39 Most Popular C# Interview Questions and Answers (2025)

Blog / 39 Most Popular C# Interview Questions and Answers (2025)

C# is more than just syntax sugar for braces and semicolons—it’s a language engineered around generics, async/await, pattern-matching, and Span-powered memory safety that all compile down to airtight IL.

When you grasp why these features exist and how the compiler, CLR, and runtime weave them together, you write cleaner, faster code and you breeze through interviews.

Q1.

What is C#?

Junior

C# (pronounced as "C-sharp") is a modern, object-oriented programming language developed by Microsoft.
It is designed for building Windows applications and is widely used for web, desktop, mobile, and cloud-based applications.

Q2.

How is C# different from C?

Junior

The most significant difference between C# and C is that C# is an object-oriented programming language, while C is a procedural programming language.
C# supports automatic garbage collection by the CLR, whereas C does not.
C# primarily requires the.NET framework to execute, while C is generally considered a platform-agnostic language.

Q3.

Explain the difference between value types and reference types in C#.

Junior

Value types store data directly, while reference types store references to data in memory.
Value types include primitive data types like int, float, and bool, while reference types include objects, classes, and arrays.

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Q4.

What is the purpose of access modifiers (e.g., public, private, protected) in C#?

Junior

Access modifiers determine the visibility and accessibility of class members (fields, methods, properties, etc.). For example:

public: The member is accessible from any code.
private: The member is only accessible from within the defining class.
protected: The member is accessible from within the defining class and its subclasses.

Q5.

What does the static keyword do in C#?

Junior

The static keyword makes a member belong to the type itself rather than an instance.
A static field or static method is shared across all instances and can be accessed without creating an object.
A static class cannot be instantiated and can only contain static members.
For example:
- Here, Add is a static method, so you call it on the class MathHelper directly. You can also have a static constructor (a constructor with no parameters and marked static); it initializes static fields and runs only once, before the first use of the class.

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Q6.

What are properties in C#?

Junior

Properties provide a way to access class data (fields) with getters and setters. They look like fields to the user but can contain logic.
Auto-implemented properties allow you to skip writing a backing field.

Q7.

What is an exception in C# and how are exceptions handled?

Junior

An exception is an abnormal event or error condition that occurs during the execution of a program. Exceptions can be handled using try, catch, finally, and throw keywords.
The try block contains the code that may throw an exception, the catch block catches and handles exceptions, and the finally block is used for cleanup code that runs regardless of whether an exception is thrown or not.

Q8.

What is the var keyword in C#, and when is it used?

Junior

The var keyword is used for implicit type declaration in C#.
It allows the compiler to infer the data type of a variable based on its initialization value.

Q9.

What are arrays and lists in C#?

Junior

An array is a fixed-size, zero-based collection of elements of the same type.
An array’s size cannot change after creation. A List<T> (from System.Collections.Generic) is a dynamic array that can grow or shrink.
Lists provide methods like Add, Remove, Count, etc., and are more flexible. Use an array when you know the size ahead of time; use List<T> for a resizable collection.

Q10.

What is method overloading in C#?

Mid

Overloading: Defining multiple methods in the same class with the same name but different parameter lists (signatures). The compiler determines which method to call based on the arguments.
Overriding: Providing a new implementation of a method in a derived class, using the override keyword. The method in the base class must be marked virtual or abstract. At runtime, the appropriate override is chosen based on the actual object type.
Overloading is compile-time (same scope), while overriding is runtime polymorphism via inheritance.

Q11.

Explain the concept of inheritance in C#.

Mid

Inheritance is a fundamental concept in object-oriented programming (OOP) that allows a class (subclass or derived class) to inherit properties and behaviors from another class (superclass or base class).
In C#, you can use the : symbol to specify inheritance.

Q12.

Explain the concept of polymorphism in C#.

Mid

Polymorphism is one of the core principles of object-oriented programming (OOP).
It allows objects of different classes to be treated as objects of a common base class.
Polymorphism enables dynamic method invocation and method overriding. For example:
- Here Dog inherits Speak() from Animal but overrides it. The variable myPet is of type Animal but holds a Dog object; calling Speak() results in the Dog version. This is runtime polymorphism.

Q13.

What is an interface in C#, and how does it differ from an abstract class?

Mid

An interface defines a contract of methods and properties that a class must implement.
An abstract class can provide some implementation and be inherited, while an interface cannot provide any implementation and is used for multiple interface inheritance.
Example:
- Here Circle implements IShape and overrides the abstract GetArea from Shape. Interfaces are for “can-do” relationships; abstract classes are for “is-a” relationships with shared code.

Q14.

What is the purpose of the using statement in C#?

Mid

The using statement ensures that an IDisposable object is disposed when the block completes. It is used for deterministic cleanup of resources (like files, streams, database connections).
This is roughly equivalent to:

Q15.

What is a nullable type in C#?

Mid

A nullable type, denoted by ?, allows a value type to have a value or be null.
For example, int? can hold an integer value or be null.
To declare a nullable variable, you can use the syntax like:

Q16.

What are the out and ref keywords used for?

