Top 90 C# Interview Questions for 2025 For Freshers & Experienced

The Top 90 C# Interview Questions for 2025 covers key areas that employers prioritize when hiring C# developers. It includes core topics like memory management, object-oriented programming, async/await, LINQ, and dependency injection to assess your problem-solving abilities and coding efficiency.

This blog offers a focused set of C# interview questions, blending theoretical knowledge with practical application to help you tackle challenging interview scenarios with confidence.

C# Interview Questions for Freshers

As a fresher preparing for C# interviews, it's essential to understand the core concepts of the language. Below are some of the key questions you may encounter in C# interview questions for freshers.

1. What Are The Key Features Of C# That Distinguishes It From Other Programming Languages?

C# is a powerful, versatile language that offers unique features compared to others like Java, Python, and C++. The key features that set C# apart include:

• Object-Oriented: C# is designed to support object-oriented programming, which helps in code reuse and scalability.
• Cross-Platform: With .NET Core, C# applications can run on Windows, macOS, and Linux.
• Memory Management: C# has automatic garbage collection to manage memory efficiently.
• Type Safety: The language enforces strict type rules, reducing the chance of runtime errors.
• Integration with .NET Framework: C# seamlessly integrates with .NET, allowing developers to leverage a broad range of libraries.
• LINQ: Language Integrated Query (LINQ) enables querying collections and databases directly in C#.

These features make C# an attractive option for both small and enterprise-level projects.

Also Read: Java vs C#: Differences ,Similarities, Features, And Advantages

2. How Does C# Handle Memory Management And Garbage Collection?

Memory management is a crucial aspect of programming. In C#, memory is managed automatically by the .NET runtime using garbage collection (GC). The garbage collector handles memory allocation and deallocation. Here’s how it works:

• Memory Allocation: When objects are created, the runtime allocates memory for them.
• Garbage Collection: The garbage collector automatically frees memory that is no longer in use by identifying unreachable objects.
• Generations: C# uses a generational garbage collection system to optimize performance. Objects are categorized into three generations (0, 1, and 2) based on their longevity.

In C#, the programmer does not need to manually free memory, reducing the likelihood of memory leaks.

3. Can You Explain The Role Of The Static Keyword In C#?

The static keyword is used to define class-level members that can be accessed without creating an instance of the class. Here are the main points:

• Static Variables: These belong to the class itself, not any particular object. They hold the same value across all instances. 
  Example:

class Example
{
    public static int Count = 0; // static variable
    
    static void Main()
    {
        // Display the value of the static variable Count
        Console.WriteLine(Count);
    }
}

• Static Methods: These can be called without an object instance. 
  Example:

class Program
{
    static void Main(string[] args)
    {
        Example.Display();  // Calling the static method
    }
}
Output:

• Static Constructors: These initialize static members of a class.

The static keyword reduces memory overhead by allowing direct access to class members.

4. What Are The Differences Between A Struct And A Class In C#?

In C#, both structs and classes are used to define data types, but there are key differences between them:

Understanding these differences is vital for optimizing performance and memory usage.

Also Read: Exploring Stack vs Heap: Decoding Memory Management

5. What Is Method Overriding In C#, And How Does It Differ From Method Overloading?

Both method overriding and method overloading allow methods to behave differently, but they work in distinct ways:

• Method Overloading: This happens when you define multiple methods with the same name but different parameters. Overloading is resolved at compile time.

Example:

Example example = new Example();
example.Display(5);      // Calls the method with an int parameter
example.Display("Hello"); // Calls the method with a string parameter

Output:

5
Hello

• Method Overriding: This occurs when a subclass provides a specific implementation for a method defined in its base class. Overriding is resolved at runtime and requires the virtual and override keywords. 

Example:

 class Animal
{
    public virtual void Speak() { Console.WriteLine("Animal speaks"); }
}

class Dog : Animal
{
    public override void Speak() { Console.WriteLine("Dog barks"); }
}

Output:

Dog barks

Method overloading is based on method signatures, while overriding is based on runtime polymorphism.

Also Read: Difference Between Overloading and Overriding in Java

6. What Is The Purpose Of The Async And Await Keywords In C#?

In C#, the async and await keywords are central to implementing asynchronous programming, which allows for non-blocking code execution. These keywords are particularly useful when handling long-running operations such as file I/O, web requests, and database queries. By using async and await, you can improve the responsiveness of applications, particularly in scenarios involving I/O-bound tasks.

• async: The async keyword marks a method as asynchronous. This tells the compiler that the method will run asynchronously and that it will return a Task or Task<T>. When you use async, the method can perform operations without blocking the main thread, which is crucial for maintaining a responsive user interface.
  Example:

public async Task<int> GetDataAsync()
{
    await Task.Delay(1000); // Simulate an async operation
    return 42;
}

Output: After 1 Second the output will be

42

• await: The await keyword pauses the execution of the method until the awaited task completes. Importantly, it doesn't block the thread, meaning the thread is free to do other work while waiting. This feature is especially helpful in applications with multiple I/O-bound operations, allowing them to run concurrently without locking up the application.

Example:

 var result = await GetDataAsync();

Together, async and await enable non-blocking execution. While async allows a method to run asynchronously, await ensures that the method waits for the completion of a task without blocking the executing thread. This is particularly advantageous in applications dealing with I/O-bound operations, where waiting for responses (e.g., file reads, web API calls) is common.

Non-Blocking Execution For Both I/O-Bound And Cpu-Bound Operations

While these keywords are widely known for improving I/O-bound tasks (such as network operations or file I/O), they can also be beneficial for CPU-bound tasks when used appropriately. For CPU-bound operations, async and await help by allowing tasks to run concurrently on different threads, ensuring that other operations can continue executing without being held up by one lengthy operation.

This leads to a more efficient use of system resources, better responsiveness, and an overall smoother user experience.

7. How Do You Implement Error Handling In C# Using Try-Catch Blocks?

Error handling in C# is crucial for managing runtime exceptions. The try-catch blocks allow you to catch exceptions and handle them gracefully.

• Try Block: This contains code that may throw an exception.
• Catch Block: This is used to handle exceptions when they occur.

Example:

try
{
    int result = 10 / 0;
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Error: " + ex.Message);
}

Output:

Error: Division by zero.

This approach ensures that your program can recover from unexpected errors without crashing.

Also Read: Top 32 Exception Handling Interview Questions and Answers [2025]

8. What Is The Difference Between Public, Private, Protected, And Internal Access Modifiers?

Access modifiers determine the visibility and accessibility of members (methods, fields, etc.) within classes. Here's a breakdown:

• Public: Members are accessible from anywhere. 
  • Example:
    public int Number;
• Private: Members are only accessible within the same class. 
  • Example:
    private int Number;
• Protected: Members are accessible within the class and its derived classes.
  • Example:
    protected int Number;
• Internal: Members are accessible within the same assembly. 
  • Example:
    internal int Number;

Choosing the right access modifier helps in encapsulating data and controlling how your code interacts with other parts of the system.

9. How Does C# Support Object-Oriented Programming (OOP)?

C# is fundamentally object-oriented as it supports the four main principles of OOP. It allows developers to use classes and objects to structure programs. 

• Encapsulation: C# allows you to bundle data (fields) and methods that operate on the data into a single unit (class). Access modifiers (public, private, etc.) control access to the data, ensuring that the internal details of a class are hidden from outside code. 

Example:

class Program
{
    static void Main(string[] args)
    {
        Car myCar = new Car();
        myCar.Model = "Tesla";  // Setting the Model
        Console.WriteLine(myCar.Model);  // Getting and printing the Model
    }
}

Output:

Tesla

• Inheritance: C# supports inheritance, which allows one class to inherit the properties and methods of another class, promoting code reuse. 

Example:

class Program
{
    static void Main()
    {
        Dog myDog = new Dog();
        myDog.Eat();  // Inherited from Animal
        myDog.Bark(); // Defined in Dog
    }
}

Output:

Eating...
Barking...

• Polymorphism: C# allows you to define methods in different classes with the same name but potentially different implementations, as shown through method overriding. 

