Polymorphism

[Richard Tappenden]

Hi!

I'm struggling a bit with Polymorphism in vb.net. There is something I know
I can do in C++, but I cannot seem to do it in vb.net (or c# for that
matter).

Basically, I have an abstract base class that deals with adding, deleting,
editing another class.

I have written an implementation of that class, and also a derived version
of the 'managed' class - so effectively my derived version manages a
specialised version of whatever the base one does. However, the compiler
complains that I havent implemented the MustInherit methods...

To explain a bit better...
Public Class CMessage
' This is the "managed" class

Public MustInherit Class CMessageManager
' This is the abstract base class
Public MustOverride Function MessageReceived (ByRef objMessage As
CMessage) As Boolean

Public Class CEmail Inherits CMessage
' Derived version

Public Class CEmailMessageMa nager Inherits CMessageManager
' Implemented version - want it to deal with emails

Public MustOverride Function MessageReceived (ByRef objMessage As CEmail)
As Boolean
' Pass in an email and deal with it
This concept is supported by C++ - it must be possible to do it in .NET, but
I really dont know how.

Any help would be greatly appreciated!

[Tom Shelton]

In article <O$************ **@TK2MSFTNGP09 .phx.gbl>, Richard Tappenden wrote:

Hi!

I'm struggling a bit with Polymorphism in vb.net. There is something I know
I can do in C++, but I cannot seem to do it in vb.net (or c# for that
matter).

Basically, I have an abstract base class that deals with adding, deleting,
editing another class.

I have written an implementation of that class, and also a derived version
of the 'managed' class - so effectively my derived version manages a
specialised version of whatever the base one does. However, the compiler
complains that I havent implemented the MustInherit methods...

To explain a bit better...
Public Class CMessage
' This is the "managed" class

Public MustInherit Class CMessageManager
' This is the abstract base class
Public MustOverride Function MessageReceived (ByRef objMessage As
CMessage) As Boolean

Public Class CEmail Inherits CMessage
' Derived version

Public Class CEmailMessageMa nager Inherits CMessageManager
' Implemented version - want it to deal with emails

Public MustOverride Function MessageReceived (ByRef objMessage As CEmail)
As Boolean
' Pass in an email and deal with it
This concept is supported by C++ - it must be possible to do it in .NET, but
I really dont know how.

Any help would be greatly appreciated!

I'm not sure what the problem is - the compiler is telling you exactly
what the problem is... If you want to be able to use
CEmailMessageMa nager then you must override the MessageReceived
function..

Public Class CEmailMessageMa nager Inherits CMessageManager

Public Overrides Function MessageReceived (ByRef objMessage As
CEmail) As Boolean
' Do stuff
End Function
End Class

Even in C++ this is the rule. You can't use a function that has no
implementation. The rules for using Abstract base classes are pretty
much the same in VB.NET as the are in C++.
--
Tom Shelton
MVP [Visual Basic]

[Richard Tappenden]

Hi Tom,

Thanks for the reply - but I think you've misunderstood the question - I'm
well aware that I can create an instance of an abstract class etc...

The problem is that I cant seem to override the base class methods unless
they all use the base message class to do the operations - i.e:

The base class method =
Public MustOverride Function MessageReceived (ByRef objMessage As CMessage)
As Boolean

My derived version
Public Overrides Function MessageReceived (ByRef objMessage As CEmailMessage)
As Boolean
' Do something
End Function.

The overridden version complains - stating that I still need to override
MessageReceived (ByRef objMessage CMessage) As Boolean.

In C++, you can do this - i.e. it resolves the virtual table correctly.

For example - check these c++ files...(apolog ies if they don't look nice, I
knocked them up quickly to prove the point)

"Tom Shelton" <to*@mtogden.co m> wrote in message
news:uQ******** ******@TK2MSFTN GP09.phx.gbl...

In article <O$************ **@TK2MSFTNGP09 .phx.gbl>, Richard Tappenden

wrote:

Hi!

