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17.11 Static constructors

王朝other·作者佚名  2006-05-21
窄屏简体版  字體: |||超大  

A static constructor is a member that implements the actions required to

initialize a class. Static constructors are

declared using static-constructor-declarations:

static-constructor-declaration:

attributesopt static-constructor-modifiers identifier ( )

static-constructor-body

static-constructor-modifiers:

externopt static

static externopt

static-constructor-body:

block

;

A static-constructor-declaration may include a set of attributes (§24) and

an extern modifier (§17.5.7).

Chapter 17 Classes

263

The identifier of a static-constructor-declaration must name the class in

which the static constructor is declared.

If any other name is specified, a compile-time error occurs.

When a static constructor declaration includes an extern modifier, the

static constructor is said to be an external

static constructor. Because an external static constructor declaration

provides no actual implementation, its

static-constructor-body consists of a semicolon. For all other static

constructor declarations, the staticconstructor-

body consists of a block, which specifies the statements to execute in

order to initialize the class. This

corresponds exactly to the method-body of a static method with a void

return type (§17.5.8).

Static constructors are not inherited, and cannot be called directly.

The static constructor for a class executes at most once in a given

application domain. The execution of a static

constructor is triggered by the first of the following events to occur

within an application domain:

? An instance of the class is created.

? Any of the static members of the class are referenced.

If a class contains the Main method (§10.1) in which execution begins, the

static constructor for that class

executes before the Main method is called. If a class contains any static

fields with initializers, those initializers

are executed in textual order immediately prior to executing the static

constructor.

[Example: The example

using System;

class Test

{

static void Main() {

A.F();

B.F();

}

}

class A

{

static A() {

Console.WriteLine("Init A");

}

public static void F() {

Console.WriteLine("A.F");

}

}

class B

{

static B() {

Console.WriteLine("Init B");

}

public static void F() {

Console.WriteLine("B.F");

}

}

must produce the output:

Init A

A.F

Init B

B.F

because the execution of A’s static constructor is triggered by the call

to A.F, and the execution of B’s static

constructor is triggered by the call to B.F. end example]

It is possible to construct circular dependencies that allow static fields

with variable initializers to be observed in

their default value state.

[Example: The example

C# LANGUAGE SPECIFICATION

264

using System;

class A

{

public static int X;

static A() { X = B.Y + 1;}

}

class B

{

public static int Y = A.X + 1;

static B() {}

static void Main() {

Console.WriteLine("X = {0}, Y = {1}", A.X, B.Y);

}

}

produces the output

X = 1, Y = 2

To execute the Main method, the system first runs the initializer for B.Y,

prior to class B’s static constructor.

Y’s initializer causes A’s static constructor to be run because the value

of A.X is referenced. The static constructor

of A in turn proceeds to compute the value of X, and in doing so fetches

the default value of Y, which is zero. A.X

is thus initialized to 1. The process of running A’s static field

initializers and static constructor then completes,

returning to the calculation of the initial value of Y, the result of which

becomes 2. end example]

 
 
 
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