C#?

string in C# is immutable. Every time you modify a string, a new object is created in memory, which makes repeated string concatenation inside loops very expensive.

StringBuilder is mutable and designed for scenarios where you need to perform many string manipulations. It modifies the existing buffer in memory rather than creating new objects.

// Bad - creates multiple string objects
string result = "";
for (int i = 0; i < 1000; i++)
    result += i.ToString();

// Good - uses a single buffer
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i++)
    sb.Append(i);
string result = sb.ToString();

Interview Tip: Use StringBuilder when performing more than 5-10 concatenations in a loop. For simple concatenations, string interpolation is perfectly fine.

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

Value types store data directly in the variable. They include int, float, double, bool, char, struct, and enum. They are stored on the stack (when local) and are copied on assignment.

Reference types store a reference (pointer) to the actual data on the heap. They include class, interface, delegate, string, object, and arrays.

The key behavioral difference is that modifying a copy of a value type does not affect the original, while modifying a reference type through any reference affects all references pointing to the same object.

4. What is boxing and unboxing in C#?

Boxing is the process of converting a value type to an object type (reference type). The value is wrapped inside a System.Object and stored on the heap.

Unboxing is the reverse — extracting the value type from the object. This requires an explicit cast and throws an InvalidCastException if the types do not match.

int num = 42;
object obj = num;       // Boxing
int result = (int)obj;  // Unboxing

Interview Tip: Boxing and unboxing have performance overhead. Avoid them in performance-critical code by using generics instead of non-generic collections like ArrayList.

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

The == operator compares references for reference types (unless overloaded) and values for value types. The .Equals() method is a virtual method defined in System.Object that can be overridden to provide custom equality logic.

For strings, both == and .Equals() compare values because the == operator is overloaded for the string class. However, for custom classes, == checks reference equality by default unless explicitly overloaded.

6. What are delegates and events in C#?

A delegate is a type-safe function pointer that holds a reference to a method with a specific signature. Delegates enable callback methods and are the foundation of event handling in C#.

An event is a mechanism built on top of delegates that provides a publish-subscribe pattern. Events add restrictions — subscribers can only add or remove handlers, not invoke the event directly or access other subscribers.

public delegate void NotifyHandler(string message);

public class Publisher
{
    public event NotifyHandler OnNotify;
    
    public void RaiseEvent(string msg)
    {
        OnNotify?.Invoke(msg);
    }
}

7. What is the difference between abstract class and interface in C#?

An abstract class can have both abstract (unimplemented) and concrete (implemented) members, constructors, fields, and access modifiers. A class can inherit from only one abstract class.

An interface defines a contract with method signatures that implementing classes must fulfill. Since C# 8.0, interfaces can have default implementations. A class can implement multiple interfaces.

Interview Tip: Use abstract classes when you want to share code among closely related classes. Use interfaces when unrelated classes need to implement a common contract.

8. What is async/await in C# and how does it work?

The async and await keywords enable asynchronous programming in C#. An async method runs synchronously until it hits an await expression, at which point it yields control back to the caller and resumes when the awaited task completes.

public async Task<string> GetDataAsync()
{
    HttpClient client = new HttpClient();
    string result = await client.GetStringAsync("https://api.example.com/data");
    return result;
}

This does not create a new thread. Instead, it uses the Task-based Asynchronous Pattern (TAP) to efficiently manage I/O-bound operations without blocking threads.

9. What is Dependency Injection in C#?

Dependency Injection (DI) is a design pattern where dependencies are provided to a class from outside rather than being created internally. This promotes loose coupling, testability, and adherence to the SOLID principles.

In .NET Core and .NET 5+, DI is built into the framework through the IServiceCollection interface. You register services at startup and the framework resolves them automatically.

// Registration in Program.cs
builder.Services.AddScoped<IUserRepository, UserRepository>();

// Constructor injection
public class UserService
{
    private readonly IUserRepository _repo;
    public UserService(IUserRepository repo)
    {
        _repo = repo;
    }
}

10. What is the difference between IEnumerable and IQueryable in C#?

IEnumerable works with in-memory collections and executes queries on the client side using LINQ to Objects. All filtering happens after the data is loaded into memory.

IQueryable is designed for out-of-process data sources like databases. It builds an expression tree and translates it to a query (like SQL) that runs on the server side, bringing back only the filtered results.

Interview Tip: Use IQueryable when querying databases through Entity Framework to avoid loading entire tables into memory. Use IEnumerable for in-memory collections.

11. Value Types vs Reference Types

Q: What are Value Types and Reference Types in C#?

Value Types store data directly on the stack (e.g., int, bool, struct). Reference Types store a reference to data on the heap (e.g., class, array, string).

12. Boxing and Unboxing

Q: What is Boxing and Unboxing?

Boxing converts a value type to a reference type on the heap. Unboxing extracts the value type back with explicit casting. Boxing causes performance overhead.

int num = 42;
object boxed = num;        // Boxing
int unboxed = (int)boxed;  // Unboxing

13. ref vs out Parameters

Q: What is the difference between ref and out?

ref requires initialization before passing. out does not but method must assign value before returning.

void RefExample(ref int x) { x += 10; }
void OutExample(out int x) { x = 42; }

14. var vs dynamic

Q: What is the difference between var and dynamic?

var is resolved at compile time with full type safety. dynamic bypasses compile-time checking and resolves at runtime.

15. const vs readonly vs static readonly

Q: What is the difference between const, readonly, and static readonly?

const is compile-time constant. readonly can be assigned in constructor. static readonly is shared across all instances, initialized once in static constructor.

16. StringBuilder vs String

Q: What is the difference between String and StringBuilder?

String is immutable — every modification creates a new object. StringBuilder is mutable and efficient for repeated concatenation.

var sb = new StringBuilder();
for (int i = 0; i < 1000; i++) sb.Append(i);
string result = sb.ToString();

17. Delegates and Events

Q: What are Delegates and Events in C#?

A Delegate is a type-safe function pointer. An Event is built on delegates providing publisher-subscriber notification.

public delegate void Notify(string message);
public class Publisher {
    public event Notify OnPublish;
    public void Publish(string msg) => OnPublish?.Invoke(msg);
}

18. Generic Methods

Q: What is a Generic Method?

A generic method uses type parameters to work with different data types while maintaining type safety.

public T Max<T>(T a, T b) where T : IComparable<T>
    => a.CompareTo(b) > 0 ? a : b;

19. using Statement

Q: What is the using statement in C#?

Ensures IDisposable objects are disposed after use, even if exceptions occur.

using (var reader = new StreamReader("file.txt"))
{
    string content = reader.ReadToEnd();
} // Dispose() called automatically

20. Access Modifiers

Q: What are the Access Modifiers in C#?

C# provides six access modifiers: public (any code, any assembly), private (same class only), protected (same class or derived), internal (same assembly), protected internal (same assembly or derived in other assemblies), and private protected (derived classes in same assembly only).