Top 20 C# Interview Questions for the Beginners

1. What is object-oriented programming?

Ans: Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code to manipulate that data. In OOP, data and code are organized into "classes", which are templates or blueprints for creating objects. Objects created from a class can interact with one another to perform tasks and solve problems. OOP encourages modularity, flexibility, and reusability of code, making it a popular approach for developing large-scale software applications.

2. What are the four pillars of object-oriented programming?

Ans: The four pillars of object-oriented programming are:

Encapsulation: Encapsulation is the process of grouping related data and behavior within a single unit, called a class. This allows for better organization of code and ensures that the data is only accessible through the defined methods of the class, providing data security.

Inheritance: Inheritance allows a class to inherit properties and methods from a parent class, also known as a superclass or base class. This allows for code reusability and enables the creation of a hierarchy of classes with shared functionality.

Polymorphism: Polymorphism allows objects of different classes to be treated as if they were objects of the same class, providing flexibility and extensibility in code. This can be achieved through method overriding or method overloading.

Abstraction: Abstraction refers to the process of hiding implementation details and focusing on essential features of an object. This allows for simpler and more modular code, and helps developers focus on the most important aspects of their code without worrying about unnecessary details.

3. What is inheritance and how is it implemented in C#?

Ans: Inheritance is a mechanism in object-oriented programming that allows a class to inherit properties and methods from another class. The class that inherits from another class is called the derived class or subclass, and the class that is inherited from is called the base class or superclass.

In C#, inheritance is implemented using the : symbol followed by the name of the base class. Here is an example:
 

class Animal {

    public void Eat() {

        Console.WriteLine("Animal is eating.");

    }

}

class Dog : Animal {

    public void Bark() {

        Console.WriteLine("Dog is barking.");

    }

}

4. What is polymorphism and how is it achieved in C#?

Ans: Polymorphism is the ability of an object to take on multiple forms. In C#, polymorphism is achieved through method overloading and method overriding.

Method overloading allows multiple methods to have the same name, but different parameters. When a method is called, the C# compiler checks the number and type of arguments passed to the method, and selects the appropriate overload to execute. For example:

 

public class Calculator

{

    public int Add(int x, int y)

    {

        return x + y;

    }

    public double Add(double x, double y)

    {

        return x + y;

    }

}

Calculator calc = new Calculator();

int result1 = calc.Add(3, 4);           // calls the int Add(int, int) method

double result2 = calc.Add(3.5, 4.5);    // calls the double Add(double, double) method

5. What is encapsulation and how is it implemented in C#?

Ans: Encapsulation is the mechanism of hiding the internal workings of an object from the outside world, and exposing a public interface through which the object can be used. This helps in achieving data security and abstraction.

In C#, encapsulation is implemented using access modifiers like public, private, protected and internal. These modifiers restrict the access to class members from outside the class. By default, all class members are private.

Public members can be accessed from anywhere, while private members can only be accessed within the same class. Protected members can be accessed within the same class or any derived class. Internal members can be accessed within the same assembly.

To implement encapsulation, we can define private fields for the class and expose public properties or methods that allow controlled access to those fields. This way, we can control how the data is accessed and modified by the users of the class.

For example:

 

public class Person

{

    private string name;

    private int age;

   

    public string Name

    {

        get { return name; }

        set { name = value; }

    }

   

    public int Age

    {

        get { return age; }

        set { age = value; }

    }

}

6. What is abstraction and how is it used in C#?

Ans: Abstraction is the process of hiding the complexity of an object and exposing only the essential features of the object to the outside world. In C#, abstraction is achieved through the use of abstract classes, interfaces, and access modifiers.

An abstract class is a class that cannot be instantiated and is meant to be inherited by other classes. It can contain both abstract and non-abstract members. An abstract member is a member that has no implementation and must be implemented by any derived class. This allows the abstract class to provide a contract for any class that derives from it.

An interface is a contract that defines a set of members that must be implemented by any class that implements the interface. An interface only defines the signature of its members and does not provide any implementation. This allows for greater flexibility in designing software, as any class that implements an interface can be used wherever that interface is expected.

Access modifiers, such as public, private, and protected, allow for encapsulation by controlling the visibility of members within a class. Public members are accessible from anywhere, while private members can only be accessed from within the same class. Protected members can be accessed from within the same class or any derived classes.

By using abstraction, developers can create more modular and maintainable code, as each class is responsible for a specific set of features and can be easily extended or modified without affecting other parts of the codebase.

7. What is the difference between abstract classes and interfaces?

Ans: In C#, abstract classes and interfaces both provide a way to define contracts that classes can implement. However, there are some differences between them:

Implementation: An abstract class can have implementation for some of its members, while an interface cannot have any implementation.

Access Modifiers: Members of an interface are implicitly public and cannot have any access modifiers, while members of an abstract class can have access modifiers.

Multiple Inheritance: A class can inherit from multiple interfaces, but can only inherit from one abstract class.

