Classes and Objects - Building Blocks of C# Programs

Vaibhav • September 10, 2025

In the previous article, we introduced the concept of object-oriented programming (OOP) and explored how it helps organize code around objects that combine data and behavior. Now it’s time to dive deeper into the two most fundamental elements of OOP in C#: classes and objects. These are the building blocks of every C# application that follows the object-oriented paradigm.

This article will walk you through how to define classes, create objects, understand their structure, and use them effectively in your programs. We’ll keep examples simple and focused so that the core ideas are clear and easy to apply.

What is a Class?

A class is a blueprint for creating objects. It defines the structure and behavior that all objects of that type will share. In C#, a class can contain fields (data), methods (behavior), properties, constructors, and more. Think of a class as a recipe - it tells you what ingredients and steps are needed to make a dish, but it’s not the dish itself.

class Car
{
    public string Make;
    public string Model;
    public int Year;

    public void Honk()
    {
        Console.WriteLine("Beep beep!");
    }
}

This class defines a Car with three fields and one method. It doesn’t do anything by itself - it’s just a definition. To use it, you need to create an object.

What is an Object?

An object is an instance of a class. When you create an object, you allocate memory and give it actual values for its fields. You can then call its methods to perform actions. In C#, you create objects using the new keyword.

Car myCar = new Car();
myCar.Make = "Toyota";
myCar.Model = "Corolla";
myCar.Year = 2020;

myCar.Honk(); // Output: Beep beep!

Here, myCar is an object of type Car. It has its own values for Make, Model, and Year, and it can perform actions like honking.

Fields and Methods Revisited

Fields are variables that belong to an object. They store the object’s state. Methods are functions that belong to an object. They define what the object can do. You access fields and call methods using dot notation.

Console.WriteLine(myCar.Make); // "Toyota"
myCar.Honk(); // "Beep beep!"

This syntax is consistent across all objects. Once you define a class, you can use its members in a predictable and readable way.

Creating Multiple Objects

You’re not limited to one object. You can create as many as you need, each with its own data.

Car car1 = new Car();
car1.Make = "Honda";
car1.Model = "Civic";
car1.Year = 2018;

Car car2 = new Car();
car2.Make = "Ford";
car2.Model = "Focus";
car2.Year = 2022;

car1.Honk(); // Beep beep!
car2.Honk(); // Beep beep!

Each object is independent. You can modify one without affecting the other. This is a key benefit of object-oriented design.

Object Initialization

C# provides a convenient syntax for initializing objects using curly braces. This is called object initializer syntax.

Car car3 = new Car
{
    Make = "Tesla",
    Model = "Model 3",
    Year = 2023
};

This syntax is cleaner and more readable, especially when setting multiple fields. It’s commonly used in real-world code.

Using Objects in Collections

You can store objects in collections like List<T>. This allows you to manage groups of objects efficiently.

List<Car> garage = new List<Car>();

garage.Add(car1);
garage.Add(car2);
garage.Add(car3);

foreach (Car c in garage)
{
    Console.WriteLine($"{c.Make} {c.Model} ({c.Year})");
}

This example shows how to store and iterate over a list of objects. You can perform operations on each object using its methods and fields.

Encapsulation with Private Fields

So far, we’ve used public fields. But in real applications, you often want to hide internal details and expose only what’s necessary. This is called encapsulation. You can make fields private and use methods to access them.

class Car
{
    private string make;
    private string model;
    private int year;

    public void SetDetails(string m, string mo, int y)
    {
        make = m;
        model = mo;
        year = y;
    }

    public void PrintInfo()
    {
        Console.WriteLine($"{make} {model} ({year})");
    }
}

This design protects the internal state of the object. You control how data is set and retrieved, which helps prevent bugs and misuse.

Null Objects and Safety

When working with objects, it’s important to check for null before accessing members. A null object means no instance has been created, and accessing its fields or methods will cause a runtime error.

Car car4 = null;

if (car4 != null)
{
    car4.Honk();
}
else
{
    Console.WriteLine("No car to honk.");
}

Always check for null when working with objects that might not be initialized. This prevents crashes and improves reliability.

Objects as Parameters

You can pass objects to methods as parameters. This allows you to write reusable functions that operate on different instances.

void DisplayCar(Car c)
{
    Console.WriteLine($"{c.Make} {c.Model} ({c.Year})");
}

DisplayCar(car1);
DisplayCar(car2);

This pattern is common in real-world code. You define methods that accept objects and perform actions based on their data.

Returning Objects from Methods

Methods can also return objects. This is useful when creating or modifying data and passing it back to the caller.

Car CreateCar(string make, string model, int year)
{
    Car c = new Car();
    c.Make = make;
    c.Model = model;
    c.Year = year;
    return c;
}

Car newCar = CreateCar("Hyundai", "Elantra", 2021);
newCar.Honk();

This approach keeps your code modular and testable. You can isolate logic into methods and reuse them across your application.

Summary

In this article, we explored the core concepts of classes and objects in C#. A class is a blueprint that defines the structure and behavior of objects. An object is an instance of a class that holds actual data and can perform actions. We learned how to define classes, create objects, use fields and methods, initialize objects, store them in collections, and apply encapsulation for safety and clarity.

These concepts form the foundation of object-oriented programming in C#. As you build larger applications, you’ll rely on classes and objects to organize your code, model real-world entities, and create reusable components.

In the next article, we’ll explore Fields and Properties - how to manage object state more effectively using C#’s property syntax, and how to enforce rules around data access and mutation.