Object-oriented programming

In this part of the C# tutorial, we will talk about object oriented programming in C#.

There are three widely used programming paradigms: procedural programming, functional programming and object-oriented programming. C# supports both procedural and object-oriented programming.

Object-oriented programming (OOP) is a programming paradigm that uses objects and their interactions to design applications and computer programs.

There are some basic programming concepts in OOP:

The abstraction is simplifying complex reality by modeling classes appropriate to the problem. The polymorphism is the process of using an operator or function in different ways for different data input. The encapsulation hides the implementation details of a class from other objects. The inheritance is a way to form new classes using classes that have already been defined.

Objects

Objects are basic building blocks of a C# OOP program. An object is a combination of data and methods. The data and the methods are called members of an object. In an OOP program, we create objects. These objects communicate together through methods. Each object can receive messages, send messages and process data.

There are two steps in creating an object. First, we define a class. A class is a template for an object. It is a blueprint which describes the state and behavior that the objects of the class all share. A class can be used to create many objects. Objects created at runtime from a class are called instances of that particular class.

using System;

public class Being {}

public class Objects
{
    static void Main()
    {
        Being b = new Being();
        Console.WriteLine(b);
    }
}

In our first example, we create a simple object.

public class Being {}

This is a simple class definition. The body of the template is empty. It does not have any data or methods.

Being b = new Being();

We create a new instance of the Being class. For this we have the new keyword. The b variable is the handle to the created object.

Console.WriteLine(b);

We print the object to the console to get some basic description of the object. What does it mean, to print an object? When we print an object, we in fact call its ToString() method. But we have not defined any method yet. It is because every object created inherits from the base object. It has some elementary functionality which is shared among all objects created. One of this is the ToString() method.

$ ./simpleobject.exe 
Being

We get the object class name.

Object attributes

Object attributes is the data bundled in an instance of a class. The object attributes are called instance variables or member fields. An instance variable is a variable defined in a class, for which each object in the class has a separate copy.

using System;

public class Person 
{
    public string name;
}

public class ObjectAttributes
{
    static void Main()
    {
        Person p1 = new Person();
        p1.name = "Jane";

        Person p2 = new Person();
        p2.name = "Beky";

        Console.WriteLine(p1.name);
        Console.WriteLine(p2.name);
    }
}

In the above C# code, we have a Person class with one member field.

public class Person 
{
    public string name;
}

We declare a name member field. The public keyword specifies that the member field will be accessible outside the class block.

Person p1 = new Person();
p1.name = "Jane";

We create an instance of the Person class and set the name variable to "Jane". We use the dot operator to access the attributes of objects.

Person p2 = new Person();
p2.name = "Beky";

We create another instance of the Person class. Here we set the variable to "Beky".

Console.WriteLine(p1.name);
Console.WriteLine(p2.name);

We print the contents of the variables to the console.

$ ./person.exe 
Jane
Beky

We see the output of the program. Each instance of the Person class has a separate copy of the name member field.

Methods

Methods are functions defined inside the body of a class. They are used to perform operations with the attributes of our objects. Methods bring modularity to our programs.

Methods are essential in the encapsulation concept of the OOP paradigm. For example, we might have a Connect() method in our AccessDatabase class. We need not to be informed how exactly the method Connect() connects to the database. We only have to know that it is used to connect to a database. This is essential in dividing responsibilities in programming, especially in large applications.

Objects group state and behavior, methods represent the behavioral part of the objects.

using System;

public class Circle 
{
    private int radius; 

    public void SetRadius(int radius)
    {
        this.radius = radius;
    }

    public double Area()
    {
        return this.radius * this.radius * Math.PI;
    }
}

public class Methods
{
    static void Main()
    {
        Circle c = new Circle();
        c.SetRadius(5);

        Console.WriteLine(c.Area());
    }
}

In the code example, we have a Circle class. We define two methods.

private int radius;

We have one member field. It is the radius of the circle. The private keyword is an access specifier. It tells that the variable is restricted to the outside world. If we want to modify this variable from the outside, we must use the publicly available SetRadius() method. This way we protect our data.

public void SetRadius(int radius)
{
    this.radius = radius;
}

This is the SetRadius() method. The this variable is a special variable which we use to access the member fields from methods. The this.radius is an instance variable, while the radius is a local variable, valid only inside the SetRadius() method.

