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Class

Class

A class in C# is defined as a reference type. In order to instantiate a variable of a reference type you must specify the new keyword, otherwise the variable will have the default value of null. See below for an example.

// The value of variableOne is null at this point.
NewClass variableOne;

// Now the value of variableOne will be an instance of the class NewClass
variableOne = new NewClass();

At runtime, when the class is instantiated, enough memory is allocated onto the heap for that specific instance of the class held in the variable.

Creating Classes

To create a class in C# we need to use the class keyword followed by a unique identifier.

Like other languages, C# creates a default constructor that accepts no parameters. We can also specify our own constructor if we need to take in special parameters or have custom initialization steps in our constructor.

public class NewClass
{
    NewClass(string name)
    {
        // Initialization steps...
    }
}

A class is a prototype or blueprint from which objects are created. In C#, the class is defined by using the keyword class. A class is used to combine together some methods, properties, fields, events, and delegates into a single unit. A class may contain nested classes too.

Example: Consider the case of Employee Class below:

using System;

namespace CPrograms
{
    class Employee
    {
        private string name;
        private int employeeId;

        public Employee(string name, int employeeId)
        {
            this.name = name;
            this.employeeId = employeeId;
        }
        public void PrintEmployee()
        {
            Console.WriteLine("Employee Name: {0}, Employee ID: {1}", this.name, this.employeeId);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Employee employeeObject = new Employee("John Doe", 420156);
            employeeObject.PrintEmployee();
        }
    }
}

Output:

> Employee Name: John Doe, Employee ID: 420156

A class can inherit from one base class only. However, it can implement from more than one interface.

Example of inheriting from one class and 2 interfaces

// base class: Human
public class Human {
    public int Age;
    
    public Human (int age) {
        Age = age;
    }
}

// first interface: Student
public interface Student {
    int StudentNumber { get; set; }
}

// second interface: Employee
public interface Employee {
    int EmployeeNumber { get; set; }
    string JobTitle { get; set; } 
}

// Example of class extending a class and using 2 interfaces
public class Person : Human, Student, Employee {
   // new field for Person
   public string Name;
   
   // needed to satisfy Student interface
   public int StudentNumber { get; set; }
   
   // needed to satisfy Employee interface
   public int EmployeeNumber { get; set; }
   public string JobTitle { get; set; } 

   // set the instance variables and pass the age to the base class
   public Person(string name, int age, int studentNum, int employeeNum, string jobTitle) : base(age)
   {
       Name = name;
       StudentNumber = studentNum;
       EmployeeNumber = employeeNum;
       JobTitle = jobTitle;
   }
}

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