Visual Basic 2005 in a Nutshell, Third Edition

114

|

Chapter 8: Delegates and Events

This is the Title of the Book, eMatter Edition

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

Using a Delegate to Call a Method

To call a method using a delegate, use the Invoke method of the delegate. To illus-

trate, consider a class module with a simple method.

Public Class SimpleClass

Public Sub CallMe(ByVal content As String)

MsgBox(content)

End Sub

End Class

Delegates are one of the many .NET types. Visual Basic includes a Delegate

keyword that defines delegates. Since the goal is to call the CallMe method

through a delegate, that delegate must have the same signature as the called

method. This small example invokes the delegate from a click on a form, so the

delegate’s definition appears in the form’s class code.

Delegate Sub MyDelegate(ByVal s As String)

Finally, in the form’s Click event handler (which works via delegates, but don’t

think about that now), the call to

CallMe is made indirectly through a delegate.

Private Sub Form1_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles MyBase.Click

' ----- Get an instance of the destination class.

Dim destClass As New SimpleClass( )

Dim theDelegate As MyDelegate

' ----- Connect the delegate to its target, the CallMe method.

theDelegate = New MyDelegate(AddressOf destClass.CallMe)

' ----- Make the call.

theDelegate.Invoke("Display this!")

End Sub

It doesn’t seem