Classes and Objects

C# is fundamentally an object-oriented language. Classes are blueprints for creating objects, and they encapsulate data (fields/properties) and behaviour (methods).

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Defining a Class

public class Person
{
    // Fields (private by convention)
    private string _name;
    private int _age;

    // Constructor
    public Person(string name, int age)
    {
        _name = name;
        _age = age;
    }

    // Method
    public string Introduce()
    {
        return $"Hi, I'm {_name} and I'm {_age} years old.";
    }
}

// Creating an object
var person = new Person("Alice", 30);
Console.WriteLine(person.Introduce());

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Properties

Properties provide controlled access to fields:

public class Product
{
    // Auto-implemented property
    public string Name { get; set; }

    // Property with a backing field
    private decimal _price;
    public decimal Price
    {
        get => _price;
        set
        {
            if (value < 0) throw new ArgumentException("Price cannot be negative");
            _price = value;
        }
    }

    // Read-only property
    public string Display => $"{Name}: £{Price:F2}";

    // Init-only property (C# 9+)
    public string Sku { get; init; }
}

var product = new Product { Name = "Widget", Price = 9.99m, Sku = "W001" };
// product.Sku = "W002";  // Error! init-only property