Introduction
Preparing for a C# interview? Whether you're a beginner or an experienced developer, interviewers often test your understanding of core C# concepts rather than just syntax.
In this article, we'll cover 10 commonly asked C# interview questions with simple explanations and practical examples to help you prepare with confidence.
1. What Is the Difference Between Class and Struct?
A class is a reference type, while a struct is a value type.
| Class | Struct |
|---|
| Reference type | Value type |
| Stored on the heap | Usually stored on the stack or inline within another object |
| Supports inheritance | Does not support class inheritance |
| Can be null | Cannot be null unless declared as nullable |
Example
class Employee
{
public string Name { get; set; } = string.Empty;
}
struct Point
{
public int X;
public int Y;
}
Interview Tip: Use a struct for small, immutable data objects and a class for complex business objects.
2. What Is the Difference Between == and .Equals()?
== compares values for value types and, by default, object references for reference types (unless the operator is overloaded).
.Equals() compares object equality and can be overridden to implement custom comparison logic.
Example
string a = "Hello";
string b = "Hello";
Console.WriteLine(a == b); // True
Console.WriteLine(a.Equals(b)); // True
Interview Tip: Custom classes can override .Equals() to compare object values instead of object references.
3. What Is the Difference Between ref, out, and in?
ref
The variable must be initialized before it is passed to the method.
void Increment(ref int number)
{
number++;
}
out
The variable does not need to be initialized, but the called method must assign a value before returning.
bool GetAge(out int age)
{
age = 25;
return true;
}
in
The parameter is passed by reference but is read-only inside the method.
void Display(in int value)
{
Console.WriteLine(value);
}
Remember:
ref = Read and write
out = Output only
in = Read-only reference
4. What Is the Difference Between string and StringBuilder?
string objects are immutable, meaning every modification creates a new string instance.
StringBuilder objects are mutable and allow efficient string modifications.
Example
using System.Text;
StringBuilder builder = new StringBuilder();
builder.Append("Hello ");
builder.Append("World");
Console.WriteLine(builder.ToString());
Best Practice: Use StringBuilder when repeatedly modifying strings, especially inside loops.
5. What Are Boxing and Unboxing?
Boxing
Converting a value type into an object.
int number = 100;
object obj = number;
Unboxing
Converting an object back into its original value type.
int value = (int)obj;
Note: Excessive boxing and unboxing can negatively impact application performance because they involve additional memory allocations and type conversions.
6. What Is the Difference Between IEnumerable and IQueryable?
IEnumerable
IQueryable
Example
IQueryable<Employee> employees = dbContext.Employees;
var result = employees.Where(x => x.Department == "IT");
Performance Tip: Use IQueryable when querying databases with Entity Framework so filtering is performed by the database engine.
7. What Is the Difference Between an Abstract Class and an Interface?
Abstract Class
Can contain implementation.
Can define constructors and fields.
Supports both abstract and concrete methods.
Interface
Defines a contract.
Supports multiple interface implementations.
Declares methods, properties, events, and indexers.
Example
abstract class Animal
{
public abstract void Speak();
}
interface IFly
{
void Fly();
}
Interview Tip: Use an abstract class when sharing common implementation and an interface when defining capabilities that multiple unrelated classes can implement.
8. What Is Dependency Injection?
Dependency Injection (DI) is a design pattern in which an object's dependencies are supplied externally rather than being created inside the class.
Example
public class ProductService
{
private readonly ILogger<ProductService> _logger;
public ProductService(ILogger<ProductService> logger)
{
_logger = logger;
}
}
Benefits
9. What Is Asynchronous Programming?
Asynchronous programming allows an application to continue executing while waiting for long-running operations, such as API calls, database queries, or file operations.
Example
public async Task<string> GetDataAsync()
{
await Task.Delay(1000);
return "Completed";
}
Key Keywords
Benefit: Asynchronous programming improves application responsiveness and scalability by preventing threads from blocking during I/O operations.
10. What Is Exception Handling?
Exception handling allows applications to gracefully manage runtime errors without crashing unexpectedly.
Example
try
{
int number = 10 / 0;
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
finally
{
Console.WriteLine("Finished");
}
Best Practice: Catch specific exception types whenever possible instead of using the base Exception class.
Quick Revision Table
| Question | Key Takeaway |
|---|
| Class vs Struct | Reference type vs value type |
| == vs .Equals() | Operator comparison vs object equality |
| ref vs out vs in | Different parameter-passing mechanisms |
| string vs StringBuilder | Immutable vs mutable strings |
| Boxing vs Unboxing | Value type ↔ Object conversion |
| IEnumerable vs IQueryable | In-memory processing vs database query execution |
| Abstract Class vs Interface | Shared implementation vs contract |
| Dependency Injection | Loose coupling and testability |
| Async/Await | Non-blocking programming |
| Exception Handling | Graceful runtime error management |
Conclusion
These questions are frequently asked during C# interviews because they cover essential language features, object-oriented programming concepts, and real-world development practices. Instead of memorizing answers, focus on understanding why each feature exists, when it should be used, and the trade-offs involved.
Regularly practicing coding exercises, building real-world applications, and revisiting these core concepts will strengthen your C# skills and improve your confidence during technical interviews.