Classes and Objects

In simple words#

Learning objectives#

• Write a class with properties, a constructor and methods
• Create objects with new
• Understand that each object has its own data
• Use object initializers

Explanation#

A class groups data (properties and fields) with behaviour (methods). public members can be used from outside; private members only inside the class.

An auto-property such as public string Name { get; set; } stores a value with no extra code. { get; } makes it read-only after the constructor.

The constructor has the same name as the class and no return type. It runs when you write new Student(...) and sets up a valid starting state.

Each object created with new has its own copy of the data. Changing one student’s marks never affects another student.

Object initializers set properties right after creation: new Product { Name = "Pen", Price = 10m }.

In a top-level program, classes are declared after the statements at the top of the file. In real projects each class usually lives in its own file.

Syntax#

1public class Student
2{
3    public string Name { get; }        // read-only property
4    public int Year { get; set; }      // read-write property
5

6    public Student(string name, int year)   // constructor
7    {
8        Name = name;
9        Year = year;
10    }
11

12    public string Describe() => 
quot;{Name}, year {Year}";
13}
14

15var s = new Student("Asha", 2);

Code example: Students with their own marks#

1var asha = new Student("Asha", 2);
2var ravi = new Student("Ravi", 1);
3

4asha.AddMark(91);
5asha.AddMark(85);
6ravi.AddMark(70);
7

8Console.WriteLine(asha.Describe());
9Console.WriteLine(ravi.Describe());
10

11ravi.Year = 2;   // settable property
12Console.WriteLine(ravi.Describe());
13

14class Student
15{
16    private readonly List<int> _marks = new();
17

18    public string Name { get; }
19    public int Year { get; set; }
20

21    public Student(string name, int year)
22    {
23        Name = name;
24        Year = year;
25    }
26

27    public void AddMark(int mark) => _marks.Add(mark);
28

29    public double Average() => _marks.Count == 0 ? 0 : _marks.Average();
30

31    public string Describe() => 
quot;{Name} (year {Year}) average {Average():F1}";
32}

Asha (year 2) average 88.0
Ravi (year 1) average 70.0
Ravi (year 2) average 70.0

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. new Student("Asha", 2) runs the constructor, which fills Name and Year.
2. _marks is a private field: outside code cannot touch the list directly.
3. AddMark is the only way to add marks; each student has a separate list.
4. Average uses LINQ’s Average and guards against an empty list.
5. Year has a setter, so it can change later; Name has only get, so it cannot.

Real-world use case#

Common mistakes#

Best practices#

• One class per file, named after the class
• Use PascalCase for classes, properties and methods; _camelCase for private fields
• Make properties read-only unless they really need to change
• Initialise collections at declaration

Remember this#

Practice exercise#

1var pen = new Product("Pen", 20m);
2var bag = new Product("Bag", 500m);
3Console.WriteLine(
quot;{pen.Name}: {pen.PriceWithGst():F2}");   // Pen: 23.60
4Console.WriteLine(
quot;{bag.Name}: {bag.PriceWithGst():F2}");   // Bag: 590.00
5

6class Product
7{
8    public string Name { get; }
9    public decimal Price { get; }
10    public Product(string name, decimal price)
11    {
12        Name = name;
13        Price = price;
14    }
15    public decimal PriceWithGst() => Price * 1.18m;
16}

Mini quiz#

Want more? Practise all C# MCQs · C# interview questions

Related topics#

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