Example:

class Program
{
    static void Main()
    {
        Animal myAnimal = new Dog();
        myAnimal.MakeSound();  // This will call Dog's overridden method
    }
}

Output:

Bark!

• Abstraction: C# allows you to define abstract classes and interfaces, which enable you to focus on high-level functionalities while leaving the implementation to derived classes.

Example:

abstract class Shape
{
    public abstract void Draw();
}
class Circle : Shape
{
    public override void Draw() { Console.WriteLine("Drawing Circle"); }
}
class Program
{
    static void Main(string[] args)
    {
        Shape shape = new Circle();  // Create an instance of Circle
        shape.Draw();  // Call the Draw method
    }
}

Output:

Drawing Circle

10. What Is The Purpose Of The Using Directive In C#?

The using directive in C# serves to simplify the code by allowing access to namespaces without needing to fully qualify their names. It makes the code cleaner and more readable by reducing repetition.

• Namespace Inclusion: It helps in including namespaces in your program.

Example:

using System;
class Example
{
    public void Display() { Console.WriteLine("Using directive example"); }
}
class Program
{
    static void Main()
    {
        Example example = new Example();
        example.Display(); // This will call the Display method
    }
}

Output:

Using directive example

• Resource Management: The using statement can also be used for managing resources, ensuring that objects implementing the IDisposable interface are disposed of properly. 

Example:

using (var stream = new FileStream("file.txt", FileMode.Open))
{
    // Read file content
}  // stream will be disposed automatically

11. Can You Explain The Concept Of "Boxing" And "Unboxing" In C#?

In C#, boxing and unboxing refer to the conversion between value types and reference types.

• Boxing: It is the process of converting a value type (like int) into a reference type (like object). During boxing, the value type is wrapped inside a reference type object. 

Example:

int x = 10;
object obj = x; // Boxing

• Unboxing: It is the reverse process where a reference type is converted back into a value type. Unboxing requires explicit casting. 

Example:

object obj = 10;
int x = (int)obj; // Unboxing

Boxing incurs performance overhead because it involves copying the value from the stack to the heap. Unboxing can throw an InvalidCastException if the types do not match.

12. What Are The Differences Between Array And List In C#?

C# provides both Arrays and List for storing collections of data, but they have distinct characteristics:

• Array:
  • Fixed size, cannot be resized after declaration.
  • Directly supports multi-dimensional arrays.
  • Faster in performance as there is no overhead of resizing. 

Example:

int[] arr = new int[5];
arr[0] = 10;

• List:
  • Dynamic size, automatically resizes when elements are added or removed.
  • Offers a rich set of methods like Add(), Remove(), Find(), etc.
  • More flexible but slower than arrays due to resizing overhead. 

Example:

List<int> list = new List<int>();
list.Add(10);
list.Add(20);

13. What Is LINQ, And How Is It Used In C# To Query Data?

Language Integrated Query (LINQ) in C# enables querying collections (like arrays, lists, and databases) using a unified syntax. It simplifies data manipulation, reducing code complexity.

LINQ to Objects:

LINQ to Objects is used for querying in-memory collections such as arrays or lists.

Example:

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = from n in numbers 
                  where n % 2 == 0 
                  select n;

Output:

2
4

Best Practices:

• Use method syntax (Where) for cleaner, more flexible code.
• Avoid multiple enumerations of the same collection.
• For large data sets, consider parallel LINQ (PLINQ).

LINQ to Entity Framework (EF):

LINQ can query databases via Entity Framework (EF), which is now the recommended approach for working with databases. EF translates LINQ queries into SQL and allows you to interact with databases as strongly-typed objects.

Example:

var students = from s in dbContext.Students
               where s.Age > 18
               select s;

Output:

The output will be a collection of Student objects (or records from the Students table) where each student's age is greater than 18. 

Best Practices:

• Use async methods like ToListAsync() for better performance.
• Filter data at the database level to reduce the volume of data retrieved.
• Minimize joins and avoid the N+1 query problem by using Include() to load related entities efficiently.

Key Differences:

• LINQ to Objects: Works with in-memory collections like arrays or lists.
• LINQ to EF: Queries databases and is translated into SQL using Entity Framework.
• Performance: LINQ to Objects is faster for small datasets, while LINQ to EF benefits from database optimizations but may involve overhead due to network and database query costs.

LINQ simplifies querying, ensuring cleaner code and better performance when used with best practices.

14. What Is An Interface In C#, And How Is It Different From An Abstract Class?

An interface in C# defines a contract that classes must fulfill. It contains method signatures, properties, or events but no concrete implementation. 

Example:

public interface IOperations
{
    void Start();
    void Stop();
}

• A class that implements IOperations must provide the bodies for Start() and Stop().
• Differences from Abstract Classes
  1. Implementation: Abstract classes can contain both abstract methods and fully defined methods. Interfaces cannot contain any concrete implementations (until default interface methods introduced in newer C# versions, which still differ from abstract class methods).
  2. Multiple Inheritance: A class can implement multiple interfaces but can only inherit from one abstract class.
  3. Use Case: An interface is often used to define capabilities or behaviors that many unrelated classes can implement. An abstract class is used for classes that share a common lineage or concept.

15. Can You Explain What Delegates Are And How They Are Used In C#?

A delegate is a type that references methods with a particular parameter list and return type. It acts like a pointer to a function. 

Example:

public delegate void Notify(string message);

• Usage
  1. Callback Methods: You can pass a delegate to another method. This method can call the delegate at the right moment.
  2. Event Handling: Events rely on delegates for subscription and notification.
  3. Asynchronous Calls: Delegates can be used to initiate asynchronous operations.

16. What Is The Difference Between Ref And Out Parameters In C#?

Understanding parameter passing is a fundamental part of C# basic interview questions. This helps you handle data updates inside methods.

Below are the major differences, supported by real-world illustrations.

• Requirement for Initialization
  1. ref: The variable passed in must be initialized before it is passed to a method.
  2. out: The variable need not be initialized beforehand. The method is responsible for assigning a value before it returns.
• Typical Scenarios
  1. ref: Used when a method both reads and modifies the variable’s value.
  2. out: Used when a method’s primary goal is to produce multiple outputs.

Example:

public void ProcessData(ref int existingValue, out int newValue)
{
    existingValue += 10;   // must be initialized beforehand
    newValue = 42;        // assigned inside the method
}

Scenarios like passing hardware configuration data or game-engine stats often rely on ref or out to manage and update multiple state variables without extra overhead.

17. What Is The Difference Between Stringbuilder And String In C#?

Here's a table summarizing the differences between String and StringBuilder in C#:

18. How Does C# Handle Multiple Inheritance Through Interfaces?

Here’s a table to illustrate how C# handles multiple inheritance through interfaces:

Example of Multiple Interface Implementation:

using System;
interface IReadable
{
    void Read();
}
interface IWritable
{
    void Write(string data);
}
class FileManager : IReadable, IWritable
{
    public void Read() 
    {
        Console.WriteLine("Reading from the file...");
    }
    public void Write(string data) 
    {
        Console.WriteLine($"Writing data to the file: {data}");
    }
}
class Program
{
    static void Main(string[] args)
    {
        // Creating an instance of FileManager
        FileManager fileManager = new FileManager();
        // Calling the Read method
        fileManager.Read();
        // Calling the Write method
        fileManager.Write("Hello, World!");
    }
}

Output:

Reading from the file...
Writing data to the file: Hello, World!

19. What Is The Purpose Of The Sealed Keyword In C#?

The sealed keyword in C# is used to prevent a class from being inherited. When a class is declared as sealed, no other class can derive from it, effectively locking its design.

public sealed class FinalClass
{
    // Implementation
}

Key Use Cases:

• Prevent Inheritance: It ensures that a class cannot be extended, which is useful when you want to finalize its design and behavior.
• Control Overriding: It also prevents methods in the class from being overridden in derived classes, ensuring that the behavior remains consistent.

Sealed classes are particularly useful in frameworks or libraries where allowing further inheritance could introduce errors or unintended behavior.

20. How Does C# Handle Asynchronous Programming, And What Is The Role Of Task?

C# uses the async and await keywords to write asynchronous code that looks similar to synchronous code. The Task or Task<T> return type represents an ongoing operation.

