VB.Net Inheritance Override - vb.net

Question

I'm new to VB.Net (I'm from a foxpro background) and have had my head in a book for the last two weeks trying to get started with some of the basics.
I'm trying to master class inheritance and have what I hope is not too much of a challenging question.
I've created a class and compiled it as a DLL. It simply allows me to place a button on a form. I just want to capture the Click event - which I've managed to do but would like to override the inherited code rather than having both fire which seems to be happening at the moment.
I realise I could just double click the control and enter code directly into the MyButton1 click event but wanted to trap this programmatically instead via the handler.
I thought this would just be a case of using the overridable / overrides options.
Here's the code in my class:
Imports System.Windows.Forms
Imports System.Drawing
Public Class MyButton
Inherits Windows.Forms.Button
Sub New()
End Sub
Protected Overridable Sub MyButton_Click(sender As Object, e As System.EventArgs) Handles Me.Click
MsgBox("Base Click")
End Sub
End Class
Then I place the button on my form and name it MyButton1 and in the load event:
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
AddHandler MyButton1.Click, AddressOf Button_Click
End Sub
Private Sub Button_Click()
MsgBox("Actual Click")
End Sub
Problem is, both events fire and I want the option to override / turn off the base event.
I thought I could just add the 'overrides' keyword to the Button_Click routine i.e.:
Private Sub Overrides Button_Click()
but I get an error message Sub Button_Click() cannot be declared 'overrides' because it does not override a sub in a base class
So to clarify - at the moment my code fires both events so I get two messages. I want to be able to turn off / supress the base class event.
Any help would be much appreciated.

Answer 1

I thought this would just be a case of using the overridable /
overrides options.
The fundamental problem here is that you're trying to push a square peg into a round hole.
To override something, you need to have inheritance involved. The derived class is overriding something that was inherited from the base class. For instance, if you inherited from your MyButton class to create a new type of Button called MyButtonDerived, then you could do it as expected:
Public Class MyButton
Inherits Windows.Forms.Button
Protected Overridable Sub MyButton_Click(sender As Object, e As System.EventArgs) Handles Me.Click
MsgBox("Base Click")
End Sub
End Class
Public Class MyButtonDerived
Inherits MyButton
Protected Overrides Sub MyButton_Click(sender As Object, e As EventArgs)
' We don't call the base method...
' MyBase.MyButton_Click(sender, e)
' ... and instead do something else:
MsgBox("Derived Click")
End Sub
End Class
In contrast, when you've placed MyButton onto the Form as in your original problem description, no inheritance has taken place. Instead what you've setup is "object composition"; the form contains an instance of the button (not derived from it). While it may be possible to change what happens when the button is clicked from the form itself, this is not a case that can be solved with OOP, inheritance and overriding.
If MyButton was not designed in such a way that allows the end user to suppress its base functionality, then your options are limited in how you can use it. Here is an example of what it might look like if MyButton was designed to allow the end user to suppress its base click functionality:
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
MyButton1.SuppressDefaultClick = True
End Sub
Private Sub MyButton1_Click(sender As Object, e As EventArgs) Handles MyButton1.Click
MsgBox("Form Click Code")
End Sub
End Class
Public Class MyButton
Inherits Windows.Forms.Button
Private _suppress As Boolean = False
Public Property SuppressDefaultClick As Boolean
Get
Return _suppress
End Get
Set(value As Boolean)
_suppress = value
End Set
End Property
Protected Overridable Sub MyButton_Click(sender As Object, e As System.EventArgs) Handles Me.Click
If Not SuppressDefaultClick Then
MsgBox("Base Click")
End If
End Sub
End Class
If MyButton didn't include a way to suppress its built-in click handler like above then you'd have to resort to other means to solve your problem. In that case you'd have to prevent the button from ever receiving the message that the left mouse button has been clicked at all, and instead implement your own routine. This approach would be a considered a hack, since you are working around the limitations of something and not using it in the way it was originally intended. Here's one way the hack could be implemented:
Public Class Form1
Private WithEvents TMBC As TrapMyButtonClick
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
TMBC = New TrapMyButtonClick(Me.MyButton1)
End Sub
Private Sub TMBC_Click(sender As MyButton) Handles TMBC.Click
MsgBox("Form Click Code")
End Sub
Private Class TrapMyButtonClick
Inherits NativeWindow
Private _mb As MyButton
Private Const WM_LBUTTONDOWN As Integer = &H201
Public Event Click(ByVal sender As MyButton)
Public Sub New(ByVal mb As MyButton)
If Not IsNothing(mb) AndAlso mb.IsHandleCreated Then
_mb = mb
Me.AssignHandle(mb.Handle)
End If
End Sub
Protected Overrides Sub WndProc(ByRef m As Message)
Select Case m.Msg
Case WM_LBUTTONDOWN
RaiseEvent Click(Me._mb) ' raise our custom even that the form has subscribed to
Exit Sub ' Suppress default behavior
End Select
MyBase.WndProc(m)
End Sub
End Class
End Class
Public Class MyButton
Inherits Windows.Forms.Button
Protected Overridable Sub MyButton_Click(sender As Object, e As System.EventArgs) Handles Me.Click
MsgBox("Base Click")
End Sub
End Class

Answer 2

You are getting 2 messages because you have set 2 different event handlers for the Click event:
The MyButton_Click method defined in your MyButton class.
The Button_Click method set in your AddHandler call on the form.
As noted in a comment above, you need to override the Button.OnClick method in your MyButton class instead of creating a new method:
Imports System.Windows.Forms
Imports System.Drawing
Public Class MyButton
Inherits Windows.Forms.Button
Sub New()
End Sub
' Override the OnClick event defined in "Button" class.
Protected Overrides Sub OnClick(e As System.EventArgs)
' Call the Click event from "Button" class.
MyBase.OnClick(e)
' Some custom events.
MsgBox("MyButton Click")
End Sub
End Class
It might be a good exercise to set breakpoints in the Button_Click and MyButton.OnClick methods so you can see exactly how the stack is created.