Top C# Interview Questions and Answers for (2024)

In this article, we’ll explore some essential C# interview questions designed to sharpen your skills and boost your confidence. Whether you’re a developer or just starting, these in-depth answers and practical examples will equip you with the knowledge needed to stand out in C# programming interviews.

So, let’s get started

1. What is C#?

Answer: C# (C-sharp) is a modern, object-oriented programming language developed by Microsoft. It is designed for building Windows applications, web applications, and various types of software using the .NET framework. C# combines the power of C++ with the simplicity of Visual Basic, making it a versatile language for a wide range of applications.

2. Explain the difference between StringBuilder and String in C#.

Answer: In C#, a String is immutable, meaning its value cannot be changed after creation. Every time you modify a string, a new instance is created. StringBuilder, on the other hand, is mutable. It allows dynamic modification of the string without creating a new object each time, making it more efficient when dealing with frequent string manipulations.

Code Example:

// Using String
string str = "Hello";
str += " World";  // This creates a new string

// Using StringBuilder
StringBuilder sb = new StringBuilder("Hello");
sb.Append(" World");  // Modifies the existing StringBuilder object

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

Answer: The using statement in C# is used for automatic resource management, particularly with objects that implement IDisposable. It ensures that the resources are properly disposed of, even if an exception occurs, by calling the Dispose method. This is commonly used with file operations, database connections, and other resource-intensive tasks.

Code Example:

using (var fileStream = new FileStream("example.txt", FileMode.Open))
{
    // Perform file operations
}  // fileStream.Dispose() is called automatically

4. What are the different types of inheritance in C#?

Answer: C# supports the following types of inheritance:

1. Single Inheritance: A class can inherit from only one base class.
2. Multiple Inheritance (through interfaces): A class can implement multiple interfaces, achieving a form of multiple inheritance.
3. Multilevel Inheritance: A derived class inherits from a base class, and then another class inherits from this derived class.
4. Hierarchical Inheritance: Multiple classes inherit from a single base class.

5. Explain the concept of polymorphism in C# with an example.

Answer: Polymorphism allows objects of different types to be treated as objects of a common type. In C#, this is achieved through method overriding and interfaces.

Code Example:

// Base class
public class Shape
{
    public virtual void Draw()
    {
        Console.WriteLine("Drawing a shape.");
    }
}

// Derived class
public class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Drawing a circle.");
    }
}

// Usage
Shape myShape = new Circle();
myShape.Draw();  // Calls the overridden method in Circle class

Here, the Draw method is polymorphic, allowing the same method name to behave differently based on the actual type of the object.

6. What is the difference between IEnumerable and IEnumerator in C#?

Answer: IEnumerable represents a collection of objects that can be enumerated (iterated) one at a time. IEnumerator provides the mechanism to iterate through the collection. When you use a foreach loop, it implicitly uses these interfaces.

Code Example:

// IEnumerable example
IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
foreach (var number in numbers)
{
    Console.WriteLine(number);
}

// Behind the scenes, it's similar to:
var enumerator = numbers.GetEnumerator();
while (enumerator.MoveNext())
{
    Console.WriteLine(enumerator.Current);
}

7. What is the purpose of the async and await keywords in C#?

Answer: async and await are used in C# for asynchronous programming. The async keyword indicates that a method contains asynchronous operations, and await is used to pause the execution of the method until the awaited task is complete. This allows non-blocking execution and improves responsiveness in applications.

Code Example:

async Task<int> GetDataAsync()
{
    // Asynchronous operation, e.g., fetching data from a web service
    return await FetchDataAsync();
}

8. What is the purpose of the using directive in C#?

Answer: The using directive in C# is used to include a namespace in the program, providing access to the types and members defined in that namespace. It simplifies code by allowing the use of short type names without fully qualifying the namespace.

Code Example:

using System;

namespace MyApp
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Hello, C#!");
        }
    }
}

9. Explain the concept of delegates in C# with an example.

Answer: A delegate in C# is a type that represents references to methods. It allows methods to be passed as parameters, stored in variables, and invoked dynamically. Delegates are often used for implementing events and callbacks.

