C# Interview Questions and Answers for Junior Developers
A junior C# developer is an entry-level software engineer who uses C# and the .NET ecosystem to build, debug, and maintain applications under guidance from more experienced developers.
Expected qualifications
A junior C# developer is expected to have a solid grasp of core C# syntax, including data types, control structures, and basic object-oriented programming concepts like classes, inheritance, and encapsulation. They should understand how the .NET platform (developed by Microsoft) works at a high level, including concepts like the CLR and basic project structure. Familiarity with common tools such as Visual Studio or Visual Studio Code is typically required for writing and debugging code.
They are expected to know how to work with collections, basic LINQ queries, and simple file or database operations. A junior developer should also understand version control systems like Git and be comfortable collaborating through platforms like GitHub. Basic knowledge of debugging, error handling, and writing clean, readable code is important.
They should be familiar with fundamental programming concepts such as recursion, basic algorithms, and time complexity, even if not deeply optimized yet. Understanding how to build simple applications (console apps, basic web APIs, or small UI apps) is usually expected. Additionally, they should demonstrate willingness to learn, accept feedback, and improve their coding practices over time. Finally, communication skills and the ability to work within a team are essential, as junior developers often collaborate closely with senior engineers and other stakeholders.
Junior Level C# Interview Questions
1. What is the difference between == and .Equals()?
== compares references by default (for objects), unless overloaded.
.Equals() compares values/content (can be overridden).
2. What is the difference between ref and out parameters?
ref: Variable must be initialized before passing.
out: Variable does not need initialization; must be assigned inside the method.
void Test(ref int x, out int y)
{
x += 1;
y = 10;
}
3. What is a delegate?
A delegate is a type that represents references to methods with a specific signature. It’s commonly used for callbacks and event handling.
4. What is the difference between interface and abstract class?
Interface: Only method signatures (until default implementations in newer versions), no fields.
Abstract class: Can have both abstract and concrete methods, plus fields.
5. What is garbage collection in C#?
Garbage collection automatically manages memory by freeing objects that are no longer in use. It helps prevent memory leaks and is handled by the .NET runtime.
6. What is LINQ?
Language Integrated Query (LINQ) allows querying collections (arrays, lists, databases) using a SQL-like syntax directly in C#.
var numbers = new[] { 1, 2, 3, 4 };
var even = numbers.Where(n => n % 2 == 0);
Ref: Complete LINQ Guide
7. What is the difference between List and Array?
Array: Fixed size.
List: Dynamic size, provides more built-in methods.
8. What are value types and reference types?
Value types (e.g., int, struct) store actual data.
Reference types (e.g., class, string) store a reference to the memory location.
9. What is async and await?
They are used for asynchronous programming.
async: Marks a method as asynchronous.
await: Waits for a task to complete without blocking the thread.
public async Task GetDataAsync()
{
await Task.Delay(1000);
return 42;
}
Ref: async/await Tutorial
10. What is dependency injection?
A design pattern where dependencies are provided to a class rather than created inside it. It improves testability and maintainability.
11. What is boxing and unboxing?
Boxing: Converting a value type to an object.
Unboxing: Extracting the value type from the object.
int x = 10;
object obj = x; // Boxing
int y = (int)obj; // Unboxing
12. What is the difference between var, dynamic, and explicit typing?
var: Compile-time type inference.
dynamic: Type resolved at runtime.
Explicit: Type declared manually.
Red: var vs dynamic
13. What are properties in C#?
Properties provide controlled access to fields using getters and setters.
public int Age { get; set; }
14. What is an event in C#?
An event is a way for a class to notify other classes when something happens. It’s based on delegates.
15. What are the SOLID principles?
SOLID is a set of design principles for maintainable code:
S – Single Responsibility Principle (one class = one responsibility)
O – Open/Closed Principle (open for extension, closed for modification)
L – Liskov Substitution Principle (subtypes must be replaceable)
I – Interface Segregation Principle (small, specific interfaces)
D – Dependency Inversion Principle (depend on abstractions, not concretions)
16. What is time complexity?
Time complexity measures how execution time grows with input size, often expressed using Big-O notation (e.g., O(n), O(log n), O(n²)).
Loop through array → O(n)
Nested loops → O(n²)
17. What is recursion?
Recursion is when a method calls itself to solve smaller parts of a problem.
int Factorial(int n)
{
if (n <= 1) return 1;
return n * Factorial(n - 1);
}
Key idea: must have a base case to avoid infinite calls.
18. What is the difference between heap and stack memory?
Stack: Stores value types and method calls; fast, fixed size
Heap: Stores reference types; slower, managed by garbage collector
int x = 5; // stack
var obj = new MyClass(); // heap
19. What are the main data types in C#?
Value types: int, double, bool, struct
Reference types: string, class, object, arrays
Nullable types: int?, bool?
Ref: C# Data Types
20. What is the .NET architecture?
The .NET ecosystem (from Microsoft) consists of:
CLR (Common Language Runtime) → Executes code, handles memory
BCL (Base Class Library) → Prebuilt functionality
Languages → C#, F#, VB.NET
JIT Compiler → Converts IL (Intermediate Language) to machine code
21. What is foreach and how does it work?
foreach iterates over a collection and uses an enumerator internally.
foreach (var item in list)
{
Console.WriteLine(item);
}
It is read-only for the iteration variable.
22. Can you modify a collection inside a foreach loop?
No, modifying a collection during foreach causes a runtime exception.
Use a for loop instead if modification is needed.
23. What is the difference between const and readonly?
const: Compile-time constant, must be assigned immediately
readonly: Can be assigned at runtime (e.g., in constructor)
const int A = 10;
readonly int B;
public MyClass()
{
B = 20;
}
24. What is boxing/unboxing performance impact?
Boxing allocates memory on the heap → slower and increases GC pressure.
Unboxing requires casting → additional overhead.
25. What is the difference between IEnumerable and IQueryable?
IEnumerable: Works in memory (LINQ to Objects)
IQueryable: Executes queries remotely (e.g., database)
26. What is a Task in C#?
A Task represents an asynchronous operation, often used with async/await.
27. What is method overloading vs overriding?
Overloading: Same method name, different parameters
Overriding: Redefining a base class method using virtual + override
28. What is a struct vs class?
struct: Value type, stored on stack, lightweight
class: Reference type, stored on heap, supports inheritance
29. What is exception handling in C#?
Used to handle runtime errors safely using try-catch-finally.
try
{
int x = int.Parse("abc");
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
Ref: Exception Handling
30. What is the difference between string and StringBuilder?
string is immutable, meaning once created, it cannot be changed. Any modification creates a new object.
StringBuilder is mutable, so it can modify the same object without creating new ones, making it more efficient for frequent string changes.
string s = "Hello";
s += " World"; // creates a new object
var sb = new StringBuilder("Hello");
sb.Append(" World"); // modifies existing object
31. What is a constructor in C#?
A constructor is a special method used to initialize objects when they are created. It has the same name as the class and no return type.
class Person
{
public string Name;
public Person(string name)
{
Name = name;
}
}
32. What is the purpose of using statement in C#?
The using statement ensures that resources are properly disposed of after use, especially for objects like file handlers or database connections.
using (var file = new StreamReader("test.txt"))
{
Console.WriteLine(file.ReadToEnd());
}
It automatically calls Dispose() even if an exception occurs, helping prevent memory/resource leaks.
Ref: Using Statement
