Override Events in VB.NET

Question

Hello

I would like to shadow/override an event in VB.NET.

I do it only because I have to implement in this object an interface, that contains the same event like the base object itself, and I need to keep this base event as is it without modification nor supplementary event additions.

How can I do it?

Public Shadows Event VisibleChanged As EventHandler Implements IVisibleChanged

So, I would like to implement a interface that contains VisibleChanged event, but to keep functional the myBase VisibleChanged event too.

  Public Shadows Event VisibleChanged As EventHandler Implements IVisibleChanged
    AddHandler(ByVal value As EventHandler)
      AddHandler MyBase.VisibleChanged, value
    End AddHandler
    RemoveHandler(ByVal value As EventHandler)
      RemoveHandler MyBase.VisibleChanged, value
    End RemoveHandler
  End Event

something like this, but it seems Visual Studio does not recognize such a syntax...

I mean, in C# I realize it like this:

public new event EventHandler VisibleChanged
{
    add { base.VisibleChanged += value; }
    remove { base.VisibleChanged -= value; }
}

Answer 1

  Public Shadows Event VisibleChanged As EventHandler Implements IVisibleChanged
    AddHandler(ByVal value As EventHandler)
      AddHandler MyBase.VisibleChanged, value
    End AddHandler
    RemoveHandler(ByVal value As EventHandler)
      RemoveHandler MyBase.VisibleChanged, value
    End RemoveHandler
  End Event

something like this, but it seems Visual Studio does not recognize such a syntax...

I mean, in C# I realize it like this:

public new event EventHandler VisibleChanged
{
    add { base.Click += value; }
    remove { base.Click -= value; }
}

Answer 2

When implementing an interface in VB.NET, you don't need to name the implementation the same name as the interface.

Example interface:

Public Interface IFoo
    Event Bar As EventHandler
End Interface

Example implementation:

Public Class FooImplementation
    Implements IFoo

    ' This is the built-in Bar event, specific to this class.
    Public Event Bar As EventHandler

    ' This is the implemented IFoo.Bar event.
    Public Event IFooBar As EventHandler Implements IFoo.Bar

End Class

When somebody is attached to you as an IFoo, they will get your IFooBar event:

Dim instance As IFoo = New FooImplementation()
AddHandler instance.Bar, AddressOf IFooBarHandler

When they're attached to you as FooImplementation, they'll get the Bar event and they will also be able to attach to IFooBar:

Dim instance As New FooImplementation()
AddHandler instance.Bar, AddressOf BarHandler
AddHandler instance.IFooBar, AddressOf IFooBarHandler

Answer 3

Can you override the handler in a derived class?

Public Class OverriddenClass
Inherits OriginalClass
Protected Overrides Function ProcessVisibleChanged(ByRef msg as Message, value as ...) As Boolean
  value = value + 1
  ProcessVisibleChanged = MyBase.ProcessVisibleChanged(msg, value)
  ' or use OnVisibleChanged to avoid the base handler
End Function
end class

Answer 4

Option Strict On
Option Explicit On

Public Class Form1
  Public Sub New()
    ' This call is required by the Windows Form Designer.'
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.'
    Dim b As New MyButton
    Me.Controls.Add(b)
    b.Name = "customButton"
    b.Location = New Point(10, 10)
    b.Text = "Hit me!"
    AddHandler CType(b, IMyInterface).Click, AddressOf MyButton1_Click
  End Sub

  Private Sub MyButton1_Click( _ 
      ByVal sender As System.Object, ByVal e As System.EventArgs)
    Debug.Print("{0} clicked!; ", CType(sender, Control).Name)
  End Sub
End Class

' ------- interface'
Public Interface IMyInterface
  Event Click As EventHandler
End Interface

' ------- class'
Public Class MyButton
  Inherits System.Windows.Forms.Button
  Implements IMyInterface
  ' ============ HERE IS THE SOLUTION'
  Private Event Click1 As EventHandler Implements IMyInterface.Click

  Private Sub ResendClick( _ 
      ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Click
    RaiseEvent Click1(sender, e)
  End Sub
  ' END HERE IS THE SOLUTION ============ '
End Class