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. When you have not created a constructor in the class, the compiler will automatically create a default constructor in the class. The default constructor initializes all numeric fields in the class to zero and all string and object fields to null.

Some of the key points regarding the Constructor 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 parametrized constructor.
  • Within a class you can create only one static constructor. 
Constructors can be divided into 5 types:
  1. Default Constructor
  2. Parametrized Constructor
  3. Copy Constructor
  4. Static Constructor
  5. Private Constructor 
Now let us see  each constructor type with example as below
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 DefaultConstractor
 {
    class addition
    {
        int a, b; 
        public addition()   //default contructor
        {
            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 parametrized constructor. The advantage of a parametrized constructor is that you can initialize each instance of the class to different values.
 
using System;
namespace Constructor
{
    class paraconstrctor
    {
      public  int a, b;
      public paraconstrctor(int x, int y)  // decalaring Paremetrized Constructor with passing x,y parameter
        {
            a = x;
            b = y;
        }
   }
    class MainClass
    {
        static void Main()
        {
            paraconstrctor v = new paraconstrctor(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 copyConstractor
{
    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 deatils 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.

Some key points of a static constructor is:
  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 staticConstractor
{
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 constrctor 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. 
Now let us see it practically.
using System;
namespace defaultConstractor
{
    public class Counter
    {
        private Counter()   //private constrctor declaration
        {
        }
        public static int currentview;
        public static int visitedCount()
        {
            return ++ currentview;
        }
    }
    class viewCountedetails
    {
        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).
Summary
I hope this article is useful for all readers, if you have any suggestion then please contact me.