• Role of Task
  1. Representation: A Task symbolizes a future piece of work.
  2. Chaining Operations: You can attach continuations to a task, making it easier to manage complex workflows.
  3. Scalability: In a microservices scenario, tasks help each service handle multiple requests without blocking.

Example:

public async Task FetchDataAsync()
{
    // Simulate a delay for a network call
    await Task.Delay(1000);
    // Continue after the delay
}

Output:

After the 1-second delay, the method resumes execution and completes.

21. What Are Generics, And How Are They Used In C#?

Generics let you define classes, methods, and structures with placeholders for types. This eliminates the need for boxing and unboxing, thus improving efficiency.

Example:

using System;
public class GenericContainer<T>
{
    private T _value;
    public void Add(T item) { _value = item; }
    public T Get() { return _value; }
}
class Program
{
    static void Main()
    {
        // Create a container for an integer
        GenericContainer<int> intContainer = new GenericContainer<int>();
        intContainer.Add(42);
        Console.WriteLine(intContainer.Get());  // Output: 42
        // Create a container for a string
        GenericContainer<string> stringContainer = new GenericContainer<string>();
        stringContainer.Add("Hello, Generics!");
        Console.WriteLine(stringContainer.Get());  // Output: Hello, Generics!
    }
}

Output:

42
Hello, Generics!

• Practical Utility
  1. Collections: List<T>, Dictionary<TKey, TValue> are generic and type-safe.
  2. Algorithm Libraries: Generics allow you to write reusable and efficient libraries for sorting, searching, or specialized data structures.

22. Can You Explain The Difference Between A Task And A Thread In C#?

Understanding concurrency is vital in C# basic interview questions. This often includes clarifying how tasks differ from raw threads.

Below is a table that highlights the key differences.

Also Read: Thread Priority in Java: Explained with Examples

23. What Is An Event In C#, And How Does It Differ From A Delegate?

Events are central to many C# interview questions, particularly those involving user interfaces or real-time notifications. An event is a specialized delegate that follows the publisher-subscriber model, where a class acting as the publisher notifies its subscribers when something significant occurs. 

This mechanism is crucial in scenarios such as GUI interactions, IoT alerts, and real-time enterprise applications, where timely updates and notifications are essential.

• Key Differences
  1. Delegate: A function pointer that can be invoked directly.
  2. Event: A restricted delegate. Outside code can only subscribe (+=) or unsubscribe (-=) to an event, but cannot invoke it directly.

Example:

public class Alarm
{
    public event Action OnAlarmTriggered;
    public void TriggerAlarm()
    {
        OnAlarmTriggered?.Invoke();
    }
}
class Program
{
    static void Main(string[] args)
    {
        Alarm alarm = new Alarm();
        // Subscribe to the event
        alarm.OnAlarmTriggered += () => Console.WriteLine("Alarm triggered!");
        // Trigger the alarm
        alarm.TriggerAlarm();
    }
}

Output:

Alarm triggered!

24. What Is The Significance Of The Readonly Keyword In C#?

Fields marked readonly can only be assigned during declaration or in a constructor. They cannot be modified afterward.

public readonly int ID = 100;

• Use Case
  1. Immutable Data: Ideal for constants or values that should not change once set.
  2. Thread Safety: Reduces data race conditions in concurrent operations.

Keep reading for more C# interview questions that target your understanding of immutability and secure coding.

25. How Is Polymorphism Implemented In C#?

Polymorphism is crucial in C# basic interview questions because it enables you to write flexible and maintainable code.

Below are concise points to help you connect theory with practice.

1. Compile-Time Polymorphism (Overloading)
You achieve this by defining multiple methods with the same name but different signatures.

public void Print(int number) { /* ... */ }
public void Print(string text) { /* ... */ }

2. Runtime Polymorphism (Overriding)
You achieve this using virtual and override.

public virtual void Execute() { /* ... */ }
public override void Execute() { /* ... */ }

Polymorphism is widely used in game development (e.g., various NPC types sharing a single interface) and advanced analytics platforms (various data processors sharing a unified processing method). 

26. How Does C# Handle Multi-Threading, And What Are The Potential Issues You Might Face?

Handling concurrency is often discussed in C# interview questions for freshers, especially when aiming to build efficient and scalable systems.

Below are significant aspects, along with challenges you might encounter.

• Multi-Threading Approaches
  1. Thread Class: Manually create and manage threads.
  2. Thread Pool: Reuse threads for efficient resource utilization.
  3. Task Parallel Library (TPL): High-level abstraction using tasks for parallel operations.
• Potential Issues
  1. Deadlocks: Two or more threads waiting on each other indefinitely.
  2. Race Conditions: Unpredictable behavior when multiple threads access shared resources simultaneously.
  3. Thread Synchronization: Needing locks or other mechanisms to ensure thread-safe operations.

27. What Is A Virtual Method In C#, And How Is It Different From A Static Method?

A good understanding in fundamental questions, often highlighted in c# interview questions for freshers.

• Virtual Method
  1. Declared with the virtual keyword inside a class.
  2. Allows subclasses to override its implementation.
  3. Resolves at runtime, enabling polymorphic behavior.
• Static Method
  1. Associated with the class itself rather than an instance.
  2. Cannot be overridden in derived classes.
  3. Called using the class name, not an object reference.

Example:

using System;
public class BaseClass
{
    // Virtual method that can be overridden in derived classes
    public virtual void Display() 
    {
        Console.WriteLine("Base display");
    }
    // Static method that can't be overridden, but can be called from derived class
    public static void Show() 
    {
        Console.WriteLine("Static show");
    }
}
public class DerivedClass : BaseClass
{
    // Override the Display method
    public override void Display() 
    {
        Console.WriteLine("Derived display");
    }
}
class Program
{
    static void Main()
    {
        // Create an instance of the base class and call its methods
        BaseClass baseObj = new BaseClass();
        baseObj.Display();  // Will print: "Base display"
        BaseClass.Show();   // Will print: "Static show"
        // Create an instance of the derived class and call its methods
        BaseClass derivedObj = new DerivedClass();
        derivedObj.Display();  // Will print: "Derived display" (method overriding)
        BaseClass.Show();      // Will print: "Static show" (static method can't be overridden)
    }
}

Output:

Base display
Static show
Derived display
Static show

28. What Are Tuples, And How Do They Work In C#?

A tuple is a finite ordered list of elements. It allows you to return multiple values from a method without creating a separate class or struct. 

Example:

(int age, string name) personInfo = (25, "Alice");
Console.WriteLine(personInfo.age);  
// Output: 25

Use Cases

• Quick Data Grouping: Combine multiple related pieces of data.
• Temporary Data Holding: Pass small sets of data around without overhead.

Tuples are especially useful in functional-style programming or quick prototypes, which can be common in modern app development where speed and agility are paramount.

29. What Are The Key Differences Between A Hashtable And A Dictionary In C#?

Data structures often pop up in C# interview questions. Here is the table summarizing the key differences between Hashtable and Dictionary in C#:

30. How Does C# Handle Exception Handling With Custom Exceptions?

Building robust applications often requires handling specific errors in a tailored way. Custom exceptions provide a way to handle error conditions that aren't adequately captured by standard exceptions. This is an essential topic in many C# interview questions, especially for freshers who are looking to understand best practices in error handling.

