Constructors In C Sharp

Constructors :-
v Constructor is  a special method, which initializes the class member with default values or the value that is passed by the constructors.

v Constructors must have the same name as class name.

v Constructors does not return any types, not even void.

v Constructor can’t be get inherited, although a derived class constructor can call the base class constructor using “base” keyword

v A class has atleast one constructor also known as default constructor.(a constructor without parameter)

v We have to explicitly define a default constructor in case of overloading constructors.

v  Constructor overloading means defining multiple constructors of a class with different sets of parameters.

v A constructor can be called another constructor using “this” keyword with in single class.

Types of Constructors

vInstance Constructors :- Instance constructors are specific to an instance of a type and are used to initialize both static and instance fields of the type. 

vPublic Constructors (parameterless or parameterized)

vPrivate Constructors (parameterless or parameterized)

vProtected Constructors(pameterless or parameterized)

vStatic Constructors :-  Static constructors are used to initialize static field of the type. We can initialize the non-static fields also but for this we require the object reference of that type.

Private Constructor (Singleton Pattern)

v The Singleton pattern defines the class in which only a single instance can exist.

v  It is useful for exposing read-only data, and static methods that do not rely on instance data.

v Rather than creating an instance of the class each time, the application obtains a reference to an existing instance using a static method of the class that implements the Singleton pattern.

v If this is the first call to the method that returns the instance, the Singleton creates an instance, populates it with any required data, and returns that instance.

v Subsequent calls to the method simply return this instance. The instance lifetime is that of the application domain.

v It is not possible to inherit  a class that has only a private constructor.

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Example of Private Constructor (Singleton Pattern).



using System;
sealed class Singleton
{
private static Singleton instance = null;
private static bool flag = false;


public static Singleton CreateInstance()
{
if(!flag)
{
flag = true;
instance = new Singleton();
}
return instance;
}
private Singleton(){}
public void Display()
{
Console.WriteLine("I am instance member");
}
}


class PrivateConstructorTestOne
{
static void Main()
{
                //Singleton s_instance = new Singleton();   this generate an error due to private constructor.
Singleton s_instance = Singleton.CreateInstance();
s_instance.Display();
}
}


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Second Example of Private Constructor.


using System;
sealed class Singleton
{
private static Singleton instance = null;
private string name="";


public static Singleton CreateInstance()
{
if(instance == null)
{
instance = new Singleton();
}
return instance;
}
private Singleton(){}
public void SetName(string name)
{
               // use this keyword when class field member name and parameter name is same.
this.name = name; 
}


public void DisplayName()
{
Console.WriteLine("Value at name is " + name);
}
}


class PrivateConstructorTestTwo
{
static void Main()
{
Singleton s1_instance = Singleton.CreateInstance();
s1_instance.SetName("Amit");
Console.WriteLine("Call DisplayName by first reference variable");
s1_instance.DisplayName();


Singleton s2_instance = Singleton.CreateInstance();
s2_instance.SetName("Gopal");
Console.WriteLine("Call DisplayName by second reference variable");
s2_instance.DisplayName();


Console.WriteLine("Call DisplayName again by first reference variable");
s1_instance.DisplayName();
}
}

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