Junior-Level C# Interview Questions and Answers

Q1. What is C#?

Answer: C# (pronounced "C-Sharp") is a modern programming language developed by Microsoft. It is used for building applications like websites, desktop apps, and mobile apps. It runs on the .NET framework, which helps developers write efficient and secure code.

Q2. What is the difference between a Class and an Object?

Answer: A Class is like a blueprint, while an Object is the actual thing built from that blueprint . An object can also be referred to as an instance of a class.
Example: If "Car" is a class, then a specific car like "Toyota Camry" is an object of that class.

Q3. What are the different data types in C#?

Answer:C# has different types of data types:
Integer (int) – Whole numbers like 10, 100, -5.
Floating-point (float, double) – Numbers with decimals like 10.5, 3.14.
Character (char) – A single letter like ‘A’.
Boolean (bool) – True or False.
String (string) – A group of characters like "Hello World".

Q4. What is the difference between == and = in C#?

Answer:
= is used to assign a value. Example: int x = 5;
== is used to compare values. Example: if (x == 5) checks if x is equal to 5.

Q5. What are loops in C# and why do we use them?

Answer: Loops help repeat a block of code multiple times. Common loops in C#:
for loop – Used when you know how many times to repeat.
while loop – Runs as long as a condition is true.
do-while loop – Runs at least once before checking the condition.
Example:

for (int i = 1; i <= 5; i++) {
    Console.WriteLine(i);
}

This prints numbers 1 to 5.

Q6. What is an if statement?

Answer: An if statement checks a condition and runs code only if the condition is true.
Example:

int age = 18;
if (age >= 18) {
    Console.WriteLine("You can vote.");
} else {
    Console.WriteLine("You cannot vote yet.");
}

Q7. What is an array?

Answer: An array is used to store multiple values in a single variable.
Example:

int[] numbers = {1, 2, 3, 4, 5};
Console.WriteLine(numbers[0]); // Outputs 1

Q8. What is a method in C#?

Answer: A method is a block of code that performs a task.
Example:

void SayHello() {
    Console.WriteLine("Hello, World!");
}

To call this method, you simply write SayHello();

Q9. What is the difference between public, private, and protected in C#
Answer:

public – Can be accessed from anywhere.
private – Can only be accessed inside the same class.
protected – Can be accessed in the same class and inherited classes.
Example:

class Car {
    private string color = "Red";  // Only accessible inside Car class
}

Q10. What is the difference between break and continue in loops?

Answer:
break – Stops the loop completely.
continue – Skips the current iteration and moves to the next one.
Example:

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }
    Console.WriteLine(i);
}

This skips printing 3 but continues the loop.

Q11. What is exception handling in C#?

Answer: Exception handling helps manage errors in a program.
try – Code that might have an error.
catch – Handles the error if one occurs.
finally – Runs whether there’s an error or not.
Example

try {
    int x = 10 / 0; // This will cause an error
} catch (Exception e) {
    Console.WriteLine("An error occurred: " + e.Message);
} finally {
    Console.WriteLine("This will always run.");
}

Q12. What is Object-Oriented Programming (OOP)?

Answer: OOP is a way of organizing code using objects. The four main principles are:
Encapsulation – Hiding details inside a class.
Abstraction – Hiding complexity and showing only necessary features.
Inheritance – A child class can use the properties of a parent class.
Polymorphism – The ability to have multiple forms (e.g., method overloading).

Example:

class Animal {
    public void MakeSound() {
        Console.WriteLine("Animal makes a sound");
    }
}
class Dog : Animal {
    public void Bark() {
        Console.WriteLine("Dog barks");
    }
}

Dog inherits from Animal, so it can use MakeSound() and its own method Bark().

Q13. What is a constructor in C#?

Answer: A constructor is a special method that runs when an object is created.
Example:

class Car {
    public Car() {
        Console.WriteLine("A new car is created!");
    }
}
Car myCar = new Car(); // Outputs: A new car is created!

Q14. What is the difference between == and .Equals()?

Answer:
== checks if two variables refer to the same object.
.Equals() checks if two objects have the same value.
Example:

string a = "hello";
string b = "hello";
Console.WriteLine(a == b);     // True
Console.WriteLine(a.Equals(b)); // True

Q15. What is a static keyword in C#?

Answer:
A static method or variable belongs to the class, not an object.
Example:

class MathHelper {
    public static int Add(int a, int b) {
        return a + b;
    }
}
Console.WriteLine(MathHelper.Add(2, 3)); // Outputs 5

You can call Add() without creating an object of MathHelper.

Q16: How does encapsulation improve security in financial applications?

Answer:
Encapsulation hides sensitive data (e.g., account balance, PIN) inside a class, only allowing controlled access via methods. This prevents direct data manipulation, reducing fraud risks.

Q17: What is the best collection to store multiple customers' transaction histories?

Answer:
A Dictionary<string, List<double>> is ideal:
The key is the customer’s account number.
The value is a list of transactions.

Q18: Why are threads important in financial applications?

Answer:
Threads enable:
Real-time transaction processing (handling multiple requests at once).
Live stock price updates in trading applications.
Parallel banking operations (handling deposits, withdrawals, and loan processing simultaneously).

Q19. What is the difference between ref and out parameters in C#?

Answer:
Both ref and out are used to pass parameters by reference, but they have different use cases:

• ref: The parameter must be initialized before it is passed to the method. The method can modify the value of the parameter, and the modified value will be reflected in the calling method.
• out: The parameter does not need to be initialized before passing. It is meant to be used when a method is expected to assign a value to the parameter. The method guarantees that the parameter will be assigned a value before it returns.

Q20. What are nullable types in C#, and when would you use them?