Creating Custom Exceptions

To create a custom exception in C#, you generally follow these steps:

1. Derive from the Exception class: All custom exceptions must derive from the base Exception class. You can do this by either directly inheriting from Exception or from any of its other derived classes (e.g., ApplicationException).

using System;
public class InvalidOrderException : Exception
{
    // Constructor accepting a message
    public InvalidOrderException(string message)
        : base(message)
    {
    }
    // Constructor accepting a message and an inner exception
    public InvalidOrderException(string message, Exception innerException)
        : base(message, innerException)
    {
    }
    // Optional: Add a default constructor if needed
    public InvalidOrderException() 
        : base("An invalid order was detected.")
    {
    }
}

Output:

try
{
    // Throwing the custom exception with a message
    throw new InvalidOrderException("Order ID is missing");
}
catch (InvalidOrderException ex)
{
    Console.WriteLine(ex.Message);  // Output: Order ID is missing
}
try
{
    // Throwing the custom exception with a message and an inner exception
    Exception innerException = new InvalidOperationException("The order amount is invalid");
    throw new InvalidOrderException("Invalid order detected", innerException);
}
catch (InvalidOrderException ex)
{
    Console.WriteLine(ex.Message);  // Output: Invalid order detected
    Console.WriteLine(ex.InnerException.Message);  // Output: The order amount is invalid
}

2. Adding Custom Fields: You can enhance your custom exception by adding custom properties or fields. These can store additional information related to the error, such as error codes, timestamps, or specific business logic-related details.

public class InvalidOrderException : Exception
{
    public int ErrorCode { get; set; }
    public DateTime Timestamp { get; set; }
    public InvalidOrderException(string message, int errorCode)
        : base(message)
    {
        ErrorCode = errorCode;
        Timestamp = DateTime.Now;
    }
}

Sample Output (depends on when the exception is thrown)

Error Message: Invalid order placed.
Error Code: 404
Timestamp: 4/3/2025 10:15:30 AM  (this will be the current date and time when the exception is thrown)

3. Throwing Custom Exceptions: You can now throw your custom exception when specific conditions in your code are met.

 if (order == null)
{
    throw new InvalidOrderException("Order is null", 1001);
}

4. Catching Custom Exceptions: Once custom exceptions are thrown, they can be caught like any standard exception using try-catch blocks.

using System;
public class InvalidOrderException : Exception
{
    public int ErrorCode { get; }
    public DateTime Timestamp { get; }
    // Constructor accepting a message and error code
    public InvalidOrderException(string message, int errorCode)
        : base(message)
    {
        ErrorCode = errorCode;
        Timestamp = DateTime.Now;
    }
    // Constructor accepting a message, error code, and an inner exception
    public InvalidOrderException(string message, int errorCode, Exception innerException)
        : base(message, innerException)
    {
        ErrorCode = errorCode;
        Timestamp = DateTime.Now;
    }
    // Optional: Add a default constructor
    public InvalidOrderException()
        : base("An invalid order was detected.")
    {
        ErrorCode = 0;
        Timestamp = DateTime.Now;
    }
}

Code to handle the exception

try
{
    // Simulating code that might throw the custom exception
    ProcessOrder("Invalid Order");
}
catch (InvalidOrderException ex)
{
    // Handle the exception
    Console.WriteLine($"Error: {ex.Message}, Code: {ex.ErrorCode}, Time: {ex.Timestamp}");
}

Output:

Error: Invalid order detected, Code: 1001, Time: 4/3/2025 10:35:22 AM

31. What Is Null Propagation In C#?

Null reference issues commonly appear in C# basic interview questions because they can lead to exceptions at runtime. Null propagation uses the ?. operator to safely access members of an object that might be null.

Example:

Person person = null;
int? age = person?.Age;

• If ‘person’ is null, the expression returns null instead of throwing a NullReferenceException.

32. What Are The Basic C# Interview Questions For Freshers?

You might encounter a variety of C# interview questions for freshers. These usually cover fundamental topics, language constructs, and practical coding scenarios. By now, you have reviewed many of these concepts in depth. The basic questions often include:

Below are some of them, accompanied by brief illustrations.

• Value Types vs. Reference Types
  Inquires about how memory is allocated and managed.
• Control Flow Statements
  Checks familiarity with if, for, while, and other loops.
• Data Structures
  Explores knowledge of arrays, lists, dictionaries, and their use cases.

These C# basic interview questions lay the groundwork for more advanced queries about performance optimization and large-scale architecture.

33. Explain Memory Management In C#?

In C#, Memory Management is largely handled by the Garbage Collector (GC), which automatically reclaims memory for unused objects. Here's a breakdown of how it works:

1. Garbage Collection Generations:

C# uses generational garbage collection, which categorizes objects into three generations (0, 1, and 2) based on their lifespan:

• Generation 0: New objects, collected frequently.
• Generation 1: Objects that survived Generation 0, collected less often.
• Generation 2: Long-lived objects, collected during full GC cycles, reducing overhead.

Older generations (1 and 2) are only collected during full GC to optimize performance. Objects that survive multiple collections are promoted to higher generations, reducing their collection frequency.

• Manual Garbage Collection:

While GC is automatic, you can trigger it manually with GC.Collect(), though it’s generally not recommended unless necessary.

2. Large Object Heap (LOH):

• Objects > 85 KB are stored in the LOH.
• These objects are collected less frequently, but LOH fragmentation can occur if not managed properly. However, fragmentation can occur in the LOH, leading to inefficient memory usage.

3. Fragmentation Management:

• Manual Fragmentation Management: One way to manage LOH fragmentation is by explicitly nullifying references to large objects when they are no longer needed, allowing the GC to reclaim memory more effectively.
• Object Pooling: To minimize the creation of large objects and reduce LOH fragmentation, consider object pooling, where objects are reused instead of being frequently allocated and deallocated.

4. Weak References:

• The WeakReference class allows objects to be referenced without preventing garbage collection when no strong references exist.
• Ideal for caching scenarios, where you want to hold onto an object but allow the GC to reclaim memory when necessary.

5. High-Throughput Systems:

• In real-time data processing or high-frequency trading, efficient memory management is vital.
• Optimizations like memory pooling, minimizing large object allocations, and reducing GC pauses are crucial for maintaining performance.

6. Monitoring & Optimization:

• Use tools like dotMemory to track memory usage, pinpoint bottlenecks, and ensure your application scales efficiently.

34. What Is The Difference Between Abstract Class And Interface In C#?

This question appears in countless C# interview questions. Here’s a table that summarizes the key differences between Abstract Classes and Interfaces in C#:

Having discussed entry-level concepts, let’s look at C# interview questions for those with more experience.

C# Interview Questions for Experienced

C# shapes many complex systems and innovative software solutions. These C# advanced interview questions will guide you through core concepts, best practices, and modern developments that are relevant for 2025 and beyond.

35. Can You Explain The Differences Between Arraylist And List In C#?

Here's a concise explanation of the differences between ArrayList and List in C#:

36. How Do You Manage Concurrency In C# Applications?

Concurrency management in C# is essential for building efficient, responsive applications, especially as modern systems demand parallel operations. Key tools like async/await, the Task Parallel Library (TPL), and synchronization primitives such as lock, Mutex, and Semaphore provide developers with the ability to control thread execution and data integrity.

For practical applications, locks can be used to control access to shared resources. For instance, locking a shared dictionary before reading or modifying it ensures that only one thread interacts with the resource at any given time, preventing race conditions.

On the other hand, concurrent collections like ConcurrentDictionary allow thread-safe operations without needing explicit locks. This is ideal for systems with high throughput, where performance and scalability are key.

37. What Is The Significance Of The Async And Await Keywords In C# For I/O Operations?

In C#, the async and await keywords enable non-blocking I/O operations, freeing up the calling thread while tasks like data fetching or file I/O are processed in the background. This improves the responsiveness of applications, especially in environments where I/O operations can be time-consuming, such as web and mobile apps.

By marking a method as async, you allow asynchronous execution, and using await pauses the method’s execution until the asynchronous operation completes. This keeps the application responsive, ensuring the UI remains fluid without locking up during lengthy operations.

For example, consider this snippet:

await httpClient.GetStringAsync(url);

The line fetches data from a URL asynchronously, preventing the UI from freezing while waiting for the response.

38. Can You Explain What Dependency Injection Is In C# And Why Is It Important?

Dependency Injection (DI) is a design pattern that decouples class dependencies by providing them from an external source, rather than creating them inside the class itself. This approach enhances modularity, improves testability, and makes configurations more flexible.

Here’s why DI is essential:

• Modular Design: DI allows you to easily swap out dependencies, promoting a more flexible and maintainable codebase.
• Enhanced Testability: By injecting dependencies, you can easily replace them with mock objects during unit tests, eliminating the need to alter class code.
• Flexible Configurations: DI frameworks, like Microsoft.Extensions.DependencyInjection, enable you to register and resolve interfaces with their concrete implementations at runtime, making your application easily adaptable to changes.

