What is a constructor?

A special method of the class that will be automatically invoked when an instance of the class is created is called a constructor. The main use of constructors is to initialize private fields of the class while creating an instance for the class. If you have not already coded a constructor for a class, the compiler will automatically create a default constructor. The default constructor initializes all numeric fields in the class to zero and all string and object fields to null.

Key points

Some of the key points regarding Constructors are:

  • A class can have any number of constructors.
  • A constructor doesn't have any return type, not even void.
  • A static constructor can not be a parameterized constructor.
  • Within a class you can create only one static constructor.

Constructor Types

Constructors can be divided into 5 types:

  1. Default Constructor
  2. Parameterized Constructor
  3. Copy Constructor
  4. Static Constructor
  5. Private Constructor

Now let us consider each constructor type with an example. 

Default Constructor

A constructor without any parameters is called a default constructor; in other words this type of constructor does not take parameters. The drawback of a default constructor is that every instance of the class will be initialized to the same values and it is not possible to initialize each instance of the class to different values. The default constructor initializes:

  1. All numeric fields in the class to zero.
  2. All string and object fields to null. 

Example


using System; namespace DefaultConstructor
 {
 
class addition   {     int a, b;
     public addition()  //default constructor
    {
      a = 100;
      b = 175;
    
     public static void Main()
    {
      addition obj = new addition(); //an object is created , constructor is called
      Console.WriteLine(obj.a);
      Console.WriteLine(obj.b);
      Console.Read();
    }
   }
  }



Now run the application, the output will be as in the following:

default.png

Parameterized Constructor

A constructor with at least one parameter is called a parameterized constructor. The advantage of a parameterized constructor is that you can initialize each instance of the class to different values. 

using System;
namespace Constructor
{
   class paraconstructor
{
    public int a, b;
    public paraconstructor( int x, int y)  // decalaring Parametrized Constructor with passing x,y parameter
    {
      a = x;
      b = y;
    }
  }
   class MainClass
  {
     static void Main()
    {
      paraconstructor v = new paraconstructor(100, 175);  // Creating object of Parameterized Constructor and passing values
      Console.WriteLine( "-----------parameterized constructor example by Vithal Wadje---------------" );
      Console.WriteLine( "\t" );
      Console.WriteLine( "value of a=" + v.a );
      Console.WriteLine( "value of b=" + v.b);
      Console.Read();
    }
  }
}

Now run the application, the output will be as in the following:

paraconstrctor.png

Copy Constructor

The constructor which creates an object by copying variables from another object is called a copy constructor. The purpose of a copy constructor is to initialize a new instance to the values of an existing instance.

Syntax


public employee(employee emp)
{
name=emp.name; age=emp.age;
}


The copy constructor is invoked by instantiating an object of type employee and passing it the object to be copied.

Example

employee emp1=new employee (emp2);

Now, emp1 is a copy of emp2.  

So let us see its practical implementation. 



using System;
namespace copyConstructor
{
   class employee
  {
     private string name;
     private int age;
     public employee(employee emp)  // declaring Copy constructor.
    {
      name = emp.name;
      age = emp.age;
    }
     public employee( string name, int age)  // Instance constructor.
    {
       this .name = name;
       this .age = age;
    }
     public string Details   // Get details of employee
    {
       get
      {
         return  " The age of " + name + " is " + age.ToString();
      }
    }
  }
   class empdetail
  {
     static void Main()
    {
      employee emp1 = new employee( "Vithal" , 23);  // Create a new employee object.
      employee emp2 = new employee(emp1);      // here is emp1 details is copied to emp2.
      Console.WriteLine(emp2.Details);
      Console.ReadLine();
    }
  }
}


Now run the program, the output will be as follows:

Copyconstrctor.png

Static Constructor

When a constructor is created as static, it will be invoked only once for all of instances of the class and it is invoked during the creation of the first instance of the class or the first reference to a static member in the class. A static constructor is used to initialize static fields of the class and to write the code that needs to be executed only once.

Key points

Some key points of a static constructor are:

  1. A static constructor does not take access modifiers or have parameters.
  2. A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
  3. A static constructor cannot be called directly.
  4. The user has no control on when the static constructor is executed in the program.
  5. A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file. 

Syntax



class employee { // Static constructor static employee(){} }
  
Now let us see it with practically
  
using System;
namespace staticConstructor
  
{
public class employee
{
   static employee() // Static constructor declaration{Console.WriteLine("The static constructor ");
}
public static void Salary()
{
  Console.WriteLine();
  Console.WriteLine( "The Salary method" );
}
}
class details
{   
   static void Main()
    {
    Console.WriteLine( "----------Static constructor example by vithal wadje------------------" );
    Console.WriteLine();
    employee.Salary();
    Console.ReadLine();
  }
}
}



Now run the program the output will be look as in the following:

Static.png

Private Constructor 

When a constructor is created with a private specifier, it is not possible for other classes to derive from this class,  neither is it possible to create an instance of this class. They are usually used in classes that contain static members only. Some key points of a private constructor are:
  1. One use of a private constructor is when we have only static members
  2. It provides an implementation of a singleton class pattern
  3. Once we provide a constructor that is either private or public or any, the compiler will not add the parameter-less public constructor to the class.
Let us see it in practical use:.


using System;
namespace defaultConstructor
{
   public class Counter
  {
     private Counter()  //private constructor declaration
    {
    }
     public static int currentview;
     public static int visitedCount()
    {
       return ++ currentview;
    }
  }
   class viewCounterdetails
  {
     static void Main()
    {
       // Counter aCounter = new Counter(); // Error
      Console.WriteLine( "-------Private constructor example by vithal wadje----------" );
      Console.WriteLine();
      Counter.currentview = 500;
      Counter.visitedCount();
      Console.WriteLine( "Now the view count is: {0}" , Counter.currentview);
      Console.ReadLine();
    }
  }
}


Now run the application; the output is:

Private.png

If you uncomment the preceding statement that is commented in the above program then it will generate an error because the constructor is inaccessible (private).