Answer:
Nullable types allow value types (like int, double, bool, etc.) to hold null as a valid value. This is useful when you need to represent the absence of a value in situations where value types cannot naturally be null.
A nullable type is defined using the ? symbol after the value type, e.g., int? or bool?.
Example:

int? age = null;  // Nullable integer
if (age.HasValue)
{
    Console.WriteLine(age.Value);
}
else
{
    Console.WriteLine("Age is not provided.");
}

Q21. What is the difference between == and Equals() in C#?

Answer:*

• ==: The == operator compares two operands. For value types, it compares their actual values. For reference types, it compares the references (i.e., the memory addresses). Example:

int a = 5, b = 5;
Console.WriteLine(a == b); // true, compares values

• Equals(): The Equals() method is used to compare objects for equality based on their content or state. It is generally overridden in custom classes to define equality based on the object’s properties rather than just memory addresses. Example:

object obj1 = new object();
object obj2 = new object();
Console.WriteLine(obj1.Equals(obj2));  // false, compares content of the objects

When comparing objects, Equals() is typically preferred for a deeper comparison, while == can be used for reference or primitive types comparison.

Q22. What is the difference between String and StringBuilder in C#?
Answer:

• String: Immutable, meaning once a string is created, it cannot be modified. Any operation that appears to modify a string actually creates a new instance. Example:

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

• StringBuilder: Mutable, designed for scenarios where you need to perform multiple string manipulations (like concatenation) efficiently. It avoids the overhead of creating new string objects for each modification. Example:

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

Use StringBuilder when performing multiple string operations to improve performance.

Q23. What is the difference between value type and reference type in C#?
Answer:

• Value Type: Holds the actual data. When a value type is assigned to another, a copy of the value is created. Examples: int, float, bool, struct
• Reference Type: Holds a reference (memory address) to the data. When a reference type is assigned to another, both refer to the same object. Examples: class, string, array, delegate

Q24. What is a delegate in C#?

Answer:
A delegate is a type-safe function pointer in C#. It is used to reference methods with a specific signature and invoke them indirectly. Delegates are particularly useful for event handling or callback functions.
Example:

delegate void MyDelegate(string message);
void ShowMessage(string message)
{
    Console.WriteLine(message);
}
MyDelegate del = new MyDelegate(ShowMessage);
del("Hello, Delegate!");  // Invokes ShowMessage

Q25. What is the purpose of the using statement in C#?

Answer:
The using statement is used to manage resources such as file streams, database connections, or network sockets. It ensures that objects that implement IDisposable are disposed of correctly, releasing any resources when done.
Example:

using (StreamWriter writer = new StreamWriter("file.txt"))
{
    writer.WriteLine("Hello, world!");
}  // Automatically calls Dispose() when done

Q26. What is the difference between abstract class and interface in C#?

Answer:

• Abstract Class: Can provide both method declarations and implementations. A class can inherit from only one abstract class.
• Interface: Only provides method declarations (no implementation). A class can implement multiple interfaces. Example:

// Abstract Class
abstract class Animal
{
    public abstract void Speak();  // Abstract method
}
class Dog : Animal
{
    public override void Speak() { Console.WriteLine("Bark"); }
}
// Interface
interface IAnimal
{
    void Speak();
}
class Cat : IAnimal
{
    public void Speak() { Console.WriteLine("Meow"); }
}

Q27. What is the purpose of the static keyword in C#?

Answer:
The static keyword is used to define members (fields, methods, properties) that belong to the type itself, rather than to instances of the type. Static members can be accessed without creating an instance of the class.
Example:

class MathUtility
{
    public static int Add(int a, int b) => a + b;
}
Console.WriteLine(MathUtility.Add(2, 3));  // No need to create an instance

Q28. What is the try-catch block in C#?

Answer:
A try-catch block is used to handle exceptions in C#. Code inside the try block is executed, and if an exception occurs, it is caught by the catch block, preventing the program from crashing.
Example:

try
{
    int x = 10, y = 0;
    int result = x / y;  // This will throw a DivideByZeroException
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Error: Cannot divide by zero.");
}

Q29. What is the difference between continue and break in loops in C#?

Answer:

• continue: Skips the current iteration of the loop and moves to the next iteration.
• break: Exits the loop entirely, terminating it. Example:

// Example of continue
for (int i = 0; i < 5; i++)
{
    if (i == 2) continue;  // Skips the iteration when i is 2
    Console.WriteLine(i);   // Will print 0, 1, 3, 4
}
// Example of break
for (int i = 0; i < 5; i++)
{
    if (i == 2) break;  // Breaks the loop when i is 2
    Console.WriteLine(i);   // Will print 0, 1
}

Q30. What is the difference between Task and Thread in C#?

Answer:

• Thread: Represents a separate path of execution in a program. A thread is a low-level construct provided by the operating system. You can create a new thread by creating an instance of the Thread class. It is more manual and requires careful management (e.g., starting, stopping, synchronization). Example:

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

• Task: Represents an asynchronous operation and is a higher-level construct provided by the Task Parallel Library (TPL). It’s part of the System.Threading.Tasks namespace and is typically used for parallel programming. Tasks are more efficient than threads because they are managed by the .NET runtime, making it easier to handle asynchronous operations without directly managing threads. Example:

Task.Run(() =>
{
    Console.WriteLine("Task is running");
});

Key differences:

• Thread is lower level, while Task is higher level and works with the Task Parallel Library.
• Thread creates and manages a new OS-level thread, whereas a Task may use thread pooling, making it more efficient for short-lived asynchronous operations.
• Task is better suited for asynchronous programming, while Thread is more suitable for when you need a long-running operation or need to manually control execution.

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