Circle c = new Circle();
c.SetRadius(5);

We create an instance of the Circle class and set its radius by calling the SetRadius() method on the object of the circle. We use the dot operator to call the method.

public double Area()
{
    return this.radius * this.radius * Math.PI;
}

The Area() method returns the area of a circle. The Math.PI is a built-in constant.

$ ./circle.exe 
78.5398163397448

Running the example gives this outcome.

Access modifiers

Access modifiers set the visibility of methods and member fields. C# has four access modifiers: public, protected, private and internal. The public members can be accessed from anywhere. The protected members can be accessed only within the class itself and by inherited and parent classes. The private members are limited to the containing type, e.g. only within its class or interface. The internal members may be accessed from within the same assembly (exe or DLL).

Access modifiers protect data against accidental modifications. They make the programs more robust.

using System;

public class Person 
{
    public string name;
    private int age;

    public int GetAge()
    {
        return this.age;
    }

    public void SetAge(int age)
    {
        this.age = age;
    }
}

public class AccessModifiers
{
    static void Main()
    {
        Person p = new Person();
        p.name = "Jane";
 
        p.SetAge(17);

        Console.WriteLine("{0} is {1} years old", 
            p.name, p.GetAge());
    }
}

In the above program, we have two member fields. One is declared public, the other private.

public int GetAge()
{
    return this.age;
}

If a member field is private, the only way to access it is via methods. If we want to modify an attribute outside the class, the method must be declared public. This is an important aspect of data protection.

public void SetAge(int age)
{
    this.age = age;
}

The SetAge() method enables us to change the private age variable from outside of the class definition.

Person p = new Person();
p.name = "Jane";

We create a new instance of the Person class. Because the name attribute is public, we can access it directly. However, this is not recommended.

p.SetAge(17);

The SetAge() method modifies the age member field. It cannot be accessed or modified directly because it is declared private.

Console.WriteLine("{0} is {1} years old", 
    p.name, p.GetAge());

Finally, we access both members to build a string.

$ ./modifiers.exe 
Jane is 17 years old

Running the example gives this output.

Member fields with private access modifiers are not inherited by derived classes.

using System;

public class Base 
{
    public string name = "Base";
    protected int id = 5323;
    private bool isDefined = true;
}

public class Derived : Base 
{
    public void info()
    {
        Console.WriteLine("This is Derived class");
        Console.WriteLine("Members inherited");
        Console.WriteLine(this.name);
        Console.WriteLine(this.id);
        // Console.WriteLine(this.isDefined);
    }
}

public class CSharpApp
{
    static void Main()
    {
        Derived drv = new Derived();
        drv.info();
    }
}

In the preceding program, we have a Derived class which inherits from the Base class. The Base class has three member fields. All with different access modifiers. The isDefined member is not inherited. The private modifier prevents this.

public class Derived : Base 

The class Derived inherits from the Base class. To inherit from another class, we use the colon (:) operator.

Console.WriteLine(this.name);
Console.WriteLine(this.id);
// Console.WriteLine(this.isDefined);

The public and the protected members are inherited by the Derived class. They can be accessed. The private member is not inherited. The line accessing the member field is commented. If we uncommented the line, the code would not compile.

$ ./protected.exe 
This is Derived class
Members inherited
Base
5323

Running the program, we receive this output.

The constructor

A constructor is a special kind of a method. It is automatically called when the object is created. Constructors do not return values. The purpose of the constructor is to initiate the state of an object. Constructors have the same name as the class. The constructors are methods, so they can be overloaded too.