Code Example:

// Declaration of a delegate
delegate void PrintDelegate(string message);

class Program
{
    static void Main()
    {
        // Instantiating the delegate with a method
        PrintDelegate printMethod = PrintMessage;

        // Invoking the delegate
        printMethod("Hello, Delegates!");
    }

    static void PrintMessage(string message)
    {
        Console.WriteLine(message);
    }
}

10. What is the purpose of the sealed keyword in C#?

Answer: The sealed keyword in C# is used to prevent a class from being inherited. When a class is marked as sealed, it cannot be used as a base class for other classes. This is often used to secure a class’s implementation or to prevent unintended modifications.

Code Example:

sealed class FinalClass
{
    // Class implementation
}

11. Explain the concept of the try, catch, and finally blocks in exception handling.

Answer: In C#, the try block is used to enclose a block of code that might raise an exception. The catch block is used to handle the exception, and the finally block is used to execute code regardless of whether an exception is thrown or not. This ensures proper resource cleanup.

Code Example:

try
{
    // Code that might throw an exception
    int result = 10 / int.Parse("0");
}
catch (DivideByZeroException ex)
{
    // Handle specific exception
    Console.WriteLine("Cannot divide by zero.");
}
catch (Exception ex)
{
    // Handle other exceptions
    Console.WriteLine($"An error occurred: {ex.Message}");
}
finally
{
    // Cleanup code (executed whether an exception occurs or not)
}

12. How does garbage collection work in C#?

Answer: Garbage collection in C# is an automatic process where the runtime environment manages the memory by reclaiming unused objects. The .NET garbage collector runs in the background, identifies objects without references, and releases their memory. Developers do not need to explicitly free memory, enhancing application stability and reducing memory-related issues.

13. What is the purpose of the static keyword in C#?

Answer: The static keyword in C# is used to declare members (fields, methods, properties) that belong to the class rather than instances of the class. These members can be accessed without creating an instance of the class.

Code Example:

public class MathOperations
{
    public static int Add(int a, int b)
    {
        return a + b;
    }
}

// Usage
int result = MathOperations.Add(5, 3);

14. Explain the concept of an interface in C# with an example.

Answer: An interface in C# defines a contract of methods, properties, and events that a class must implement. It provides a way to achieve multiple inheritance and promotes code consistency.

Code Example:

// Interface definition
public interface IShape
{
    void Draw();
}

// Class implementing the interface
public class Circle : IShape
{
    public void Draw()
    {
        Console.WriteLine("Drawing a circle.");
    }
}

15. What is the purpose of the out keyword in C#?

Answer: The out keyword in C# is used in method parameters to indicate that the parameter is an output parameter. It is used to return multiple values from a method.

Code Example:

public class MathOperations
{
    public static void Divide(int numerator, int denominator, out int result, out int remainder)
    {
        result = numerator / denominator;
        remainder = numerator % denominator;
    }
}

// Usage
int quotient, remainder;
MathOperations.Divide(10, 3, out quotient, out remainder);

16. Explain the concept of boxing and unboxing in C#.

Answer: Boxing is the process of converting a value type to an object type, and unboxing is the reverse process. It involves implicitly or explicitly converting between value types (e.g., int, char) and reference types (e.g., object).

Code Example:

int number = 42;

// Boxing: Converting value type to reference type
object boxedNumber = number;

// Unboxing: Converting reference type back to value type
int unboxedNumber = (int)boxedNumber;

17. What are lambda expressions in C#? Provide an example.

Answer: Lambda expressions in C# provide a concise way to write anonymous methods or functions. They are often used in LINQ queries and functional programming.

Code Example:

// Without lambda expression
List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.FindAll(delegate (int n) { return n % 2 == 0; });

// With lambda expression
var evenNumbersLambda = numbers.FindAll(n => n % 2 == 0);

Lambda expressions simplify the syntax for defining small, inline functions.

18. Explain the difference between ref and out parameters in C#.

Answer: Both ref and out parameters in C# are used to pass parameters by reference, allowing the method to modify the value of the parameter. However, there are key differences.

• ref parameters must be initialized before passing to the method, while out parameters do not require initialization.
• out parameters are used when the method is expected to assign a value to the parameter.

Code Example:

// Using ref
void ModifyWithRef(ref int value)
{
    value += 10;
}

// Using out
void ModifyWithOut(out int value)
{
    value = 10;
}

// Usage
int a = 5;
ModifyWithRef(ref a);  // a is modified
int b;
ModifyWithOut(out b);  // b is assigned a value

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

Answer: The yield keyword in C# is used in iterator methods to simplify the implementation of custom iterators. It allows a method to return a sequence of values, one at a time, without having to create an entire collection.

