C# Classes & Objects - C# Tutorial

C# is a class-based, object-oriented language. Classes define blueprints; objects are instances of those blueprints.

Basic Class

CSHARP

class Book
{
    // Properties (data)
    public string Title { get; set; }
    public string Author { get; set; }
    public int Pages { get; set; }

    // Constructor (initialises the object)
    public Book(string title, string author, int pages)
    {
        Title = title;
        Author = author;
        Pages = pages;
    }

    // Method (behaviour)
    public string Summary()
    {
        return $"{Title} by {Author} ({Pages} pages)";
    }
}

// Creating an object
var book = new Book("Palpasa Cafe", "Narayan Wagle", 312);
Console.WriteLine(book.Summary());
// "Palpasa Cafe by Narayan Wagle (312 pages)"

Auto-Properties

Shorthand for simple properties — C# creates the backing field automatically:

CSHARP

public string Title { get; set; }          // read-write
public int Age { get; private set; }       // read-only from outside
public string Name { get; init; }          // set only during initialisation

Access Modifiers

Modifier	Class	Subclass	Assembly (project)	World
`public`	✅	✅	✅	✅
`protected`	✅	✅	❌	❌
`internal`	✅	✅	✅	❌
`private`	✅	❌	❌	❌

Default is private for members and internal for classes.

Inheritance

CSHARP

class Animal
{
    public string Name { get; set; }
    public virtual void Speak() => Console.WriteLine("...");
}

class Dog : Animal
{
    public override void Speak() => Console.WriteLine("Woof!");
}

var rex = new Dog { Name = "Rex" };
rex.Speak(); // "Woof!"

virtual = subclasses can override this method
override = this method replaces the parent's version
sealed = subclasses cannot override further

Interfaces

Interfaces define a contract — a set of methods and properties a class must implement:

CSHARP

interface IShape
{
    double Area();
    string Name { get; }
}

class Circle : IShape
{
    public double Radius { get; set; }
    public string Name => "Circle";
    public double Area() => Math.PI * Radius * Radius;
}

class Rectangle : IShape
{
    public double Width { get; set; }
    public double Height { get; set; }
    public string Name => "Rectangle";
    public double Area() => Width * Height;
}

Convention: prefix interface names with I (IShape, IRepository).

Records (C# 9+)

Immutable data carriers — ideal for DTOs and value objects:

CSHARP

record Person(string Name, int Age);
record Point(double X, double Y);

var ram = new Person("Ram", 22);
Console.WriteLine(ram); // Person { Name = Ram, Age = 22 }

// Records support equality out of the box
var ram2 = new Person("Ram", 22);
Console.WriteLine(ram == ram2); // True

Structs vs Classes vs Records

Feature	Struct	Class	Record
Type	Value	Reference	Reference
Inheritance	❌	✅	✅
Mutable	✅	✅	❌ (default)
Use for	Small data	Behaviour	Data transfer

Rule of thumb: start with class, use record for DTOs, use struct only for small, immutable value types.

Tips

Prefer record for data objects that do not need mutation.
Use init instead of set when properties should only be set during construction.
Keep classes small and focused — one responsibility per class.
Use interfaces for loose coupling — depend on abstractions, not concrete classes.

Next: collections — lists, dictionaries, and more.

Basic Class

Auto-Properties

Access Modifiers

Inheritance

Interfaces

Records (C# 9+)

Structs vs Classes vs Records

Tips

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