17.7.4 Virtual, sealed, override, and abstrac

王朝other·作者佚名  2006-05-15
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A virtual event declaration specifies that the accessors of that event are

virtual. The virtual modifier applies

to both accessors of an event.

An abstract event declaration specifies that the accessors of the event are

virtual, but does not provide an

actual implementation of the accessors. Instead, non-abstract derived

classes are required to provide their own

implementation for the accessors by overriding the event. Because an

accessor for an abstract event declaration

provides no actual implementation, its accessor-body simply consists of a

semicolon.

Chapter 17 Classes

251

An event declaration that includes both the abstract and override modifiers

specifies that the event is

abstract and overrides a base event. The accessors of such an event are

also abstract.

Abstract event declarations are only permitted in abstract classes (§17.1.1.

1).

The accessors of an inherited virtual event can be overridden in a derived

class by including an event declaration

that specifies an override modifier. This is known as an overriding event

declaration. An overriding event

declaration does not declare a new event. Instead, it simply specializes

the implementations of the accessors of an

existing virtual event.

An overriding event declaration must specify the exact same accessibility

modifiers, type, and name as the

overridden event.

An overriding event declaration may include the sealed modifier. Use of

this modifier prevents a derived class

from further overriding the event. The accessors of a sealed event are also

sealed.

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

new modifier.

Except for differences in declaration and invocation syntax, virtual,

sealed, override, and abstract accessors

behave exactly like virtual, sealed, override and abstract methods.

Specifically, the rules described in §17.5.3,

§17.5.4, §17.5.5, and §17.5.6 apply as if accessors were methods of a

corresponding form. Each accessor

corresponds to a method with a single value parameter of the event type, a

void return type, and the same

modifiers as the containing event.

 
 
 
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