Constructors cannot be inherited. They are called in the order of inheritance. If we do not write any constructor for a class, C# provides an implicit default constructor. If we provide any kind of a constructor, then a default is not supplied.

using System;

public class Being 
{
    public Being() 
    {
        Console.WriteLine("Being is created");
    } 

    public Being(string being)
    {
        Console.WriteLine("Being {0} is created", being);

    }
}

public class Constructor
{
    static void Main()
    {
        new Being();
        new Being("Tom");
    }
}

We have a Being class. This class has two constructors. The first one does not take parameters; the second one takes one parameter.

public Being(string being)
{
    Console.WriteLine("Being {0} is created", being);
}

This constructor takes one string parameter.

new Being();

An instance of the Being class is created. This time the constructor without a parameter is called upon object creation.

$ ./constructor.exe 
Being is created
Being Tom is created

This is the output of the program.

In the next example, we initiate data members of the class. Initiation of variables is a typical job for constructors.

using System;

public class MyFriend 
{
    private DateTime born;
    private string name;
   
    public MyFriend(string name, DateTime born)
    {
        this.name = name;
        this.born = born;
    }

    public void Info() 
    {
        Console.WriteLine("{0} was born on {1}",
            this.name, this.born.ToShortDateString());
    } 
}

public class Constructor2
{
    static void Main()
    {
        string name = "Lenka";
        DateTime born = new DateTime(1990, 3, 5);

        MyFriend fr = new MyFriend(name, born);
        fr.Info();
    }
}

We have a MyFriend class with data members and methods.

private DateTime born;
private string name;

We have two private variables in the class definition.

public MyFriend(string name, DateTime born)
{
    this.name = name;
    this.born = born;
}

In the constructor, we initiate the two data members. The this variable is a handler used to reference the object variables.

MyFriend fr = new MyFriend(name, born);
fr.Info();

We create a MyFriend object with two arguments. Then we call the Info() method of the object.

$ ./constructor2.exe 
Lenka was born on 3/5/1990

We execute the constructor2.exe program.

Constructor chaining

Constructor chaining is the ability of a class to call another constructor from a constructor. To call another constructor from the same class, we use the this keyword.

using System;

public class Circle 
{
    public Circle(int radius)
    {
        Console.WriteLine("Circle, r={0} is created", radius);
    }

    public Circle() : this(1) 
    {

    }
}

public class ConstructorChaining
{
    static void Main()
    {
        new Circle(5);
        new Circle();
    }
}

We have a Circle class. The class has two constructors. One that takes one parameter and one that does not take any parameters.

public Circle(int radius)
{
    Console.WriteLine("Circle, r={0} is created", radius);
}

This constructor takes one parameter — the radius.

public Circle() : this(1) 
{

}

This is the constructor without a parameter. It simply calls the other constructor and gives it a default radius of 1.

$ ./constructorchaining.exe 
Circle, r=5 is created
Circle, r=1 is created

This is the output of the constructorchaining.exe program.

Class constants

C# enables to create class constants. These constants do not belong to a concrete object. They belong to the class. By convention, constants are written in uppercase letters.

using System;

public class Math 
{
    public const double PI = 3.14159265359;
}

public class ClassConstants
{
    static void Main()
    {
        Console.WriteLine(Math.PI);
    }
}

We have a Math class with a PI constant.

public const double PI = 3.14159265359;

The const keyword is used to define a constant. The public keyword makes it accessible outside the body of the class.

$ ./classconstant.exe 
3.14159265359

Running the example we see this output.

