# Defining an Object

An object's responsibilities split into what it knows (its state) and what it does (its behavior), and a class is the blueprint that describes both.

Apprentice · C# · OOP · Updated November 3, 2026

## After this lesson you can

- Separate what an object knows (its state) from what it does (its behavior)
- Read a class drawn as a UML class diagram

## Key takeaways

- What an object knows is its state, stored in variables; what it does is its behavior, written as methods.
- A class is a blueprint for a kind of object. Each object made from it is an instance, with its own state.
- A UML class diagram is a box with three sections: the class's name, its state with types, then its methods.
- In UML, `-` marks a private member and `+` a public one. State is usually private, and the methods that work with it public.

Once you know an object's responsibilities, you can sort them into two groups: the things the object **knows**, and the things it can **do**.

![A refrigerator with open doors](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/09/c5902147680b4685a6fd3b38209bdb2c.png)

## Things an object knows: its state

What an object knows is its **state**: the facts it must remember. A refrigerator needs to remember:

- its current temperature
- its capacity (how much it can hold)
- how much space is used

In code, an object's state is stored in variables that belong to it.

## Things an object does: its behavior

What an object can do, or what can be done to it, is its **behavior**. A refrigerator can:

- open its door
- add an item
- remove an item

In code, an object's behavior is its methods. Notice that behavior usually uses or changes state: adding an item changes how much space is used, and the refrigerator should refuse an item that doesn't fit.

## A class is a blueprint

A **class** describes a kind of object: what every object of that kind knows and does. It's a blueprint, or a template. The class `Refrigerator` describes refrigerators in general; each actual refrigerator you make from it is an **object**, also called an **instance** of the class.

From one blueprint you can make many objects. Two refrigerators made from the same class each have their own temperature and their own contents.

## Drawing a class: UML

Developers sketch classes before they write them, using **UML** (the Unified Modeling Language). A class is drawn as a box with three sections:

![A UML class diagram of Refrigerator: its name, then its fields, then its methods](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/09/50eeba04b24a4fca9c7e98cf38fe1e0c.png)

| Section | Holds | Refrigerator |
|---|---|---|
| Top | The class's name | `Refrigerator` |
| Middle | What it knows: its state, with types | `- temperature: int`, `- capacity: int`, `- usedSpace: int` |
| Bottom | What it does: its methods | `+ AddItem(size: int): bool`, `+ RemoveItem(size: int)` |

The symbol in front of each member says who can use it: `-` means **private** (only the object itself) and `+` means **public** (anyone). State is usually private, and the methods that work with it are public. You'll see why in the next lessons.

## Check yourself

2 questions to try on the lesson page.

## So what

Every class you write answers two questions: what does this object need to know, and what does it need to do? Answer them on paper, in a UML box, before you write a line of code.

## Key terms

- [State](https://www.ask-codey.com/glossary/state)
- [Behavior](https://www.ask-codey.com/glossary/behavior)
- [Class](https://www.ask-codey.com/glossary/class-oop)
- [Instance](https://www.ask-codey.com/glossary/instance-oop)
- [UML Class Diagram](https://www.ask-codey.com/glossary/uml-class-diagram)

## Cheat sheets

- [Defining an Object](https://www.ask-codey.com/cheat-sheets/working-with-objects-in-csharp-defining-an-object-cheat-sheet)

Source: https://www.ask-codey.com/lessons/working-with-objects-in-csharp-defining-an-object