C# class

Summary: in this tutorial, you’ll learn about the C# class and how to define a custom class.

Introduction to the objects and classes

Objects are one of the essential concepts in object-oriented programming. Objects have states and behaviors:

• States represent the data that the object holds at a particular point in time.
• Behaviors represent the actions that the object can perform to manipulate its states.

C# uses class-based object-oriented programming. Before creating objects, you need to define a class. A class is a blueprint for creating objects.

Define a class

To define a new class, you use the class keyword followed by the class name. By convention, a class name is in the Pascal case, such as Person, SalesPerson, etc.

To create a new class, you follow these steps:

First, create a new Person.cs file.

Second, define the Person class in the Person.cs file:

class Person
{
}Code language: C# (cs)

Create objects from the class

To create objects from the Person class, you follow these steps:

First, create the Program.cs file.

Second, create a new object from the Person class by using the new keyword:

Person p1 = new Person();Code language: C# (cs)

Or you can use the var keyword:

var p1 = new Person();Code language: C# (cs)

Or

Person p1 = new();Code language: C# (cs)

In this example, the p1 is a new object of the Person class. It is also called an instance of the Person class.

Add fields to the class

The following add three fields to the Person class: FirstName, LastName, and Age. These fields represent the states of a person object:

class Person
{
    public string FirstName;
    public string LastName;
    public byte Age;
}Code language: C# (cs)

The public keyword controls the access level to access the fields from both inside and outside the Person class. See the public & private access modifier tutorial for more information.

Since the Person class has three fields, and all of its objects can access them. For example, the following creates a new instance of the Person class and assigns the values to each field:

using HR;

Person p1 = new();

p1.FirstName = "John";
p1.LastName = "Doe";
p1.Age = 25;Code language: C# (cs)

From the Person class, you can create as many objects as you want to. For example, the following creates two Person’s objects with the names p1 and p2:

using HR;

Person p1 = new();
p1.FirstName = "John";
p1.LastName = "Doe";
p1.Age = 25;


Person p2 = new();
p2.FirstName = "Jane";
p2.LastName = "Doe";
p2.Age = 22;Code language: C# (cs)

Note that p1 and p2 have the same set of fields. However, their field values are different.

Add methods to a class

The following adds the GetFullName() method to the Person class:

// Person.cs

class Person
{
    public string FirstName;
    public string LastName;
    public byte Age;

    public string GetFullName()
    {
        return $"{FirstName} {LastName}";
    }
}Code language: C# (cs)

The GetFullname() is like a function with the public keyword. When you define a function inside a class, it is called a method.

The GetFullName() method concatenates the first name with the last name and returns the full name as a string.

The public keyword indicates that it is accessible from both inside and outside the Person class.

To call the GetFullName() method from a Person object, you use the object name, dot operator, and the method name. For example:

// Program.cs

Person p1 = new();
p1.FirstName = "John";
p1.LastName = "Doe";
p1.Age = 25;
Console.WriteLine(p1.GetFullName());


Person p2 = new();
p2.FirstName = "Jane";
p2.LastName = "Doe";
p2.Age = 22;
Console.WriteLine(p2.GetFullName());Code language: C# (cs)

Output:

John Doe
Jane DoeCode language: C# (cs)

Summary

• Objects have states and behaviors.
• C# uses a class-based object-oriented programming approach.
• A class is a blueprint for creating objects. Objects are instances of a class.
• Use the class keyword to define a class.
• A class has fields and methods.
• Use the new keyword to create a new object from a class.