39. What Is The Role Of Reflection In C#?

Reflection in C# enables you to inspect and manipulate types at runtime. It allows for discovering assemblies, types, methods, and properties, and even invoking members dynamically. This powerful feature is widely used in modern frameworks, especially for dynamic API generation, automation, and plugin systems.

Reflection is commonly utilized in applications that need to load plugins or call methods from external assemblies at runtime, offering flexibility in extensible application design. However, it can be slower than direct method calls, so it’s recommended to use it when flexibility outweighs performance needs.

40. What Is The Singleton Design Pattern, And How Do You Implement It In C#?

The Singleton Design Pattern ensures that a class has only one instance throughout the application’s lifecycle. This pattern is commonly used for managing global states, such as configuration settings.

To implement a Singleton in C#, the constructor is made private, and a static property exposes the single instance. 

class Program
{
    static void Main()
    {
        // Accessing the single instance of MySingleton
        MySingleton instance1 = MySingleton.Instance;
        MySingleton instance2 = MySingleton.Instance;
        // Both references point to the same instance
        Console.WriteLine(object.ReferenceEquals(instance1, instance2)); // Output: True
    }
}

In this implementation:

• The static field _instance holds the only instance of the class.
• The private constructor prevents external creation of the class.
• The Instance property provides access to the unique instance.

Thread-Safety: This approach is thread-safe by default, thanks to C#’s static constructor initialization. For additional control in multi-threaded scenarios, double-checked locking can be used to ensure that the instance is only created once.

This pattern is frequently asked in C# OOP interview questions, as it demonstrates an understanding of design patterns, object creation, and thread-safety in real-world applications.

41. How Would You Implement A Factory Pattern In C#?

The factory pattern abstracts object creation, allowing you to instantiate objects without knowing their specific construction details. This is particularly useful in scenarios where objects require complex setups or come in different variants. Commonly applied in industries like gaming or e-commerce, the factory pattern allows for dynamic product creation.

You can start by defining an IProduct interface, followed by concrete classes like ProductA and ProductB. A ProductFactory class would have a CreateProduct method, which returns an IProduct.

This approach supports scalability—when new product variations are needed, you simply add new classes and extend the factory logic, minimizing changes to existing code.

42. What Is The Difference Between A Shallow Copy And A Deep Copy In C#?

Here’s a table summarizing the key differences between a shallow copy and a deep copy in C#:

43. How Do You Work With Collections In C#, Such As List, Dictionary<Tkey, Tvalue>, And Hashset?

In C#, collections like List, Dictionary<TKey, TValue>, and HashSet are essential for organizing data.

• List is an ordered collection, ideal for sequential or index-based access.
• Dictionary<TKey, TValue> maps unique keys to values, perfect for efficient key-based lookups.
• HashSet stores unique items without any specific order, ensuring no duplicates.

For example, List works well for maintaining user profiles, Dictionary excels at managing large-scale key-based lookups like user IDs, and HashSet ensures no duplicates in a set of unique IDs.

44. What Is The Difference Between Throw And Throw Ex In C# Exception Handling?

Here's a table summarizing the differences between throw and throw ex in C# exception handling:

45. How Do You Define An Extension Method In C#?

An extension method is a static method in a static class that extends the functionality of an existing type without modifying its source code. You define an extension method by using the ‘this’ keyword before the type of the first parameter.

For instance, to extend the Person class with a method GetFullName, you can create a static class PersonExtensions with a method like:

public static void Main()
{
    // Creating a new Person object
    Person person = new Person
    {
        FirstName = "John",
        LastName = "Doe"
    };
    // Calling the extension method GetFullName
    string fullName = person.GetFullName();
    // Output the full name
    Console.WriteLine(fullName);  // Output: John Doe
}

Extension methods make it easy to add new functionality to existing types, which is especially useful in development cycles. The methods must reside in the same namespace or be included via a using statement.

46. What Is Dependency Injection, And How Do You Implement It In C#?

Dependency Injection (DI) is a design pattern used to manage dependencies in an application by injecting them at runtime rather than hardcoding them. It improves code flexibility, maintainability, and testability. In C#, DI can be implemented using a DI container such as Microsoft.Extensions.DependencyInjection or third-party libraries like Autofac and StructureMap.

Advanced DI Techniques:

• Property Injection: Allows setting properties via the DI container. Useful when constructor injection isn’t viable or optional dependencies are required.
• Method Injection: Injects dependencies through method parameters, often used in scenario-driven architectures where dependencies are specific to certain operations.

Dependency Lifetime Types:

• Transient: Creates a new instance for every request.
• Scoped: Creates a new instance per scope (e.g., per HTTP request).
• Singleton: Creates a single instance for the entire application lifecycle.

47. Can You Explain How A Try-Finally Block Works In C#?

A try-finally block guarantees that the code in the finally block always executes, even if an exception is thrown within the try block. It is primarily used for resource cleanup (e.g., closing files or releasing database connections).

Example:

• File Handling: Ensures file.Close() or stream.Dispose() is always called to release resources, even when an exception occurs.

Even in asynchronous environments, the finally block is still valuable for cleanup. Alternatively, you can use the using statement or await using for simpler resource management.

48. What Is A Partial Class In C#, And How Does It Differ From A Normal Class?

A partial class allows a class to be split across multiple files. It provides a way to organize large classes into smaller, logical segments, improving readability and maintainability.

Key Benefits:

• Separation of Concerns: You can split large classes into parts like UI logic, business logic, and auto-generated code (e.g., from designer tools).
• Single Identity at Compile-Time: All parts of the partial class are compiled as a single unit, ensuring no conflicts as long as method and property names match.

This technique is commonly used in enterprise solutions to handle large auto-generated code files.

49. How Does C# Handle Memory Allocation For Large Objects?

Objects larger than 85,000 bytes are allocated on the Large Object Heap (LOH). The LOH reduces fragmentation for large allocations but comes with some performance considerations, as garbage collection for large objects is more expensive.

Optimization Tips:

• Frequent Large Allocations: Avoid allocating large objects or arrays frequently. Instead, reuse objects or use pooled buffers to minimize performance hits.
• Garbage Collection: The LOH is collected during full garbage collections, and objects on the LOH do not move around frequently, but this can slow down the GC process.

50. What Is A Multicast Delegate In C#?

A multicast delegate can hold references to multiple methods and call each of them in its invocation list when invoked. This feature is useful for event handling and scenarios where multiple methods need to react to the same event.

Important Points:

• You can add multiple methods to a multicast delegate using the += operator (e.g., myDelegate += MethodA; myDelegate += MethodB;).
• Order of Execution: If the first method in the invocation list throws an exception, subsequent methods will not be called unless exceptions are handled within each delegate call.

51. What Is The Purpose Of The Override Keyword In C#?

The override keyword allows a derived class to modify or extend the behavior of a method, property, or indexer from a base class marked as virtual or abstract. It is central to polymorphism in C# and enables customization of inherited members.

For example, when a method is marked as virtual in the base class, you can override it in the subclass to change its functionality:

public override void DoWork()
{
    // New definition in derived class
}

Similarly, abstract methods in abstract classes must be overridden, ensuring that derived classes provide their own implementation.

52. How Does C# Handle Garbage Collection, And How Can You Influence It?

C# uses a generational garbage collector that organizes objects into generations based on their lifetimes. You can influence garbage collection using GC.Collect(), though it’s generally discouraged unless absolutely necessary for performance optimization. For specialized scenarios, like real-time applications or containerized environments, you can configure GC settings in project files or environment variables.

53. What Is A Const Variable, And How Is It Different From A Readonly Field In C#?

A const is a compile-time constant, while a readonly field is assigned at runtime, either in the constructor or field initializer. const values are replaced with literals during compilation, whereas readonly values can be set only once and retain their integrity throughout the application’s lifetime.

For instance:

const double Pi = 3.14159;  // Compile-time constant
readonly int instanceId;    // Runtime value

readonly is ideal for values that are set once at runtime but need to remain constant afterward

54. How Do You Implement A Thread-Safe Collection In C#?

C# provides built-in thread-safe collections like ConcurrentDictionary, ConcurrentQueue, and ConcurrentBag from the System.Collections.Concurrent namespace. These collections allow concurrent reads and writes, which is crucial for multi-threaded applications. Alternatively, you can implement custom synchronization mechanisms using locks around standard collections.

