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Answer Posted / yosh
C# supports two types of constructor, a class constructor
(static constructor) and 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 :
private static int id = 10;
private static string name = "jack";
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 :
//File Name : Test.cs
using System;
namespace Constructor
{
class Test
{
//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("Test.id = " + id);
}
static void Main(string[] args)
{
//Print the value of id
Test.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 :
Test.id = 5
Lets create one more class similar to class Test but this
time the value of its static data member would depend on the
value of static data member <id> of class Test.id.
//File Name : Test1.cs
using System;
namespace Constructor
{
class Test1
{
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 Test1()
{
if( Test.Id < 10 )
{
id = 20;
}
else
{
id = 100;
}
Console.WriteLine("Static<Class> Constructor for Class Test1
Called..");
}
public static void print()
{
Console.WriteLine("Test1.id = " + id);
}
static void Main(string[] args)
{
//Print the value of id
Test1.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 :
Static<Class> Constructor for Class Test1 Called..
id = 20
Since <id> in class Test was initialized with a value of 5,
therefore <id> in class Test1 got initialized to a value of 20.
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
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