Defining classes

Learn to define classes with the class keyword and instantiate with new.

Defining a class

A class is a template used to create objects. In C# a class starts with the class keyword, followed by a name and a body in curly braces.

Example: class Book { }. Here Book is the class name; the body is empty for now. Class names use PascalCase: UserAccount, ShoppingCart. Defining a class does not run it; it only prepares the template. To use the template you create an instance with new.

Creating instances with new

To create an object from a class you use the new keyword. new allocates memory, calls the constructor and returns a reference to the new object.

Example: Book book = new Book();. The left side declares that the book variable is of type Book; the right side creates a new Book object. The parentheses are the constructor call. Now book holds a reference to that object.

Exercise Define the Book class and create an instance

Define a Book class with an empty body. Inside Main create an instance with new Book() and print "Book created" with Console.WriteLine.

Show example solution

Define the class with class Book { }. In Main add Book book = new Book(); and Console.WriteLine("Book created");.
Example solution:
using System;

class Book
{
}

class Program
{
    static void Main()
    {
        Book book = new Book();
        Console.WriteLine("Book created");
    }
}

Test yourself

Which is a valid class definition?
- Book class { }
- function Book() { }
- class Book { }
- class Book()
Show answer

class Book { } — Correct. The class keyword comes first, then the name and curly braces.

Write the new keyword

Book book = ____ Book();

Show answer

Book book = new Book();

Match each keyword to its meaning

class

Class template
new

Creates an instance
void

No return value
public

Access from anywhere

Next lesson: Fields and properties

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