Visual Basic 2005 in a Nutshell, Third Edition

Events and Event Binding | 119

Delegates and

Events

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

Private Sub stuffHappens_AnEvent(ByVal eventData As Integer) _

Handles stuffHappens.AnEvent

MsgBox("Event raised: " & eventData)

End Sub

Some code added to a button’s Click event completes the demonstration.

Private Sub Button1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles Button1.Click

' ----- Raise an event in an ActiveClass instance.

stuffHappens = New ActiveClass( )

stuffHappens.RaiseTheEvent(7)

End Sub

The WithEvents keyword approach to event handling has one slight drawback in

that the

New keyword cannot be used in the declaration, as in:

' ----- This doesn't work.

Public WithEvents stuffHappens As New ActiveClass

Thus, the object must be instantiated separately from the variable declaration, as

demonstrated in the example.

AddHandler and RemoveHandler

Binding events to event handlers with the Handles keyword is easy, but it’s not the

only way to use events. Visual Basic also includes the

AddHandler statement (and its

counterpart, the

RemoveHandler statement) to bind an event to an event handler at

runtime instead of at design time. For instance, you could connect a button’s Click

event to a handler at runtime. First, create the event handler in the form’s code.

Private Sub LonelyHandler(ByVal sender As ...