Constructors: An abstract class can have constructors, but an interface cannot.

Fields: An interface cannot have fields, while an abstract class can.

Default Implementation: An abstract class can provide default implementation of its members, while an interface cannot.

Versioning: Adding a new method to an interface does not break the code, while adding a new member to an abstract class can break the code.

In summary, abstract classes are used when there is a need for common implementation for the derived classes, while interfaces are used when there is a need for a common contract that multiple classes can implement.

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

Ans: In C#, the sealed keyword is used to indicate that a class or method cannot be derived or overridden by any derived class.

When the sealed keyword is used before a class definition, it prevents the class from being used as a base class for any other class, meaning that it cannot be inherited from. Similarly, when the sealed keyword is used before a method definition, it prevents the method from being overridden by any derived class.

The sealed keyword is useful in situations where you want to restrict the behavior of a class or method to its current implementation, preventing any future derived classes from modifying its behavior. It can also be used to improve performance by allowing the compiler to generate more efficient code for the sealed class or method.

9. What is the difference between value types and reference types in C#?

Ans: In C#, variables can either be of value type or reference type.

Value types are those that hold the actual value of the data they represent, and they are stored on the stack. Examples of value types include basic data types such as int, float, double, and bool, as well as structs and enums. When you create a new variable of a value type, the memory is allocated to hold the value of that variable.

Reference types, on the other hand, hold a reference to an object or a value stored on the heap. They are stored on the heap and a pointer to the memory location of the value is stored on the stack. Examples of reference types include classes, delegates, and arrays. When you create a new variable of a reference type, memory is allocated to hold a reference to that variable.

The main difference between the two is that value types hold the actual value of the data they represent, while reference types hold a reference to the memory location where the value is stored. Additionally, value types are typically smaller in size and are often faster to access than reference types.

10. What is a constructor and what is its purpose in C#?

Ans: In C#, a constructor is a special method of a class that is called when an instance of the class is created. Its purpose is to initialize the object's state and prepare it for use.

Constructors can take parameters or have no parameters, and they are used to assign initial values to the fields or properties of the object. When an object is created, the constructor is automatically called, and it can perform any necessary initialization before the object is used.

Constructors are used to enforce object creation rules, initialize fields, and set default values. They also ensure that the object is in a valid state before it is used. A constructor can be overloaded, allowing for multiple constructors with different parameter lists to be defined within a class.

11. What is the difference between a class and an object?

Ans: In C#, a class is a blueprint or template that defines the data and behavior of a certain type of object. It is a user-defined data type that contains fields, properties, methods, and events. Classes are used to create objects.

An object, on the other hand, is an instance of a class that can be created and manipulated at runtime. It is a concrete entity that contains actual data values and can invoke methods defined in the class.

In simple terms, a class is like a cookie cutter that defines the shape and attributes of a cookie, while an object is like an actual cookie that is created from the cookie cutter and can be decorated or eaten.

12. What is a namespace in C#?

Ans: In C#, a namespace is a container that provides a way to organize related types, such as classes, interfaces, enums, and delegates. A namespace can be defined using the "namespace" keyword, followed by a unique name.

A namespace can contain other namespaces, as well as classes and other types. By using namespaces, you can avoid naming conflicts between types with the same name that are defined in different parts of your code.

For example, the .NET Framework has many namespaces, such as "System" and "System.IO", that group related types together. When you want to use a type that is defined in a namespace, you can either use the fully qualified name of the type (including the namespace), or you can include a "using" directive at the top of your code file to indicate which namespace you want to use.

13. What is the difference between private and protected access modifiers?

Ans: In C#, private and protected are access modifiers that control the visibility of class members from outside the class.

private: members that are marked as private can only be accessed within the same class in which they are declared. They are not visible outside the class.

protected: members that are marked as protected can be accessed within the same class, as well as from derived classes (i.e., subclasses). They are not visible outside the class hierarchy.

In summary, the main difference between the two is that private members are only accessible within the same class, while protected members can also be accessed by derived classes.

14. What is a delegate in C# and how is it used?

Ans: In C#, a delegate is a type that represents a reference to a method. It is similar to a function pointer in C or C++. A delegate can be thought of as a container for a method that can be passed as an argument to another method or stored as a field or property.

Delegates are commonly used in event handling, where an object exposes an event and other objects can register to be notified when the event occurs. The object raising the event maintains a list of delegates that are invoked when the event is raised.

To declare a delegate, you use the delegate keyword followed by the return type and parameter types of the method it will reference. For example:

delegate int Operation(int a, int b);

This declares a delegate named Operation that takes two int parameters and returns an int.

15. What is an event in C#?

Ans: In C#, an event is a way for a class to notify other classes or objects when something interesting happens. It is based on the publisher-subscriber pattern, where the class that generates the event is the publisher and the class that receives the event notification is the subscriber.

