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17.12 Destructors

王朝other·作者佚名  2006-05-22
窄屏简体版  字體: |||超大  

A destructor is a member that implements the actions required to destruct an

instance of a class. A destructor is

declared using a destructor-declaration:

destructor-declaration:

attributesopt externopt ~ identifier ( ) destructor-body

destructor-body:

block

;

A destructor-declaration may include a set of attributes (§24).

The identifier of a destructor-declarator must name the class in which the

destructor is declared. If any other

name is specified, a compile-time error occurs.

When a destructor declaration includes an extern modifier, the destructor

is said to be an external destructor.

Because an external destructor declaration provides no actual

implementation, its destructor-body consists of a

semicolon. For all other destructors, the destructor-body consists of a

block, which specifies the statements to

execute in order to destruct an instance of the class. A destructor-body

corresponds exactly to the method-body of

an instance method with a void return type (§17.5.8).

Destructors are not inherited. Thus, a class has no destructors other than

the one which may be declared in that

class.

[Note: Since a destructor is required to have no parameters, it cannot be

overloaded, so a class can have, at most,

one destructor. end note]

Destructors are invoked automatically, and cannot be invoked explicitly. An

instance becomes eligible for

destruction when it is no longer possible for any code to use that

instance. Execution of the destructor for the

instance may occur at any time after the instance becomes eligible for

destruction. When an instance is

destructed, the destructors in that instance?s inheritance chain are

called, in order, from most derived to least

derived [Example: The output of the example

Chapter 17 Classes

265

using System;

class A

{

~A() {

Console.WriteLine("A’s destructor");

}

}

class B: A

{

~B() {

Console.WriteLine("B’s destructor");

}

}

class Test

{

static void Main() {

B b = new B();

b = null;

GC.Collect();

GC.WaitForPendingFinalizers();

}

}

is

B?s destructor

A?s destructor

since destructors in an inheritance chain are called in order, from most

derived to least derived. The GC.Collect

method is not required, but may be provided by an implementation. end

example]

Destructors may be implemented by overriding the virtual method Finalize on

System.Object. In any event,

C# programs are not permitted to override this method or call it (or

overrides of it) directly. [Example: For

instance, the program

class A

{

override protected void Finalize() {} // error

public void F() {

this.Finalize(); // error

}

}

contains two errors. end example]

The compiler behaves as if this method, and overrides of it, does not exist

at all. [Example: Thus, this program:

class A

{

void Finalize() {} // permitted

}

is valid and the method shown hides System.Object?s Finalize method. end

example]

For a discussion of the behavior when an exception is thrown from a

destructor, see §23.3.

 
 
 
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