Constructor is a special
method of a class which will invoke automatically whenever instance or object
of class is created. Constructors are responsible for object initialization and
memory allocation of its class. If we create any class without constructor, the
compiler will automatically create one default constructor for that class.
There is always at least one constructor in every class.
Here you need to remember
that a class can have any number of constructors and constructors don’t have
any return type, not even void and within a class we can create only one static
constructor.
Generally constructor name
should be same as class name. If we want to create constructor in a class we
need to create a constructor method name same as class name check below sample
method for constructor
Types of Constructors
Basically constructors are 5
types those are
1.
Default Constructor
2.
Parameterized Constructor
3.
Copy Constructor
4.
Static Constructor
5.
Private Constructor
class SampleA
{
public SampleA()
{
Console.WriteLine("Sample
A Test Method");
}
}
using System;
namespace
ConsoleApplication3
{
class Sample
{
public string param1,
param2;
public
Sample() // Default Constructor
{
param1 = "Welcome";
param2 = "Welcome to
Technology Sharing Blog";
}
}
class Program
{
static void Main(string[]
args)
{
Sample obj=new Sample();
// Once object of class created automatically constructor will be called
Console.WriteLine(obj.param1);
Console.WriteLine(obj.param2);
Console.ReadLine();
}
}
}
Parameterized Constructors
A
constructor with at least one parameter is called as parameterized constructor.
In parameterized constructor we can initialize each instance of the class to
different values like as shown below
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample(string x, string y) // Declaring Parameterized constructor with Parameters
{
param1
= x;
param2
= y;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj=new Sample("Welcome","Parameterized constructor"); //
Parameterized Constructor Called
Console.WriteLine(obj.param1
+" to "+ obj.param2);
Console.ReadLine();
}
}
}
Constructor Overloading
In
c# we can overload constructor by creating another constructor with same method
name and different parameters like as shown below
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample() // Default Constructor
{
param1
= "Hi";
param2
= "I am Default Constructor";
}
public Sample(string x, string y) // Declaring Parameterized constructor with Parameters
{
param1
= x;
param2
= y;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj
= new Sample(); // Default Constructor will Called
Sample obj1=new Sample("Welcome","Constructor Over Loading"); //
Parameterized Constructor will Called
Console.WriteLine(obj.param1
+ ", "+obj.param2);
Console.WriteLine(obj1.param1
+" to " + obj1.param2);
Console.ReadLine();
}
}
Copy Constructor
A
parameterized constructor that contains a parameter of same class type is
called as copy constructor. Main purpose of copy constructor is to initialize
new instance to the values of an existing instance. Check below example for
this
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
public Sample(string x, string y)
{
param1
= x;
param2
= y;
}
public Sample(Sample obj) // Copy Constructor
{
param1
= obj.param1;
param2
= obj.param2;
}
}
class Program
{
static void Main(string[] args)
{
Sample obj
= new Sample("Welcome", "Constructor"); // Create instance to class
Sample
Sample obj1=new Sample(obj); // Here obj details will copied to obj1
Console.WriteLine(obj1.param1
+" to " + obj1.param2);
Console.ReadLine();
}
}
}
Static Constructor
When
we declared constructor as static it will be invoked only once for any number
of instances of the class and it’s during the creation of first instance of the
class or the first reference to a static member in the class. Static
constructor is used to initialize static fields of the class and to write the
code that needs to be executed only once.
using System;
namespace ConsoleApplication3
{
class Sample
{
public string param1, param2;
static Sample()
{
Console.WriteLine("Static Constructor");
}
public Sample()
{
param1
= "Sample";
param2
= "Instance Constructor";
}
}
class Program
{
static void Main(string[] args)
{
//
Here Both Static and instance constructors are invoked for first instance
Sample obj=new Sample();
Console.WriteLine(obj.param1
+ " " +
obj.param2);
//
Here only instance constructor will be invoked
Sample obj1
= new Sample();
Console.WriteLine(obj1.param1
+" " + obj1.param2);
Console.ReadLine();
}
}
}
Importance points of static constructor
- Static constructor will not accept any parameters because it is
automatically called by CLR.
- Static constructor will not have any access modifiers.
- Static constructor will execute automatically whenever we create
first instance of class
- Only one static constructor will allowed.
Private Constructor
Private
constructor is a special instance constructor used in a class that contains
static member only. If a class has one or more private constructor and no
public constructor then other classes is not allowed to create instance of this
class this mean we can neither create the object of the class nor it can be
inherit by other class. The main purpose of creating private constructor is
used to restrict the class from being instantiated when it contains every
member as static.
using System;
namespace ConsoleApplication3
{
public class Sample
{
public string param1, param2;
public Sample(string a,string b)
{
param1
= a;
param2
= b;
}
private Sample() // Private Constructor Declaration
{
Console.WriteLine("Private Constructor with no prameters");
}
}
class Program
{
static void Main(string[] args)
{
//
Here we don't have chance to create instace for private constructor
Sample obj
= new Sample("Welcome","to Constructor");
Console.WriteLine(obj.param1
+" " + obj.param2);
Console.ReadLine();
}
}
}
In
above method we can create object of class with parameters will work fine. If
create object of class without parameters it will not allow us create.
// it will works fine
Sample obj = new Sample("Welcome","to Constructor");
// it will not work because
of inaccessability
Sample obj=new Sample();
|
Important points of private constructor
- One use of private construct is when we have only static member.
- Once we provide a constructor that is either private or public or
any, the compiler will not allow us to add public constructor without
parameters to the class.
- If we want to create object of class even if we have private
constructors then we need to have public constructor along with private
constructor
