VB.Net How to attach a handler to a generic class - vb.net

Question

I have a class 'oBnd' defined as Object that can be assigned as type clsBound or clsClaim.
Claim and bound are identical from the outside, same methods etc.
I call the various properties using 'CallByName'
ie Dim Current As String = CallByName(oBnd, PropName, CallType.Get)
When a property is changed in either class the DirtyStatus event is raised by that class.
I am having a problem attaching to this event.
If I try
AddHandler oBnd.DirtyStatus, AddressOf oBnd_DirtyStatus
I get the error "DirtyStatus is not an event of Object" I guess that makes sense as clearly object know nothing of my dirtystatus.
I tried using:
AddHandler DirectCast(oBnd, clsBound).DirtyStatus, AddressOf oBnd_DirtyStatus
While this does fix the error it does not get called when the DirtyStatus event is raised.
oBnd is defined as
Private WithEvents oBnd As Object
It is global to the form
oBnd gets set as
oBnd = New clsBound(mvarBUDConnection)
AddHandler oBnd.DirtyStatus, AddressOf oBnd_DirtyStatus
oBnd.Load(CInt(txtTrans.Text))
BuildPage(oBnd)
Or
oBnd = New clsClaim(mvarBUDConnection)
AddHandler oBnd.DirtyStatus, AddressOf oBnd_DirtyStatus
oBnd.Load(CInt(txtTrans.Text))
BuildPage(oBnd)
The oBnd_DirtyStatus sub, that I am trying to attach to, looks like this
Private Sub oBnd_DirtyStatus(IsDirty As Boolean) ' Handles oBnd.DirtyStatus
Me.Text = "QFix"
If IsDirty Then
Me.Text = "QFix - Pending Save"
btnSave.Enabled = True
Else
btnSave.Enabled = False
End If
End Sub
How can I attach a handle to this event?

Answer 1

Here is how you can both get Events working and get away from using Reflection to access properties. Even given the public methods are similar but the data being carried is very different it should still possible to use OOP/Inheritance.
Public Enum ClaimBoundType
None ' error!!!!
Claim
Bound
End Enum
Public MustInherit Class ClaimBase
' type tracker usually rather handy
Public Property ItemType As ClaimBoundType
Public Sub New(t As ClaimBoundType)
ItemType = t
End Sub
' low rent INotifyPropertyChanged
Public Event DataChanged(sender As Object, e As EventArgs)
' "universal" prop: works the same for all derived types
Private _name As String = ""
Public Property Name As String
Get
Return _name
End Get
Set(value As String)
If value <> _name Then
_name = value
BaseDataChanged(Me)
End If
End Set
End Property
' props which must be implemented; 1 or 100 doesnt matter
MustOverride Property CurrentValue As Integer
' methods which must be implemented
MustOverride Function DoSomething() As Integer
' raise the changed event for base or derived classes
Protected Friend Sub BaseDataChanged(sender As Object)
RaiseEvent DataChanged(sender, New EventArgs())
End Sub
End Class
You'd have to do some basic data analysis to figure out which Properties and Methods can be implemented in the base class (as with Name above) and which in the inherited classes. There are usually at least some which can be done in the base class.
Your derived classes can implement the methods in totally different ways and load data from where ever:
Public Class Claim
Inherits ClaimBase ' the IDE will add all the MustInherits when
' you press enter
Public Sub New()
MyBase.New(ClaimBoundType.Claim)
End Sub
Public Overrides Function DoSomething() As Integer
' what happens here can be completely different
' class to class
End Function
Private _CurValue As Integer = 0
Public Overrides Property CurrentValue As Integer
Get
Return _CurValue
End Get
Set(Value As Integer)
If _CurValue <> Value Then
_CurValue = Value
OnDataChanged("CurrentValue")
End If
End Set
End Property
' name of prop that changed not actually used here, but
' is usually good to know (use custom args or INotifyPropertyChanged)
Public Sub OnDataChanged(pname As String)
' fire shared datachanged event
MyBase.BaseDataChanged(Me)
End Sub
End Class
How to Use Them
Now you can implement them without resorting to Object, subscribe to the event and not have to use Reflection to get/set properties:
' 'generic' object variable: DONT/CANT USE [New] w/ClaimBase
Private myCB As ClaimBase
...
' set it as a Claim instance...
' This is perfectly legal because Claim is also a ClaimBase Type:
myCB = New Claim
' hook up the event handler
AddHandler myCB.DataChanged, AddressOf cb_DataChanged
You can declare your object variables as ClaimBase, but you cannot create an instance of ClaimBase since it is abstract/MustInherit. Since the event is part of the base class, there is no problem with syntax. The form level handler:
' Use standard (sender, e) signature
' (CA will object to other signatures:)
Private Sub cb_DataChanged(sender As Object, e As EventArgs)
' do change stuff here
...
End Sub
Best of all, you can reference properties directly:
cbObj.Name = "Ziggy" ' will fire the event from the base class
cbObj.CurrentValue = 42 ' fires event from the Claim class
I added the ItemType property so you can tell them apart at run time (ie when you hold the mouse over a ClaimBase object variable). If/when there are Type specific properties/methods to access, cast it (from what you said, there cant be any of these now):
If cbObj.ItemType = ClaimBoundType.Claim Then
CType(cbObj, Claim).ClaimSomething = 5
End If
Also use ClaimBase as the declaration Type for Lists and method signatures also to allow either type to be passed rather than boxing them (converting to Object):
Private cbList As New List(Of ClaimBase)
...
' just an example of the declaration
Private Sub AddThingToList(cb As ClaimBase)
cbList.Add(cb)
End Sub
I did not go into INotifyProperty in order to focus on Inheritance, though the basics of it are in that base class. It is a more systemic way to implement the DataChanged/DirtyStatus event and detection.