For example, a ConcurrentDictionary is ideal for scenarios requiring high throughput and concurrency, such as real-time data processing or distributed applications.

55. What Are Anonymous Methods, And How Do They Differ From Lambda Expressions In C#?

Anonymous methods allow you to define inline method bodies without a name. They are less concise than lambda expressions, which offer a more readable and flexible syntax. Both serve the purpose of creating inline code for event handling or callbacks.

For instance:

// Anonymous Method
button.Click += delegate { Console.WriteLine("Clicked!"); };
// Lambda Expression
button.Click += (sender, e) => Console.WriteLine("Clicked!");

While anonymous methods still appear in older codebases, lambda expressions are more commonly used in modern development due to their succinctness.

56. What Is Thread Pooling In C#, And How Is It Beneficial For Multithreading?

Thread pooling in C# is a technique that allows the reuse of worker threads from the ThreadPool rather than creating new threads for each individual task. This reduces the overhead associated with thread creation, leading to better performance and resource utilization, particularly in scenarios with high concurrency, such as web servers or cloud-based applications.

For example, you can queue tasks to the thread pool using:

ThreadPool.QueueUserWorkItem(DoWork);

Benefits of Thread Pooling:

1. Reduced Overhead: Creating and destroying threads can be expensive in terms of system resources. Thread pooling minimizes this cost by reusing threads that have already been created.
2. Optimized Resource Management: Thread pooling helps manage resources more efficiently, especially for I/O-bound operations, where threads can often be idle waiting for external operations (e.g., file I/O, network requests).
3. Improved Performance in High-Concurrency Environments: By reusing threads, thread pooling reduces the time spent in thread creation and destruction, making it ideal for applications that need to handle many concurrent tasks, such as web servers.

Task Parallel Library (TPL) and Thread Pooling

While ThreadPool provides low-level access to thread management, the Task Parallel Library (TPL) abstracts thread pool management, making it easier for developers to handle tasks. With TPL, you don't need to manually manage threads or worry about the underlying details of thread pooling.

Example with TPL:

Task.Run(() => DoWork());

The TPL automatically uses the thread pool for task execution, simplifying the threading process by handling synchronization and thread management for you. This not only makes the code more readable but also ensures that thread usage is optimized without manual intervention.

Also Read: Multithreading in Python [With Coding Examples] 

57. Can You Explain Reflection In C# And Its Common Uses?

Reflection enables type inspection, dynamic code generation, and runtime analysis of attributes. It is often used for advanced scenarios like dynamic proxy creation or generating code during runtime with Reflection.Emit.

Example:

Type type = typeof(MyClass);
PropertyInfo prop = type.GetProperty("MyProperty");

Reflection is crucial for frameworks that automate tasks like dependency injection or entity mapping.

58. What Is The Dynamic Type In C#, And How Does It Differ From Var?

The dynamic type defers type checking until runtime, while var infers the type at compile time. dynamic is useful when working with dynamic languages or interop with COM objects, while var maintains compile-time type safety.

Example:

dynamic obj = GetDynamicObject();
var number = 10;  // Infers type as int

dynamic can introduce runtime errors if a member doesn’t exist, whereas var ensures type safety at compile time.

59. How Do You Perform Type Checking At Runtime In C#?

In C#, you can use the is keyword or pattern matching to check an object's type at runtime. Pattern matching simplifies this by automatically casting the object if the check succeeds, making your code cleaner.

Example:

if (obj is MyClass myObj)
{
    // myObj is now cast to MyClass
}

You can also use typeof to obtain a Type object for comparison or dynamic object creation.

60. Can You Explain How Null References Are Handled In C#?

C# handles null references by throwing a NullReferenceException when attempting to access members of a null object. The null-conditional operator (?.) and the null-coalescing operator (??) help prevent these exceptions. Nullable reference types, introduced in C# 8.0, provide compile-time checks to avoid null errors.

Example:

person?.Address?.City; // Safely returns null if any part is null

Nullable reference types enhance safety by notifying developers of potential null dereference issues during compilation.

61. What Are Delegates In C#, And How Do They Enable Event Handling?

Delegates define method signatures and are used in event handling to notify subscribers when an event occurs. This is crucial in building event-driven architectures for GUIs, services, and IoT systems.

Example:

public delegate void MyEventHandler();
public event MyEventHandler SomethingHappened;

Delegates enable flexible and decoupled communication between components.

62. What Is The Purpose Of Yield Return In C#?

yield return generates an iterator without loading an entire collection into memory. This is ideal for scenarios where you need to process large datasets or streams efficiently, such as in data pipelines.

Example:

foreach (var item in GetItems())
{
    yield return item;
}

It saves memory by returning elements one at a time, making it ideal for large-scale data processing.

63. What Is The Difference Between IEnumerable And IQueryable In C#?

Here’s a detailed table comparing IEnumerable and IQueryable in C#:

64. How Can You Prevent Method Overriding In C#?

To prevent method overriding in derived classes, mark the method as sealed. This ensures that the method’s implementation remains fixed and cannot be modified further.

Example:

public sealed override void DoWork()
{
    // Fixed implementation
}

Sealing methods are useful for security or performance reasons, ensuring that critical behavior cannot be altered.

Now, let's dive into some of the most commonly asked coding questions in C# interviews.

Commonly Asked Coding Questions in C# Interviews

C# coding interview questions are a staple when you are preparing for technical roles. You might also encounter c# tricky interview questions that test deep language features and best practices. Additionally, many recruiters will check your knowledge with c# dot net interview questions to ensure your mastery of modern .NET trends.

65. What Are Extension Methods, And How Are They Implemented In C#?

Extension methods enable you to add functionality to existing classes without altering their source code. This is especially useful for enhancing third-party libraries or sealed classes.

How to implement:

• Write the extension method as a static method in a static class.
• Use the ‘this’ keyword to specify the type you are extending.

Example:

class Program
{
    static void Main()
    {
        string str1 = "madam";
        string str2 = "hello";
        string str3 = "A man a plan a canal Panama";
        string str4 = " ";
        Console.WriteLine(str1.IsPalindrome());  // Output: True
        Console.WriteLine(str2.IsPalindrome());  // Output: False
        Console.WriteLine(str3.IsPalindrome());  // Output: True
        Console.WriteLine(str4.IsPalindrome());  // Output: False
    }
}

Once declared, you can call IsPalindrome() on any string instance, extending its functionality without modifying the string class itself.

66. How Can You Create A Thread In C#?

Creating threads is essential for parallelism and performance in C#. Here's how to do it:

• Use the System.Threading.Thread class and pass a delegate to define the work.

Example:

Thread myThread = new Thread(() =>
{
    Console.WriteLine("Thread is running.");
});
myThread.Start();

Output:

Thread is running.

Threads are useful in CPU-bound tasks, like real-time analytics or image processing, and enhance the responsiveness of your application.

Also Read: Life Cycle of Thread in Java

67. What Is Thread Synchronization, And How Is It Implemented In C#?

Thread synchronization prevents multiple threads from accessing shared resources at the same time, ensuring data integrity.

Methods for synchronization: Use the lock keyword to enforce exclusive access.

private static object _lockObj = new object();
lock (_lockObj)
{
    // Critical section
}

• For advanced control, use Monitor, Mutex, or Semaphore.

These synchronization techniques ensure safe, concurrent operations in multi-threaded applications.

68. What Is The Purpose Of The Volatile Keyword In C#?

The volatile keyword ensures that a field’s value is always read from the main memory, not cached by the CPU, which is crucial in multi-threaded environments.

Example:

public volatile bool _isActive;

This ensures that each thread sees the most up-to-date value of _isActive.

69. How Does C# Handle Memory Leaks, And How Can You Avoid Them?

C# relies on a garbage collector (GC) to manage memory. However, leaks can still occur if you retain references unnecessarily.