Code Example:

public IEnumerable<int> GenerateNumbers()
{
    for (int i = 0; i < 5; i++)
    {
        yield return i;
    }
}

// Usage
foreach (var number in GenerateNumbers())
{
    Console.WriteLine(number);
}

The yield keyword makes it easier to work with large data sets without loading everything into memory at once.

20. What is the purpose of the params keyword in C#?

Answer: The params keyword in C# allows a method to accept a variable number of parameters. It simplifies method calls by allowing the caller to provide a variable number of arguments.

Code Example:

public static int Sum(params int[] numbers)
{
    int result = 0;
    foreach (int num in numbers)
    {
        result += num;
    }
    return result;
}

// Usage
int sum = Sum(1, 2, 3, 4, 5);

21. Explain the purpose of the using statement with databases in C#.

Answer: In the context of databases, the using statement is crucial for managing resources like database connections. It ensures that the connection is properly closed and resources are released, even if an exception occurs.

Code Example:

using (SqlConnection connection = new SqlConnection(connectionString))
{
    connection.Open();
    // Perform database operations
} // Connection is automatically closed and resources released

This prevents resource leaks and improves the overall reliability of database operations.

22. What is the role of the DataAnnotations namespace in C#?

Answer: The DataAnnotations namespace in C# is used for declarative data validation. It provides attributes that can be applied to model properties to define validation rules, such as required fields, string lengths, and regular expressions.

Code Example:

public class Person
{
    [Required]
    public string Name { get; set; }

    [Range(1, 100)]
    public int Age { get; set; }
}

These annotations are often used in ASP.NET MVC for client and server-side validation.

23. Explain the concept of extension methods in C# with an example.

Answer: Extension methods in C# allow developers to add new methods to existing types without modifying them. This is achieved by creating static methods in static classes and using the this keyword to specify the extended type.

Code Example:

// Extension method class
public static class StringExtensions
{
    public static string Reverse(this string input)
    {
        char[] charArray = input.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }
}

// Usage
string originalString = "hello";
string reversedString = originalString.Reverse();

In this example, the Reverse method is added to the string type without modifying the original string class.

24. What is the purpose of the async modifier in C#? Provide an example.

Answer: The async modifier in C# is used to define asynchronous methods. Asynchronous methods allow non-blocking execution, enabling the program to continue performing other tasks while waiting for an asynchronous operation to complete.

Code Example:

public async Task<int> FetchDataAsync()
{
    // Simulating an asynchronous operation, e.g., fetching data from a web service
    await Task.Delay(2000); // Represents some async operation
    return 42;
}

// Usage
Task<int> resultTask = FetchDataAsync();
int result = await resultTask;

The async keyword is used to mark the method, and the await keyword is used to wait for the asynchronous operation to complete.

25. Explain the concept of Dependency Injection (DI) in C#.

Answer: Dependency Injection (DI) is a design pattern in C# that promotes loose coupling between components by externalizing the dependencies of a class. Instead of a class creating its dependencies internally, they are provided (injected) from the outside. This enhances testability, maintainability, and flexibility.

Code Example:

// Without Dependency Injection
public class ProductService
{
    private Logger logger = new Logger();

    public void ProcessOrder(Order order)
    {
        // Business logic
        logger.Log("Order processed successfully.");
    }
}

// With Dependency Injection
public class ProductServiceDI
{
    private ILogger logger;

    // Constructor injection
    public ProductServiceDI(ILogger logger)
    {
        this.logger = logger;
    }

    public void ProcessOrder(Order order)
    {
        // Business logic
        logger.Log("Order processed successfully.");
    }
}

Dependency injection allows swapping implementations (e.g., different loggers) without modifying the core logic.

26. Explain the purpose of the Lock statement in C#.

Answer: The lock statement in C# is used for synchronization in a multithreaded environment. It ensures that only one thread can enter a critical section of code at a time, preventing race conditions and ensuring data consistency.

Code Example:

public class Counter
{
    private int count = 0;
    private object lockObject = new object();

    public void Increment()
    {
        lock (lockObject)
        {
            // Critical section
            count++;
        }
    }

    public int GetCount()
    {
        lock (lockObject)
        {
            // Critical section
            return count;
        }
    }
}

The lock statement uses a specified object as a lock, and only one thread can acquire the lock at a time, preventing concurrent access to the critical section of code.

Whether you’re preparing for an interview, expanding your skills, or seeking to deepen your understanding of C#, this article would serve you as a first step in your path.

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