C# Classes and Objects

A small program can store productName, productPrice, and productStock as separate locals. With ten products, parallel variables or lists make it easy to combine one product's name with another product's price. A type can define that these facts travel together and which operations make sense for them.

Object-oriented programming is one way to organize a system around collaborating types. It is not the only way to program C#, and a class is not automatically better than a method and a few locals. Use a class when it creates a useful boundary around state and behavior.

The Product class describes which members every Product object has. new Product() creates an object and returns a reference to it. notebook and pen can both have type Product while holding references to different objects with different values.

The dot operator accesses a member through an object reference. notebook.Price reads that object's Price field. TotalFor is an instance method, so inside it Price means the Price belonging to the current Product object.

using System;

class Counter
{
  public int Value;

  public void Increment()
  {
      Value++;
  }
}

class Program
{
  static void Main()
  {
      Counter pageViews = new Counter();
      Counter downloads = new Counter();

      pageViews.Increment();
      pageViews.Increment();
      downloads.Increment();

      Console.WriteLine(pageViews.Value);
      Console.WriteLine(downloads.Value);
  }
}

2
1

An instance member belongs to one object and is accessed through a reference. A static member belongs to the type itself. Math.Round is static because rounding does not require a particular Math object; account.Withdraw is an instance operation because it changes one account.

Do not make everything static to avoid creating objects. Static functions are good for operations with no object state. Objects are useful when state, identity, and behavior form a coherent concept.

Classes are reference types. If Product second = first;, both variables initially refer to the same Product object. Changing second.Price is visible through first.Price because only one object changed. Use another new expression or an explicit copying strategy when you need an independent object.

Identity matters for entities: two customer objects may contain the same name but represent different people. Value-like data has different equality needs, which is why C# also offers records and structs. Those choices come after you can reason about class identity.