Constructors in C#

Introduction

A constructor is one that has special method which runs automatically when we create instance of the class. Please note following points here which is applicable:

(i) Constructors has same name as the class.

(ii) Constructors take parameters.

(iii) Constructors never returns any value not even void.

(iv) Every class must have a constructor, if we don’t write constructor, compiler generates a default one which doesn't actually do anything.

(vi) Default constructor has public accessibility.

//Example of constructor

class ClassRoom

{

    private int boycount;   //field

    public ClassRoom()     //default constructor

    {

        boycount = 30;

    }

 

    public double Avg()     //method

    {

        //statements goes here

    }

}

In above example, constructor is marked as public. If we omit this keyword, the constructor will be private and it will have property like methods and fields. If we put constructor as private then obviously, cannot be used outside the class, which will prevent from being able to create ClassRoom objects from methods that are not part of the ClassRoom class. Sometimes we will get some exception here. We might therefore think that private constructors are not that valuable, never, they do have their uses. Don't get confused here too, to check this, change any constructor to private and look at the result.

Now, to use ClassRoom class and its Area method, dot notation will be used to invoke the Area method on a ClassRoom object:

ClassRoom r;    //creating a class variable, object

r = new ClassRoom();   //initializing class variable

double newAvg = r.Avg();   //invoking class method

In above example, we have created a new class variable named r and in just next line we are initializing all components of ClassRoom class to new r class variable. After this, we are invoking the class method by using dot(.) operator and assigning the value in newAvg.

Constructors Overloading

Till now we discussed on basics of contractors, let's have some discussion on its overloading techniques. In above example, ClassRoom objects will always be 30 because the default constructor sets the boycount to 30 and it stays at 30; the boycount field is private and there is no easy way of changing its value after it has been initialized once. However, we should realize that a constructor is just a special kind of method and that's it. We knew that methods can be overloaded and constructor too. Constructor overloading is a type of Static Polymorphism. Using constructor overloading, we can define any number of constructors to the same class. But ensure that each constructor has a different number and type of parameters defined.

//Example of constructor overloading

class ClassRoom

{

    private int boycount;   //field

    public ClassRoom()     //default constructor

    {

        boycount = 30;

    }

    public ClassRoom(int bcount)     //overloaded constructor

    {

        boycount = bcount;

    }

 

    public double Avg()     //method

    {

        //statements goes here

    }

}

In above example, we have created a new constructor having one parameter and we are assigning parameter value in field.

Now, to use above new constructor we need to pass the parameter value, look at example:

ClassRoom r;

r = new ClassRoom(33);

Now compiler will decide which constructor it should call based on the parameters that we have passed. In above example we have passed an integer value that is 33. Please note the order of defined constructors can be anything, like in above class example, we have two different constructors and anyone may be first.