Working sample of Control.VisibleChanged Event in vb.net - vb.net

Question

I'm struggling to make the MSDN code sample for the Control.VisibleChanged event work: I don't see the MsgBox.
Private Sub Button_HideLabel(ByVal sender As Object, ByVal e As EventArgs)
myLabel.Visible = False
End Sub 'Button_HideLabel
Private Sub AddVisibleChangedEventHandler()
AddHandler myLabel.VisibleChanged, AddressOf Label_VisibleChanged
End Sub 'AddVisibleChangedEventHandler
Private Sub Label_VisibleChanged(ByVal sender As Object, ByVal e As EventArgs)
MessageBox.Show("Visible change event raised!!!")
End Sub 'Label_VisibleChanged

Answer 1

You need to "wire up" the events to the event handlers.
To start with, to get the code in HideLabel_Click to be called you need it to respond to a click on the button named "HideLabel".
There are two ways to do that: you can use AddHandler or the Handles clause.
To demonstrate the latter:
Option Strict On
Public Class Form1
Private Sub HideLabel_Click(sender As Object, e As EventArgs) Handles HideLabel.Click
myLabel.Visible = False
End Sub
Private Sub myLabel_VisibleChanged(sender As Object, e As EventArgs) Handles myLabel.VisibleChanged
MessageBox.Show("Visible change event raised!!!")
End Sub
End Class
However, you will notice that the message is shown even before the form appears. That is because of what goes on behind the scenes to create the form.
To avoid that happening, you can add the handler after the form has been shown:
Option Strict On
Public Class Form1
Private Sub HideLabel_Click(sender As Object, e As EventArgs) Handles HideLabel.Click
myLabel.Visible = False
End Sub
Private Sub myLabel_VisibleChanged(sender As Object, e As EventArgs)
MessageBox.Show("Visible change event raised!!!")
End Sub
Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles MyBase.Shown
AddHandler myLabel.VisibleChanged, AddressOf myLabel_VisibleChanged
End Sub
End Class
Another way, in VB2015 and later, is to use a "lambda expression" instead of a separate method, although then you cannot disassociate the handler from the event with RemoveHandler:
Option Strict On
Public Class Form1
Private Sub HideLabel_Click(sender As Object, e As EventArgs) Handles HideLabel.Click
myLabel.Visible = False
End Sub
Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles MyBase.Shown
AddHandler myLabel.VisibleChanged, Sub() MessageBox.Show("Visible change event raised!!!")
End Sub
End Class
Craig was kind enough to [and I quote verbatim] call attention to the importance of Option Strict when you add handlers manually using AddHandler. Without it, the "relaxed delegate convention" may allow adding handlers which don't exactly match the event signature that you won't be able to remove later.
Having said that, Option Strict On isn't a complete safeguard: notice how my last example compiles and works even with the wrong method signature for the handler.
[I suspect that the MSDN code sample was first created in C# as part of a larger example, so some parts have been lost in the translation and excerption.]

Answer 2

I get this is old but came across this post when looking for more information on VisibleChanged and couldn't help but notice that the accept answer may be misleading. If you are using a designer to create your Form and place objects on it, then the accepted answer will be fine. In fact you can get rid of the addHandler because the designer handles that for you. All you would need to do is use a handles clause with your label.
Private Sub Button_HideLabel(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
myLabel.Visible = False
End Sub 'Button_HideLabel
Private Sub Label_VisibleChanged(ByVal sender As Object, ByVal e As EventArgs) Handles myLabel.VisibleChanged
MessageBox.Show("Visible change event raised!!!")
End Sub 'Label_VisibleChanged
Where the issue lies with the accepted answer is if you arn't using a designer. Adding handle clauses to "wire up" simply won't work (we can make it work and if anyone is interested in that I'll be happy to post a code snippet of that, but it's not how the accepted answer lays it out). In your case all you need to do is call AddVisibleChangedEventHandler() to set up the handler. that's it. you could have done this by calling it in MyBase.Load
Private Sub Load_Form(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
AddVisibleChangedEventHandler()
End Sub
Private Sub Button_HideLabel(ByVal sender As Object, ByVal e As EventArgs)
myLabel.Visible = False
End Sub 'Button_HideLabel
Private Sub Label_VisibleChanged(ByVal sender As Object, ByVal e As EventArgs)
MessageBox.Show("Visible change event raised!!!")
End Sub 'Label_VisibleChanged
Private Sub AddVisibleChangedEventHandler()
AddHandler myLabel.VisibleChanged, AddressOf Label_VisibleChanged
End Sub
Once again I know this is dated but couldn't help but notice that (more or less assuming) that you are trying to get a msgBox to appear when you click a label. That is you click a label and then toggled the visibility of another label. The other label is the one where the event handler is on for visibility change. So that inevitably gets called when clicking the original label. IF you only want this msgBox to appear when clicking that label and not when the form loads as well, you should change the addHandler statement so that you are adding a handler on the click event.
Private Sub Load_Form(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
AddVisibleChangedEventHandler()
End Sub
Private Sub Label_VisibleChanged(ByVal sender As Object, ByVal e As EventArgs)
MessageBox.Show("Visible change event raised!!!")
End Sub 'Label_VisibleChanged
Private Sub AddVisibleChangedEventHandler()
AddHandler otherLabel.Click, AddressOf Label_VisibleChanged
End Sub 'AddVisibleChangedEventHandler
Also Option Strict On has nothing to do with addhandler (From my understanding, could be wrong. please enlighten me if that is the case). Option Strict On is only checking to see that you arn't implicitly typecasting. So for example:
Dim a As Double
Dim b As Integer
a = 10
b = a
results in an error when Option Strict is On but is totally legal if it is off. So in the case of you leaving off the handles clause, you'll never be implicitly typecasting and therefore is not needed.
Hope this helps anyone who sees this question