Using a Delegate to Simulate Invoking Events in Base Class

[Charles Law]

I have a base class and derived classes that relate to a set of documents I
process. e.g. DocBase, DocA, DocB, DocC. The processing of each document is
handled in teh derived classes, as you might imagine, but the base class has
properties and methods that are common to all documents.

The processing of each docuemnt can result in a range of document specific
errors, which I want to raise as events on the document object, but there
are some events that are common to all documents, so I would like to have
those raised in the base class. In fact, I want to raise all events through
a delegate in the base class so that the object owners don't have to do too
much work.

So, I have in mind something like this for the caller:

Main
Sub ProcessDocA
Dim doc As DocBase = New DocA(path)

doc.Process(AddressOf DocA_ProcessError) <--- Problem here
End Sub

Sub ProcessDocB
Dim doc As DocBase = New DocB(path)

doc.Process(AddressOf DocB_ProcessError) <--- Problem here
End Sub

Sub DocA_ProcessError(ByVal sender As Object, ByVal e As
DocAProcessErrorEventArgs)
' Prompt User
End Sub

Sub DocB_ProcessError(ByVal sender As Object, ByVal e As
DocBProcessErrorEventArgs)
' Prompt User
End Sub

My base class looks like this

Public MustInherit Class DocBase

Delegate Sub ProcessErrorEventHandler(ByVal sender As Object, ByVal
e As EventArgs)

Protected ProcessErrorCallBack As ProcessErrorEventHandler

MustOverride Sub Process(ByVal callback As ProcessErrorEventHandler)

Protected Sub OnProcessError(ByVal e As EventArgs)
ProcessErrorCallBack.Invoke(Me, e)
End Sub

End Class

My derived class looks like this

Public Class DocA

Inherits DocBase

Public Overrides Sub Process(ByVal processErrorCallback As
ProcessErrorEventHandler)
MyBase.ProcessErrorCallback = processErrorCallback

DoProcessing()

End Sub

Private Sub Do Processing()

' Trap Error
MyBase.OnProcessError(New DocAProcessErrorEventArgs("Error in
Doc A"))

End Sub

End Class


The problem is where I have arrowed above, becase the compiler doesn't like
an implicit narrowing conversion from DocA_ProcessError() to
ProcessErrorEventHandler.

I know this all looks very complicated, and perhaps unnecessarily so, but
can anyone suggest a better, more generic way to do this.

TIA

Charles

 

[Patrice]

Have you tried or do you know about events with the standard OnXXXX
sub raising the XXXX event ? In your description you are talking
multiple times about events but you are using delegates in your code
so I'm not sure if you know about events or if you are trying to avoid
them for some reason (not sure what is this "too much work" you are
talking about)...

 

[Charles Law]

Hi Patrice

Thanks for the reply. I do know about events, but as I understand it events
in a base class can't be raised by a derived class, so that is why I'm using
delegates. They also can' te b used as a contract in an interface, which is
another goal.

The too much work I'm talking about is work the client of my Doc classes has
to do. I thought that if they just passed a reference to a call back to
handle errors then that would be the easiest solution. I also mean that I
want to impose some sort of contract on the user so that I can use the
compiler to pickup errors at design time if the classes are not used
correctly. Also, if I can create an interface in a particular way, it will
ensure that the rest of the code is written correctly.

Charles

 

[Patrice]

Try :

MustInherit Class DocBase
Event ProcessingError()
Protected Overridable Sub OnProcessingError()
Debug.WriteLine("Here in DocBase.OnProcessingError")
RaiseEvent ProcessingError()
End Sub
MustOverride Sub Processing()
End Sub
End Class

Class DocA
Inherits DocBase
Sub Processing()
OnProcessingError()
End Sub
Sub Test() Handles Me.ProcessingError
Debug.WriteLine("Here in DocA.Test that handles
ProcessingError")
End Sub
End Class

It will display both messages. Is this what you are looking for ?

If not the problem in your first code is that you seems to use a
specific argument type for each of your child class. It should work if
you replace DocAProcessEventArgs, DocB... by just EventArgs so that it
match what you used in the base class.

 

[Charles Law]

Hi Patrice

I can see what you have done there, and you are right that I am using a
specific argument type for each of my derived classes.

I agree that that is the problem, so perhaps I have to find a way to make
the argument common, even if it is not EventArgs. Currently, each derived
class returns specific information relating to the document it is
processing, so I'm not sure how to make the argument common, but I will try.
It would be nice to find a generic way to do it where the argument types
were different though.

Thanks.

Charles

 

[JB]

refering to the Foo event in Bill McCarthy's reply, there are several ways of
defining different event data for each derived class. One way is to define a
base class event args
Friend Class FooBaseEventArgs
Inherits System.EventArgs

Friend New( .. )

.. you can include an e.g. typeID so the comsumer can determine
.. the actual eventargs type,
.. Or use typeof or some other logic

.. properties ..
end class

then
Friend Class Derived1_FooEventArgs
Inherits FooBaseEventArgs

...
End Class

and so on...

In the base class,
Protected Sub OnFoo(ev as FooBaseEventArgs)
( you don't need the Overriable if you are only going to invoke the event)

Then in the derived class,
Dim eArgs as New Derived1_FooEventArgs(..)
OnFooEvent(eArgs)

There are other schemes for accomplishing the same thing. It is then up to
the comsumer of the event to sort out the desired data from the event data

 

[Charles Law]

Hi JB

My news reader isn't showing Bill's reply, so I have only just seen it in
your reply below.

I started down this route, and then decided that I didn't want the consumer
to have to cast the eventargs parameter received to the specific derived
eventargs type for the event. I wanted them to receive the correctly typed
parameter for the event raised by the derived class.

Charles