Objects: state plus behavior

State and behavior together

A class is a blueprint for objects: it declares the fields (state) every object carries and the methods (behavior) that operate on that state.

class Stopwatch
{
    public long elapsedMs;      // field: per-object state
    public bool running;

    public void Start()         // method: behavior over state
    {
        running = true;
    }

    public void Record(long nowMs)
    {
        if (running) elapsedMs += nowMs;   // reads/writes THIS object's fields
        running = false;
    }
}

new builds one object from the blueprint:

var a = new Stopwatch();
var b = new Stopwatch();
a.Start();
// b.elapsedMs is still 0 — each object owns its own fields

a and b are independent instances. Writing to a field on one never affects the other: state belongs to the object, not to the class.

Members and this

Inside a method, an unqualified name like elapsedMs means this object's field. Writing this.elapsedMs is explicit — you need it only when a parameter shadows a field:

public void Rename(string name)
{
    this.name = name;   // left: the field, right: the parameter
}

Everything here is per-object. Members that belong to the class itself are declared static — a later topic.