Mid

Both out and ref pass arguments by reference to methods, but with differences:

ref: The variable must be initialized before passing. The method can read and modify it, affecting the caller’s variable.
out: The variable does not need to be initialized before passing, but the called method must assign it before returning. Used to return additional values.

Q17.

Explain the difference between String and StringBuilder in C# and when to use each.

Mid

string is immutable in C# – once created, it cannot be changed. Concatenating or modifying a string actually creates a new string each time.
StringBuilder (in System.Text) is mutable, allowing efficient modifications.

Q18.

What is the difference between == and .Equals()?

Mid

The == operator and the .Equals() method both compare values, but by default == checks reference equality on reference types, whereas .Equals() is a virtual method (often overridden) to check value equality.

Q19.

What are generics in C# and why are they useful?

Mid

Generics allow you to create classes, methods, or interfaces that operate on types specified as parameters, rather than specific data types.
They provide type safety, code reusability, and improved performance by avoiding boxing/unboxing.

Q20.

Explain the concept of delegates and events in C#.

Mid

Delegates are type-safe function pointers that allow you to define and reference methods dynamically.
Events are a higher-level abstraction built on delegates and are commonly used for implementing the observer pattern.

Q21.

What is LINQ? Give an example of a LINQ query.

Mid

LINQ (Language Integrated Query) lets you query collections in a concise, SQL-like way using C# syntax. It works with any IEnumerable<T> or IQueryable<T>.
Alternatively, query syntax:
LINQ makes code more readable and expressive. It can query in-memory collections or external sources (LINQ to SQL, LINQ to XML, etc.) with deferred execution.

Q22.

What is the difference between IEnumerable and IQueryable in LINQ, and when to use each?

Mid

IEnumerable represents a sequence that can be iterated over, typically used for in-memory collections. Operations on IEnumerable are executed in memory.
IQueryable also represents a sequence but is designed for querying data sources outside of memory (like databases). It allows for query translation and execution at the data source, improving performance for large datasets.
Use IQueryable when querying databases (e.g., with Entity Framework) and IEnumerable for in-memory collections.

Q23.

Explain async and await in C#.

Mid

The async and await keywords enable asynchronous programming.
Mark a method with async to allow await inside it. An await on a Task pauses the method until the task completes, without blocking the thread.
The method returns to the caller, and when the awaited work finishes, execution resumes.

Q24.

What is the role of the Common Language Runtime (CLR) in C#?

Mid

The CLR is the virtual-machine core of Microsoft .NET. It runs IL code from languages such as C#, F#, and VB.NET and supplies a managed environment that sits between your application and the OS. Key runtime services:

Automatic memory management with a garbage collector
JIT compilation that turns IL into native machine code on the fly
Code Access Security to enforce permission rules
Structured exception handling for runtime errors
Type safety to prevent illegal memory access
Language interoperability, letting IL from any .NET language mix freely

Together, these features simplify development and boost the reliability, security, and portability of .NET applications.

Q25.

What is garbage collection in C#?

Mid

In C#, garbage collection is the automatic process of managing memory in an application.
The garbage collector automatically disposes of memory that is no longer used to make memory available for new allocations.
This process helps prevent memory leaks by reclaiming memory occupied by objects that are no longer referenced by the program.

Q26.

Explain what a lambda expression is in C# and provide an example.

Mid

A lambda expression is an anonymous function used to create delegates or expression tree types. It simplifies the syntax for defining small, inline methods.

Q27.

What is the purpose of the yield keyword in C#?

Mid

The yield keyword is used in iterator methods to indicate that the method should return an iterator, which can be used to lazily generate a sequence of values. It simplifies the creation of custom iterators.

Q28.

What are attributes in C#?

Mid

Attributes are used to add metadata to code elements (such as classes, methods, or properties). They are often used for code documentation and custom annotations.

Q29.

What are extension methods in C#?

Mid

Extension methods allow you to add new methods to existing types without modifying their source code.
They are defined in static classes and must have a special this parameter to indicate the target type.

Q30.

What is the difference between shallow copy and deep copy of objects?

Senior

A shallow copy creates a new object and copies the values of fields directly. For reference-typed fields, it copies the references (both original and copy refer to the same sub-objects).
A deep copy duplicates the object and all objects it references (recursive copy of the entire graph).

Q31.

Explain the concept of dependency injection (DI) in C#.

Senior

Dependency injection is a design pattern in which the dependencies of a class are provided from the outside (usually via constructor injection or property injection) rather than being created internally.
DI promotes loose coupling, testability, and maintainability.
Here's a simple example using the constructor injection:

Q32.

What are serialization and deserialization in C#?

Senior

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Q33.

What is the dynamic keyword and what is the DLR?

Senior

Q34.

What is the difference between Task and Thread in C#?

Senior

Q35.

What does the volatile keyword do in C#?

Senior

Q36.

How do you handle multithreading and avoid race conditions in C#?

Senior

Q37.

What are covariance and contravariance in C#?

Senior

Q38.

Explain the concept of boxing and unboxing in C#.

Senior

Q39.

What are finalizers and the IDisposable pattern?

Senior

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