To avoid memory leaks: Implement IDisposable or use using statements to automatically release unmanaged resources.

using (SqlConnection conn = new SqlConnection("..."))
{
    // Use the connection
}

• Detach event handlers to prevent lingering references.

70. What Is The Purpose Of The Is Keyword In C#?

The is keyword checks if an object is of a specific type, simplifying type casting. It's crucial for pattern matching in newer C# versions.

Example:

object item = "Hello";
if (item is string text)
{
    Console.WriteLine(text.ToUpper());
}
// Output: Hello

is enhances type safety, especially in complex systems like microservices where data types may vary.

71. How Do You Work With File I/O In C#?

File I/O operations are fundamental in applications like logging or data ingestion. Here's how you handle files in C#:

Use System.IO.File class to read or write files:

string content = File.ReadAllText("data.txt");
File.WriteAllText("output.txt", content.ToUpper());
// if data.txt contains “Hello world”
// Output: HELLO WORLD

• Utilize StreamReader or StreamWriter for more control over file streams.

Always handle exceptions and close resources to avoid file locks.

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

In C#, both == and Equals() are used to compare objects, but there are key differences in how they work:

• == Operator:
  • The == operator checks for reference equality for reference types and value equality for value types.
  • For reference types, it checks if both variables refer to the same memory location (i.e., they are the same object in memory).
  • For value types, it compares the actual values stored in the variables.
• Equals() Method:
  • The Equals() method is defined by the Object class, and its behavior can be overridden in derived classes. By default, it checks for reference equality (same as == for reference types).
  • When overridden, it can be customized to check for value equality in reference types, like comparing the contents of two objects instead of their memory addresses.

Example:

class Person
{
    public string Name { get; set; }
}
Person person1 = new Person { Name = "Alice" };
Person person2 = new Person { Name = "Alice" };
Console.WriteLine(person1 == person2);        // False (reference comparison)
Console.WriteLine(person1.Equals(person2));  // False (reference comparison, unless overridden)

When to use:

• Use == when you want to compare value types or check if two references point to the same object.
• Use Equals() when you need to compare object content, especially when working with custom types that override Equals().

73. How Do You Handle Json Data In C#?

JSON is commonly used in APIs, IoT, and microservices. Here's how you can work with JSON in C#:

• Use System.Text.Json for basic serialization and deserialization.

var jsonString = JsonSerializer.Serialize(myObject);
var newObject = JsonSerializer.Deserialize<MyClass>(jsonString);

• For advanced features, use Newtonsoft.Json.

74. What Is A Foreach Loop In C#, And How Is It Different From A For Loop?

A foreach loop simplifies iteration over collections, removing the need for index management.

• foreach:

foreach (int number in numbers)
{
    Console.WriteLine(number);
}

• for:

for (int i = 0; i < numbers.Length; i++)
{
    Console.WriteLine(numbers[i]);
}

Use foreach for simplicity and safety when iterating over collections.

75. Can You Explain The Significance Of The Using Statement In C# For Resource Management?

The using statement ensures that objects implementing IDisposable are disposed of automatically, crucial for preventing resource leaks.
Example:

using (StreamWriter writer = new StreamWriter("log.txt"))
{
    writer.WriteLine("Log entry");
}

This approach is essential in managing resources like database connections or file handles.

76. What Is The Difference Between An Abstract Method And A Virtual Method In C#?

An abstract method has no implementation and must be overridden by derived classes. A virtual method provides a default implementation, which derived classes can override if needed.

1. Abstract Method:
Forces derived classes to provide an implementation.

public abstract class Shape
{
    public abstract void Draw();
}

2. Virtual Method:
Provides a default implementation, which can be optionally overridden.

public class BaseLogger
{
    public virtual void Log(string message) => Console.WriteLine(message);
}

In large codebases, abstract methods provide a blueprint, while virtual methods offer default behaviors.

77. How Do You Implement The Observer Design Pattern In C#?

The Observer pattern allows objects to subscribe to event notifications, making it perfect for real-time systems like stock tickers or chat apps.

1. Define a Subject (Observable):

public class Subject
{
    private List<IObserver> observers = new List<IObserver>();
    public void Attach(IObserver observer) => observers.Add(observer);
    public void Detach(IObserver observer) => observers.Remove(observer);
    public void Notify() { observers.ForEach(obs => obs.Update()); }
}

2. Observers:
Implement Update() to receive notifications.

The Observer pattern enables modular and flexible design in event-driven systems.

78. How Do You Handle Database Connections In C#?

In enterprise applications, database connections are managed using ADO.NET, which connects to databases, executes commands, and reads results.

using (SqlConnection conn = new SqlConnection("connection_string"))
{
    conn.Open();
    using (SqlCommand cmd = new SqlCommand("SELECT * FROM Users", conn))
    {
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                Console.WriteLine(reader["Username"]);
            }
        }
    }
}

Use using statements to ensure proper disposal of resources.

79. What Is A Constructor In C#, And How Does Constructor Chaining Work?

A constructor initializes a new object's state, and constructor chaining calls another constructor within the same class to avoid code duplication.

public class Book
{
    public string Title { get; set; }
    public Book() : this("Untitled") { }
    public Book(string title) { Title = title; }
}

Constructor chaining helps organize initialization logic in large classes.

80. What Are Indexers In C#, And How Do They Differ From Properties?

Indexers let you treat a class like an array. Properties allow getting or setting named values.

public string this[int index]
{
    get { return names[index]; }
    set { names[index] = value; }
}

• Indexers use array-like syntax, while properties use named members.

Indexers are useful for fast lookups in data collection objects.

81. Can You Explain The Role Of Garbage Collection In C# And How Does It Work?

Garbage Collection (GC) automatically frees unused memory by identifying objects no longer referenced.

• Generations (0, 1, 2) optimize collection frequency.
• GC can be triggered manually using GC.Collect() if necessary.

Garbage collection ensures efficient memory management without manual intervention.

82. What Is The Difference Between Ref And Out Parameters In C#?

Here's a table to explain the differences between ref and out parameters in C#:

83. What Are Named Arguments In C#, And How Do They Work?

Named arguments allow specifying arguments by their parameter names, enhancing readability and avoiding confusion.

PrintDetails(age: 30, name: "John");

This improves clarity, especially when dealing with methods with many parameters.

84. How Does C# Implement Delegates And Events For Asynchronous Programming?

Delegates are type-safe function pointers, and events are specialized delegates for broadcasting notifications.

public delegate void Notify(string message);
public event Notify OnNotify;

Events help implement asynchronous and event-driven programming.

85. What Are Attributes In C#, And How Do You Use Them?

Attributes add declarative metadata to code, which is used for reflection, validation, or generating additional logic.

[Obsolete("Use NewMethod instead")]
public void OldMethod() { }

Custom attributes can be created by deriving from System.Attribute.

86. How Does C# Support Database Connectivity With Ado.Net?

ADO.NET provides classes like SqlConnection, SqlCommand, and SqlDataReader to handle database connections and operations.

using (SqlConnection conn = new SqlConnection("connection_string"))
{
    conn.Open();
    // Execute commands
}

Though ORMs like Entity Framework are popular, ADO.NET remains a powerful choice for low-level database access.

87. How Do You Implement A Callback In C#?

A callback is a function called upon the completion of a task. You can implement it using delegates.

public delegate void ProcessComplete(string result);
public static void DoWork(ProcessComplete callback)
{
    // Perform some work
    callback("Work completed");
}
// Output “Work completed”

Callbacks are widely used in event-driven systems, though async/await often replaces them in modern code.

88. What Is The Difference Between Ienumerable And Ienumerator In C#?

IEnumerable provides a sequence of items, while IEnumerator iterates through them.

public interface IEnumerable<T> { IEnumerator<T> GetEnumerator(); }

IEnumerator allows moving through the sequence with MoveNext() and Current.

89. What Is The Role Of Task Parallel Library (TPL) In C#?

The Task Parallel Library (TPL) simplifies parallel and asynchronous programming in C#. It provides a higher-level API to work with tasks and enables better scalability and performance for concurrent operations.

• Task.Run: Executes a task asynchronously, using the thread pool to run the code concurrently.

Task.Run(() => HeavyCalculation());

• Parallel.For: Executes loops in parallel, improving performance for computationally intensive operations.