Events are implemented using delegates. A delegate is a type that represents a reference to a method with a particular signature, and can be used to call the method indirectly. An event is essentially a delegate with restrictions: it can only be raised by the class that defines it, and other classes can only subscribe to it or unsubscribe from it.

Here's an example of how to declare and raise an event in C#:

public class Button {

    public delegate void ClickEventHandler(object sender, EventArgs e);

    public event ClickEventHandler Click;

    public void OnClick() {

        if (Click != null) {

            Click(this, EventArgs.Empty);

        }

    }

}

public class Form {

    private Button button;

    public Form() {

        button = new Button();

        button.Click += Button_Click;

    }

    private void Button_Click(object sender, EventArgs e) {

        Console.WriteLine("Button clicked!");

    }

}

// Usage

Form form = new Form();

button.OnClick(); // Output: "Button clicked!"

In this example, the Button class defines an event called Click, which is a delegate with the signature void ClickEventHandler(object sender, EventArgs e). The OnClick method raises the event by calling the delegate if it is not null.

The Form class subscribes to the Click event of the Button instance it creates, by registering a method called Button_Click as the event handler. When the Button is clicked, it raises the Click event, which in turn calls the Button_Click method of the Form instance.

16. What is a static class in C# and when is it used?

Ans: In C#, a static class is a class that cannot be instantiated, and all of its members and methods are also static. The purpose of a static class is to provide a collection of related methods and properties that can be accessed without having to create an instance of the class.

Static classes are often used to provide utility methods, such as mathematical functions or conversion methods, that can be used throughout an application without having to create an instance of the class. Another common use of static classes is to create extension methods, which allow developers to add new methods to existing classes without having to modify the original class.

In addition, static classes are also used to implement singleton patterns, where only one instance of a class can be created, and to provide global variables and settings that can be accessed from anywhere in the application. Because static classes cannot be instantiated, they are thread-safe and can be accessed safely from multiple threads at the same time.

17. What is the difference between a static method and an instance method in C#?

Ans: In C#, a static method is a method that is associated with the type itself rather than with an instance of the type. It can be called without creating an instance of the class and can access only static fields, properties, and methods. It can also not access instance fields, properties, or methods.

On the other hand, an instance method is a method that belongs to an instance of a class. It is called using an instance of the class and can access both static and instance fields, properties, and methods.

In summary, the main differences between static and instance methods are:

Static methods are associated with the type itself while instance methods are associated with an instance of the type.

Static methods can be called without creating an instance of the class while instance methods require an instance to be created first.

Static methods can only access static fields, properties, and methods while instance methods can access both static and instance fields, properties, and methods.

 

18. What is a collection in C# and what are some common collection types?

Ans: In C#, a collection is an object that groups together multiple elements of the same type. Collections are used to store, retrieve, manipulate, and manage data efficiently. Common collection types in C# include:

Arrays: A fixed-size collection of elements of the same type.

Lists: A dynamically resizable collection of elements of the same type.

Dictionaries: A collection of key-value pairs where each key is unique.

Sets: A collection of unique elements.

Queues: A collection of elements where the first element added is the first element retrieved (FIFO).

Stacks: A collection of elements where the last element added is the first element retrieved (LIFO).

Linked lists: A collection of elements where each element points to the next element in the list.

Each collection type has its own unique properties and methods that can be used to manipulate the data stored within it.

19. What is the difference between a List and an ArrayList in C#?

Ans: In C#, both List and ArrayList are used to store and manipulate collections of objects. However, there are some differences between them:

Type Safety: List is a generic class which allows you to specify the type of objects it can store at compile-time. On the other hand, ArrayList is a non-generic class which can store objects of any type, making it less type-safe than List.

Performance: List is generally faster than ArrayList because it is a generic class and does not require boxing/unboxing of values. Boxing is the process of converting a value type to an object type, while unboxing is the process of converting an object type to a value type.

Size and Capacity: List automatically increases its capacity as items are added, whereas ArrayList has a default capacity of 16 and will double in size whenever the capacity is reached.

Syntax: The syntax for adding and accessing elements in List is the same as for an array, using square brackets ([]). In contrast, ArrayList uses the Add() method to add elements and the [] syntax to access elements.

Compatibility: List is available in .NET 2.0 and later versions, while ArrayList is available in all versions of .NET.

Overall, List is considered to be a better choice for most scenarios due to its type safety, better performance, and more intuitive syntax. However, ArrayList can still be useful in scenarios where you need to store objects of different types or when working with legacy code.

 

20. What is a lambda expression in C#?

Ans: A lambda expression is a concise way of writing an anonymous function in C#. It is often used when you need to pass a small block of code as an argument to another method. A lambda expression has the following syntax:

(parameter list) => expression

Here, parameter list is a comma-separated list of parameters, and expression is the code that gets executed when the lambda is invoked.

For example, the following lambda expression takes two integers as input and returns their sum:

(int x, int y) => x + y

Lambda expressions are often used in LINQ queries, where they can be used to specify filtering, sorting, and grouping operations on collections of data.