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10.7 Events

Visual Studio .NET 2003

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
remove-accessor-declaration:
attributesopt   remove   block

An event-declaration may include a set of attributes (Section 17) and a valid combination of the four access modifiers (Section 10.2.3), the new (Section 10.2.2), static (Section 10.5.2), virtual (Section 10.5.3), override (Section 10.5.4), sealed (Section 10.5.5), abstract (Section 10.5.6), and extern (Section 10.5.7) modifiers.

Event declarations are subject to the same rules as method declarations (Section 10.5) with regard to valid combinations of modifiers.

The type of an event declaration must be a delegate-type (Section 4.2), and that delegate-type must be at least as accessible as the event itself (Section 3.5.4).

An event declaration may include event-accessor-declarations. However, if it does not, for non-extern, non-abstract events, the compiler supplies them automatically (Section 3.7.1); 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 event-declaration.

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 (Section 7.13.3). 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.

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
   }
   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.

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