Virtual, sealed, override, and abstract accessors
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 abstract event declaration provides no actual implementation, it cannot provide brace-delimited event-accessor-declarations. 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 (§10.1.1.1). 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 §10.6.3, §10.6.4, §10.6.5, and §10.6.6 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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