The ToString() method

Each object has a ToString() method. It returns a human-readable representation of an object. The default implementation returns the fully qualified name of the type of the Object. Note that when we call the Console.WriteLine() method with an object as a parameter, the ToString() is being called.

using System;

public class Being 
{
    public override string ToString()
    {
        return "This is Being class";
    }
}

public class ToStringMethod
{
    static void Main()
    {
        Being b = new Being();
        object o = new Object();
        
        Console.WriteLine(o.ToString());
        Console.WriteLine(b.ToString());
        Console.WriteLine(b);
    }
}

We have a Being class in which we override the default implementation of the ToString() method.

public override string ToString()
{
    return "This is Being class";
}

Each class created inherits from the base object. The ToString() method belongs to this object class. We use the override keyword to inform that we are overriding a method.

Being b = new Being();
object o = new Object();

We create one custom defined object and one built-in object.

Console.WriteLine(o.ToString());
Console.WriteLine(b.ToString()); 

We call the ToString() method on these two objects.

Console.WriteLine(b);

As we have specified earlier, placing an object as a parameter to the Console.WriteLine() will call its ToString() method. This time, we have called the method implicitly.

$ ./override.exe 
System.Object
This is Being Class
This is Being Class

This is what we get when we run the example.

Inheritance

The inheritance is a way to form new classes using classes that have already been defined. The newly formed classes are called derived classes, the classes that we derive from are called base classes. Important benefits of inheritance are code reuse and reduction of complexity of a program. The derived classes (descendants) override or extend the functionality of the base classes (ancestors).

using System;

public class Being 
{
    public Being()
    {
        Console.WriteLine("Being is created");
    }
}

public class Human : Being
{
    public Human()
    {
        Console.WriteLine("Human is created");
    }
}

public class Inheritance
{
    static void Main()
    {
        new Human();
    }
}

In this program, we have two classes. A base Being class and a derived Human class. The derived class inherits from the base class.

public class Human : Being

In C#, we use the colon (:) operator to create inheritance relations.

new Human();

We instantiate the derived Human class.

$ ./inheritance.exe 
Being is created
Human is created

We can see that both constructors were called. First, the constructor of the base class is called, then the constructor of the derived class.

A more complex example follows.

using System;

public class Being 
{
    static int count = 0;

    public Being()
    { 
        count++;
        Console.WriteLine("Being is created");
    }

    public void GetCount()
    {
        Console.WriteLine("There are {0} Beings", count);
    }
}

public class Human : Being
{
    public Human()
    {
        Console.WriteLine("Human is created");
    }
}

public class Animal : Being
{
    public Animal()
    {
        Console.WriteLine("Animal is created");
    }
}

public class Dog : Animal
{
    public Dog()
    {
        Console.WriteLine("Dog is created");
    }
}

public class Inheritance2
{
    static void Main()
    {
        new Human();
        Dog dog = new Dog();
        dog.GetCount();
    }
}

We have four classes. The inheritance hierarchy is more complicated. The Human and the Animal classes inherit from the Being class. The Dog class inherits directly from the Animal class and indirectly from the Being class. We also introduce a concept of a static variable.

static int count = 0;

We define a static variable. Static members are members that are shared by all instances of a class.

public Being()
{ 
    count++;
    Console.WriteLine("Being is created");
}

Each time the Being class is instantiated, we increase the count variable by one. This way we keep track of the number of instances created.

public class Animal : Being
...

public class Dog : Animal
...

The Animal inherits from the Being and the Dog inherits from the Animal. Indirectly, the Dog inherits from the Being as well.

new Human();
Dog dog = new Dog();
dog.GetCount();

We create instances from the Human and from the Dog classes. We call the GetCount() method of the Dog object.

$ ./inheritance2.exe 
Being is created
Human is created
Being is created
Animal is created
Dog is created
There are 2 Beings

The Human calls two constructors. The Dog calls three constructors. There are two Beings instantiated.

