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How many types of constructor in C# and what is the usages?
09-24-2010, 07:05 AM (This post was last modified: 09-24-2010 07:13 AM by arti.porwal20@gmail.com.)
Post: #1
How many types of constructor in C# and what is the usages?
C# supports two types of constructor.

1. A class constructor (static constructor)
2. An instance constructor (non-static constructor)

Static constructor is used to initialize static data members as soon as the class is referenced first time, whereas an instance constructor is used to create an instance of that class with <new> keyword. A static constructor does not take access modifiers or have parameters and can't access any non-static data member of a class.

Since static constructor is a class constructor, they are guaranteed to be called as soon as we refer to that class or by creating an instance of that class.

You may say, why not initialize static data members where we declare them in the code. Like this :
Code:
private static int id = 100;
private static string name = "arti";
Static data members can certainly be initialized at the time of their declaration but there are times when value of one static member may depend upon the value of another static member. In such cases we definitely need some mechanism to handle conditional initialization of static members. To handlesuch situation, C# provides static constructor.

Let me explain you with examples :

Code:
//File Name : StaticConstructor.cs
using System;
namespace Constructor
{
class StaticConstructor
{
//Declaration and initialization of static data member
private static int id = 5;
public static int Id
{
get
{
return id;
}
}
public static void print()
{
Console.WriteLine("StaticConstructor.id = " + id);
}
static void Main(string[] args)
{
//Print the value of id
StaticConstructor.print();
}
}
}

In the above example, static data member <id> is declared and initialized in same line. So if you compile and run this program your output would look similar to this :

Code:
StaticConstructor.id = 5

Lets create one more class similar to class StaticConstructor but this time the value of its static data member would depend on the value of static data member <id> of class StaticConstructor.id.

Code:
//File Name : StaticConstructor1.cs
using System;
namespace Constructor
{
class StaticConstructor1
{
private static int id ;
//Static constructor, value of data member id is set conditionally here.
//This type of initialization is not possible at the time of declaration.
static StaticConstructor1()
{
if( StaticConstructor.Id < 10 )
{
id = 20;
}
else
{
id = 100;
}
Console.WriteLine("Static<Class> Constructor for Class StaticConstructor1 Called..");
}
public static void print()
{
Console.WriteLine("StaticConstructor1.id = " + id);
}
static void Main(string[] args)
{
//Print the value of id
StaticConstructor1.print();
}
}
}

As you can see in the above static constructor, static data member <id> is initialized conditionally. This type of initialization is not possible at the time of declaration. This is where static constructor comes in picture. So if you compile and run this program your output would look similar to this :

Code:
Static<Class> Constructor for Class StaticConstructor1 Called..
id = 20

Since <id> in class StaticConstructor was initialized with a value of 5, therefore <id> in class StaticConstructor1 got initialized to a value of 20.
Quote:Some important point regarding static constructor from C# Language Specification and C# Programmer's Reference :

1) The static constructor for a class executes before any instance of the class is created.
2) The static constructor for a class executes before any of the static members for the class are referenced.
3) The static constructor for a class executes after the static field initializers (if any) for the class.
4) The static constructor for a class executes at most one time during a single program instantiation
5) A static constructor does not take access modifiers or have parameters.
6) A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
7) A static constructor cannot be called directly.
8) The user has no control on when the static constructor is executed in the program.
9) 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.
Hopefully, this would clear the confusion some of developers have about static constructor.



Please give reputation if my answer/post is useful for you
Best Regards
Arti Gupta
Quote


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