Parallel.For(0, 100, i => DoWork(i));

• TPL is often used in combination with async/await for building scalable and responsive applications that can handle multiple operations concurrently without blocking the main thread.

TPL vs Thread Pool

While the Task Parallel Library (TPL) abstracts away the manual thread management required when working directly with the Thread Pool, it still internally uses the thread pool for task execution. In essence, TPL leverages the ThreadPool to manage and schedule tasks efficiently, but it provides a more user-friendly and higher-level interface for concurrent programming.

• Thread Pool: Provides a pool of threads that can be reused, reducing the overhead of creating and destroying threads manually. Developers must manage thread assignment and synchronization themselves.

TPL: Uses the Thread Pool behind the scenes, but abstracts away the complexity. It makes it easier for developers to write concurrent and parallel code without worrying about thread management. TPL provides task-based parallelism with higher-level methods like Task.Run, Task.WhenAll, and Parallel.For.

90. What Are The Top C# Interview Questions?

When preparing for a C# interview, it’s essential to dive into both foundational and advanced concepts. From the intricacies of extension methods to understanding the Task Parallel Library (TPL), C# offers a wide spectrum of topics that can be tested. Additionally, tricky areas like threading, synchronization, and memory management are critical to mastering the language. 

Key Areas to Focus On:

1. Extension Methods
2. Task Parallel Library (TPL) and Asynchronous Programming
3. Threading and Synchronization
4. Advanced C# Features and Patterns
5. Garbage Collection and Memory Management
6. LINQ and Functional Programming Techniques
7. Latest .NET Features and Emerging Libraries

Also Read: Top 19 C# Projects in 2025: For Beginners & Advanced

With C# interview questions covered, it’s time to map out your learning path for 2025.

C# Learning Roadmap In 2025

Mastering C# in 2025 is essential for developers aiming to build advanced AI, cloud-native, and IoT solutions. With its continuous evolution, having a clear learning path is crucial. This roadmap ensures you’re ready to handle C# interview questions—whether you're a fresher or experienced developer. The demand for skilled C# developers in microservices, web solutions, and VR applications are at an all-time high. 

Let’s dive in!

Pre-Requisites

Practical readiness serves as the first step in your journey. Transition smoothly into these essentials and see how they solidify your foundation for advanced exploration.

Basic Programming Concepts

Start with the essentials that form the bedrock of your C# expertise:

• Control Structures: Practice with simple programs like a calculator.
• Variable Scopes: Experiment with user input handling to understand different variable scopes.

These basics set you up for deeper, more advanced C# topics.

Object-Oriented Programming (OOP)

Object-Oriented Programming (OOP) is key to creating well-structured, maintainable applications:

• Classes and Objects: Build a library system to track books.
• Inheritance and Polymorphism: Create a role-based system to manage different user levels.

These concepts flow naturally into .NET, where C# shines in enterprise solutions.

Understanding of .NET Framework

Grasping the .NET ecosystem helps you understand C#’s full potential:

• Common Language Runtime (CLR): Explore cross-language compatibility through projects.
• Assemblies: Learn how libraries and references work in real applications.

Familiarity With Basic Algorithms And Data Structures

Understanding core algorithms and data structures is essential for efficient problem-solving:

• Sorting and Searching: Implement sorting algorithms to improve e-commerce search functionality.
• Stacks and Queues: Simulate message handling in a chat application.

Basic Knowledge of Web Development

Web development is crucial for building modern systems, especially when integrating with C#:

• HTTP Protocols: Build a simple REST API that returns JSON.
• Client-Server Interaction: Connect a basic JavaScript frontend to a C# backend.

Development Environment Setup

Optimize your development workflow with the right tools:

• IDE Installation: Set up Visual Studio or Visual Studio Code and create a “Hello World” project to confirm your setup.
• Compiler Settings: Stay updated with the latest C# features.

C# Tools

Trends in 2025 show a surge in microservices and container-based deployments. Armed with the right tools, you can conquer c# interview questions for freshers and c# basic interview questions that probe your comfort with practical scenarios.

Visual Studio

Visual Studio remains a powerhouse for C# development:

• IntelliSense: Refactor repetitive code in a finance app.
• Debugger: Debug a multithreaded application to track variable states in real-time.

.NET Core SDK and Runtime

.NET Core is integral for cross-platform applications:

• Build CLI Tools: Test cross-platform functionality on Windows, Linux, and macOS.
• Versioning: Switch between different .NET Core versions to understand performance differences.

NuGet Package Manager

NuGet simplifies package management:

• Install JSON Parsers: Integrate third-party libraries like Newtonsoft.Json into your projects.
• Create Custom Packages: Share your custom libraries within your team.

LINQ (Language Integrated Query)

LINQ streamlines data manipulation:

• Transform Data: Aggregate sensor readings into meaningful results.
• Entity Framework: Use LINQ to query a sample e-commerce database.

Git and GitHub for Version Control

Version control ensures efficient collaboration:

• Initialize Git Repositories: Commit incremental changes and track your evolution.
• GitHub Collaboration: Push changes and set up pull requests for code reviews.

Step Wise Approach To Learn C#

Every step refines your skill set in a practical manner. This approach builds confidence for challenging c# interview questions for freshers and c# basic interview questions relevant to professional development in 2025.

Step 1: Learn Basic Syntax And Data Types

Build a solid foundation with C# basics:

• Variable Declarations: Automate data entry with a script.
• Operators: Implement a mini math puzzle game to practice operators.

Step 2: Master Object-Oriented Programming (OOP) Concepts

OOP is essential for clean, maintainable code:

• Employee Management System: Create classes for different job roles.
• Inheritance: Use inheritance to categorize employees (e.g., permanent or contract).

Step 3: Learn About Collections And Generics

Collections and generics handle data efficiently:

• Lists: Store temperature readings in a weather app.
• Dictionaries: Manage product IDs and pricing data.

Step 4: Understand Exception Handling And Debugging

Master error handling for stable applications:

• Try-Catch: Handle file processing errors with custom exceptions.
• Debugging: Use breakpoints and watch windows in Visual Studio to debug complex code.

Step 5: Explore Asynchronous Programming With Async And Await

Asynchronous programming improves performance:

• Async Calls: Fetch real-time stock data in a console app.
• API Calls: Use await to handle multiple API calls concurrently.

Step 6: Build Web Applications With ASP.NET Core

ASP.NET Core is central to web development:

• CRUD Application: Build a volunteer registration system for a non-profit.
• Minimal APIs: Experiment with simple APIs or Razor Pages.

Step 7: Learn Unit Testing And Test-Driven Development (TDD)

Ensure code reliability with testing:

• Unit Tests: Write tests using MSTest or xUnit for user input functions.
• TDD: Apply TDD in a to-do list app by writing tests before coding features.

Step 8: Develop And Deploy Cross-Platform Applications

Expand your reach with cross-platform solutions:

• Mobile App: Use Xamarin or .NET MAUI to create a habit-tracking app for Android and iOS.
• Containerization: Dockerize your ASP.NET Core service and deploy it on a cloud platform.

This roadmap offers a comprehensive, step-by-step structure that prepares you for dynamic industry demands. You sharpen your potential to tackle c# interview questions with the depth and confidence needed in 2025.

Conclusion

Mastering C# and preparing for interviews is an ongoing journey that requires consistent effort and focus. With the advanced C# topics covered in this blog, you now have a solid understanding of essential concepts like threading, synchronization, asynchronous programming, and design patterns. These key questions will challenge your knowledge and deepen your expertise.

To ensure you’re fully prepared for your upcoming C# interview, 

• Start by practicing these questions regularly. 
• Apply the concepts in practical coding scenarios, and work on refining your answers. 
• Create a personal question bank to track your progress and review it daily to reinforce your knowledge. 
• Simulating mock interviews is also a great way to gain confidence and polish your response strategies.

Take advantage of platforms like upGrad, where you can receive 1:1 counseling to help you pinpoint areas for improvement and perfect your interview techniques. If you prefer in-person learning, visiting upGrad center to meet with experts can provide valuable insights and help you build connections within the C# and .NET community.

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References:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13