We use the base keyword to call the parent's constructor explicitly.

using System;

public class Shape 
{
    protected int x;
    protected int y;

    public Shape() 
    {
        Console.WriteLine("Shape is created");
    } 

    public Shape(int x, int y)
    {
        this.x = x;
        this.y = y;
    }
}

public class Circle : Shape
{
    private int r;    

    public Circle(int r, int x, int y) : base(x, y)
    {
        this.r = r;
    }

    public override string ToString()
    {
        return String.Format("Circle, r:{0}, x:{1}, y:{2}", r, x, y);
    }
}

public class Shapes
{
    static void Main()
    {
        Circle c = new Circle(2, 5, 6);
        Console.WriteLine(c);
    }
}

We have two classes: the Shape class and the Circle class. The Shape class is a base class for geometrical shapes. We can put into this class some commonalities of the common shapes, like the x, y coordinates.

public Shape() 
{
    Console.WriteLine("Shape is created");
} 
public Shape(int x, int y)
{
    this.x = x;
    this.y = y;
}

The Shape class has two constructors. The first one is the default constructor. The second one takes two parameters: the x, y coordinates.

public Circle(int r, int x, int y) : base(x, y)
{
    this.r = r;
}

This is the constructor for the Circle class. This constructor initiates the r member and calls the parent's second constructor, to which it passes the x, y coordinates. Have we not called the constructor explicitly with the base keyword, the default constructor of the Shape class would be called.

$ ./shapes.exe 
Circle, r:2, x:5, y:6

This is the output of the example.

Abstract classes and methods

Abstract classes cannot be instantiated. If a class contains at least one abstract method, it must be declared abstract too. Abstract methods cannot be implemented; they merely declare the methods' signatures. When we inherit from an abstract class, all abstract methods must be implemented by the derived class. Furthermore, these methods must be declared with the same of less restricted visibility.

Unlike Interfaces, abstract classes may have methods with full implementation and may also have defined member fields. So abstract classes may provide a partial implementation. Programmers often put some common functionality into abstract classes. And these abstract classes are later subclassed to provide more specific implementation. For example, the Qt graphics library has a QAbstractButton, which is the abstract base class of button widgets, providing functionality common to buttons. Buttons Q3Button, QCheckBox, QPushButton, QRadioButton, and QToolButton inherit from this base abstract class.

Formally put, abstract classes are used to enforce a protocol. A protocol is a set of operations which all implementing objects must support.

using System;

public abstract class Drawing
{
    protected int x = 0;
    protected int y = 0;

    public abstract double Area();

    public string GetCoordinates()
    {
        return string.Format("x: {0}, y: {1}", this.x, this.y);
    }
}

public class Circle : Drawing
{
    private int r; 

    public Circle(int x, int y, int r)
    {
        this.x = x;
        this.y = y;
        this.r = r;
    }

    public override double Area()
    {
        return this.r * this.r * Math.PI;
    }

    public override string ToString()
    {
        return string.Format("Circle at x: {0}, y: {1}, radius: {2}", 
            this.x, this.y, this.r);
    }
}

public class AbstractClass
{
    static void Main()
    {
        Circle c = new Circle(12, 45, 22);
        Console.WriteLine(c);
        Console.WriteLine("Area of circle: {0}", c.Area());
        Console.WriteLine(c.GetCoordinates());
    }
}

We have an abstract base Drawing class. The class defines two member fields, defines one method and declares one method. One of the methods is abstract, the other one is fully implemented. The Drawing class is abstract because we cannot draw it. We can draw a circle, a dot or a square. The Drawing class has some common functionality to the objects that we can draw.

public abstract class Drawing

We use the abstract keyword to define an abstract class.

public abstract double Area();

An abstract method is also preceded with the abstract keyword.

public class Circle : Drawing

A Circle is a subclass of the Drawing class. It must implement the abstract Area() method.

public override double Area()
{
    return this.r * this.r * Math.PI;
}

When we implement the Area() method, we must use the override keyword. This way we inform the compiler that we override an existing (inherited) method.

$ ./abstractclass.exe 
Circle at x: 12, y: 45, radius: 22
Area of circle: 1520.53084433746
x: 12, y: 45

We see the output of the abstractclass.exe program.

This was the first part of the description of OOP in C#.