Model objects with classes and interfaces
Encapsulate state in classes and describe capabilities with interfaces.
- Create a class with properties and explain interface contracts.
A class defines a reference type with data and behavior; an object is an instance. Properties provide controlled access through get and set accessors, while constructors establish valid initial state. An interface describes members a type promises to provide. Code that depends on an interface can work with different implementations. C# 12 also supports primary constructors, while ordinary constructors remain common.
1using System;
2
3interface INotifier
4{
5 void Send(string message);
6}
7
8class ConsoleNotifier : INotifier
9{
10 public void Send(string message) => Console.WriteLine(message);
11}The class provides the interface method.
The I prefix is a common .NET interface naming convention. A class implements an interface with a colon and must provide its required members. Expression-bodied members use => for a single expression.
Key takeaways
Classes define types; objects are instances.
Properties provide controlled access to state.
Interfaces allow interchangeable implementations.
Lesson quiz
5 questions · pass with 4 correct · up to 50 XP
Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.
Practice: write C#
Write C# 12 in the editor and run it against sample input. .NET 8 compiles and runs the code through Wandbox, a free public service; your source and test input are sent there.
Create a rectangle class
Add a constructor that stores width and height and an Area method. Read two integers, create a Rectangle, and print its area.
- Three by four
- Five by two
C# 12 code is compiled and run with .NET 8 on Wandbox, a free public service. Your source code and test input are sent to that service.
Questions about this lesson
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