17.7 Events

王朝other·作者佚名  2006-05-15
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An event is a member that enables an object or class to provide

notifications. Clients can attach executable code

for events by supplying event handlers.

Events are declared using event-declarations:

event-declaration:

attributesopt event-modifiersopt event type variable-declarators ;

attributesopt event-modifiersopt event type member-name {

event-accessor-declarations }

event-modifiers:

event-modifier

event-modifiers event-modifier

event-modifier:

new

public

protected

internal

private

static

virtual

sealed

override

abstract

extern

event-accessor-declarations:

add-accessor-declaration remove-accessor-declaration

remove-accessor-declaration add-accessor-declaration

add-accessor-declaration:

attributesopt add block

Chapter 17 Classes

247

remove-accessor-declaration:

attributesopt remove block

An event-declaration may include a set of attributes (§24) and a valid

combination of the four access modifiers

(§17.2.3), the new (§17.2.2), static (§17.5.2, §17.7.3), virtual (§17.5.

3, §17.7.4), override (§17.5.4,

§17.7.4), sealed (§17.5.5), abstract (§17.5.6, §17.7.4), and extern

modifiers.

Event declarations are subject to the same rules as method declarations (§17

.5) with regard to valid combinations

of modifiers.

The type of an event declaration must be a delegate-type (§11.2), and that

delegate-type must be at least as

accessible as the event itself (§10.5.4).

An event declaration may include event-accessor-declarations. However, if

it does not, for non-extern, nonabstract

events, the compiler shall supply them automatically (§17.7.1); for extern

events, the accessors are

provided externally.

An event declaration that omits event-accessor-declarations defines one or

more events?one for each of the

variable-declarators. The attributes and modifiers apply to all of the

members declared by such an eventdeclaration.

It is a compile-time error for an event-declaration to include both the

abstract modifier and brace-delimited

event-accessor-declarations.

When an event declaration includes an extern modifier, the event is said to

be an external event. Because an

external event declaration provides no actual implementation, it is an

error for it to include both the extern

modifier and event-accessor-declarations.

An event can be used as the left-hand operand of the += and -= operators (§1

4.13.3). These operators are used,

respectively, to attach event handlers to, or to remove event handlers from

an event, and the access modifiers of

the event control the contexts in which such operations are permitted.

Since += and ?= are the only operations that are permitted on an event

outside the type that declares the event,

external code can add and remove handlers for an event, but cannot in any

other way obtain or modify the

underlying list of event handlers.

In an operation of the form x += y or x ?= y, when x is an event and the

reference takes place outside the type

that contains the declaration of x, the result of the operation has type

void (as opposed to having the type of x,

with the value of x after the assignment). This rule prohibits external

code from indirectly examining the

underlying delegate of an event.

[Example: The following example shows how event handlers are attached to

instances of the Button class:

public delegate void EventHandler(object sender, EventArgs e);

public class Button: Control

{

public event EventHandler Click;

}

public class LoginDialog: Form

{

Button OkButton;

Button CancelButton;

public LoginDialog() {

OkButton = new Button(?);

OkButton.Click += new EventHandler(OkButtonClick);

CancelButton = new Button(?);

CancelButton.Click += new EventHandler(CancelButtonClick);

}

void OkButtonClick(object sender, EventArgs e) {

// Handle OkButton.Click event

}

C# LANGUAGE SPECIFICATION

248

void CancelButtonClick(object sender, EventArgs e) {

// Handle CancelButton.Click event

}

}

Here, the LoginDialog instance constructor creates two Button instances and

attaches event handlers to the

Click events. end example]

 
 
 
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