C# 5.0 Programmer's Reference

776 ❘ Appendix K Classes and Structures
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new—Hides an event with the same name defined in an ancestor class.
virtual—If you mark an event as virtual, you can later replace it in a derived class
by overriding it.
override—If an ancestor class defines a virtual event, you can use the override
keyword to override it.
abstract—This keyword indicates the event is abstract, so derived classes must override
the event to give it an implementation. As is the case with abstract methods, a
class that contains an abstract event must be abstract and cannot be instantiated.
sealed—This keyword indicates the event is no longer virtual, so it cannot be
overridden in derived classes.
static—This indicates the class itself raises the event rather than instances of the class.
delegate—This is a delegate type that defines the parameters that will be passed to event
handlers for the event.
name—This is the name you want to give the event.
To raise an event, first determine whether any other pieces of code have registered to receive the
event by comparing the event to null. If code has registered to receive the event, invoke the event
by name, passing it any necessary parameters.
For example, suppose the Student class has a GradeChanged event. Then the following snippet
inside the Student class raises the event, passing it the current Student object as a parameter.
if (GradeChanged != null) GradeChanged(this);
To subscribe to an event, use the += operator to “add” the event handler to the event. The following
code shows how a program could register the MyPerson_NameChanged event handler to handle the
MyPerson object’s NameChanged event.
MyPerson.NameChanged += MyPerson_NameChanged;
If you subscribe an event handler to an event multiple times, it executes multiple times when the
event occurs.
To unsubscribe from an event, use the -= operator, as in the following code.
MyPerson.NameChanged -= MyPerson_NameChanged;
Unsubscribing from an event more times than the event handler was originally registered does not
throw an exception.
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