A special method of the class that will be automatically invoked when an instance of the class is created is called as constructor.
Constructors can be classified into 5 types
- Default Constructor
- Parameterized Constructor
- Copy Constructor
- Static Constructor
- Private Constructor
Default Constructor : A constructor without any parameters is called as default constructor. Drawback of default constructor is every instance of the class will be initialized to same values and it is not possible to initialize each instance of the class to different values.
Example for Default Constructor
Parameterized Constructor : A constructor with at least one parameter is called as parameterized constructor. Advantage of parameterized constructor is you can initialize each instance of the class to different values.
Example for Parameterized Constructor
using System;
namespace ProgramCall
{
class Test1
{
//Private fields of class
int A, B;
//default Constructor
public Test1()
{
A = 10;
B = 20;
}
//Paremetrized Constructor
public Test1(int X, int Y)
{
A = X;
B = Y;
}
//Method to print
public void Print()
{
Console.WriteLine("A = {0}\tB = {1}", A, B);
}
}
class MainClass
{
static void Main()
{
Test1 T1 = new Test1(); //Default Constructor is called
Test1 T2 = new Test1(80, 40); //Parameterized Constructor is called
T1.Print();
T2.Print();
Console.Read();
}
}
}
Output
A = 10 B = 20
A = 80 B = 40
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.
Example for Copy Constructor
using System;
namespace ProgramCall
{
class Test2
{
int A, B;
public Test2(int X, int Y)
{
A = X;
B = Y;
}
//Copy Constructor
public Test2(Test2 T)
{
A = T.A;
B = T.B;
}
public void Print()
{
Console.WriteLine("A = {0}\tB = {1}", A, B);
}
}
class CopyConstructor
{
static void Main()
{
Test2 T2 = new Test2(80, 90);
//Invoking copy constructor
Test2 T3 = new Test2(T2);
T2.Print();
T3.Print();
Console.Read();
}
}
}
Output
A = 80 B = 90
A = 80 B = 90
Static Constructor : You can create a constructor as static and when a constructor is created as static, it will be invoked only once for any number of instances of the class and it is 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.
Example for Static Constructor
using System;
namespace ProgramCall
{
class Test3
{
public Test3()
{
Console.WriteLine("Instance Constructor");
}
static Test3()
{
Console.WriteLine("Static Constructor");
}
}
class StaticConstructor
{
static void Main()
{
//Static Constructor and instance constructor, both are invoked for first instance.
Test3 T1 = new Test3();
//Only instance constructor is invoked.
Test3 T2 = new Test3();
Console.Read();
}
}
}
Output
Static Constructor
Instance Constructor
Instance Constructor
Private Constructor : You can also create a constructor as private. When a class contains at least one private constructor, then it is not possible to create an instance for the class. Private constructor is used to restrict the class from being instantiated when it contains every member as static.
Some unique points related to constructors are as follows
- A class can have any number of constructors.
- A constructor doesn’t have any return type even void.
- A static constructor can not be a parameterized constructor.
- Within a class you can create only one static constructor.