I'm struggling a bit with Polymorphism in vb.net. There is something I know I can do in C++, but I cannot seem to do it in vb.net (or c# for that
matter).

Basically, I have an abstract base class that deals with adding, deleting, editing another class.

I have written an implementation of that class, and also a derived version of the 'managed' class - so effectively my derived version manages a
specialised version of whatever the base one does. However, the compiler
complains that I havent implemented the MustInherit methods...

To explain a bit better...
Public Class CMessage
' This is the "managed" class

Public MustInherit Class CMessageManager
' This is the abstract base class
Public MustOverride Function MessageReceived (ByRef objMessage As
CMessage) As Boolean

Public Class CEmail Inherits CMessage
' Derived version

Public Class CEmailMessageMa nager Inherits CMessageManager
' Implemented version - want it to deal with emails

Public MustOverride Function MessageReceived (ByRef objMessage As CEmail) As Boolean
' Pass in an email and deal with it
This concept is supported by C++ - it must be possible to do it in .NET, but I really dont know how.

Any help would be greatly appreciated!

I'm not sure what the problem is - the compiler is telling you exactly
what the problem is... If you want to be able to use
CEmailMessageMa nager then you must override the MessageReceived
function..

Public Class CEmailMessageMa nager Inherits CMessageManager

Public Overrides Function MessageReceived (ByRef objMessage As
CEmail) As Boolean
' Do stuff
End Function
End Class

Even in C++ this is the rule. You can't use a function that has no
implementation. The rules for using Abstract base classes are pretty
much the same in VB.NET as the are in C++.
--
Tom Shelton
MVP [Visual Basic]

[Armin Zingler]

"Richard Tappenden" <ri******@mailk ey.com> schrieb

The problem is that I cant seem to override the base class methods
unless they all use the base message class to do the operations -
i.e:

The base class method =
Public MustOverride Function MessageReceived (ByRef objMessage As
CMessage) As Boolean

My derived version
Public Overrides Function MessageReceived (ByRef objMessage As
CEmailMessage) As Boolean
' Do something
End Function.

The overridden version complains - stating that I still need to
override MessageReceived (ByRef objMessage CMessage) As Boolean.
The derived class must contain a procedure with the same signature. In your
case, the signature is different because the type of the argument is
CEmailMessage, not CMessage as in the base class.
In C++, you can do this - i.e. it resolves the virtual table
correctly.

For example - check these c++ files...(apolog ies if they don't look
nice, I knocked them up quickly to prove the point)

--
Armin

http://www.plig.net/nnq/nquote.html
http://www.netmeister.org/news/learn2quote.html

[Tom Shelton]

In article <u1************ **@TK2MSFTNGP09 .phx.gbl>, Richard Tappenden wrote:

Hi Tom,

Thanks for the reply - but I think you've misunderstood the question - I'm
well aware that I can create an instance of an abstract class etc...

The problem is that I cant seem to override the base class methods unless
they all use the base message class to do the operations - i.e:

The base class method =
Public MustOverride Function MessageReceived (ByRef objMessage As CMessage)
As Boolean

My derived version
Public Overrides Function MessageReceived (ByRef objMessage As CEmailMessage)
As Boolean
' Do something
End Function.

The overridden version complains - stating that I still need to override
MessageReceived (ByRef objMessage CMessage) As Boolean.

Aah, now I understand and Armin hit the nail on the head. You have not
overridden the base function because the sig does not match. You are
passing a CEmailMessage - not CMessage, which is what the base class
defines.

--
Tom Shelton
MVP [Visual Basic]

[Richard Tappenden]

Hi Armin.

Thanks for the reply - I understand what you are saying, however the
CEmailMessage class is derived from CMessage - so it should work (at least
it does work that way in C++)

"Armin Zingler" <az*******@free net.de> wrote in message
news:%2******** ********@TK2MSF TNGP12.phx.gbl. ..

"Richard Tappenden" <ri******@mailk ey.com> schrieb

The problem is that I cant seem to override the base class methods
unless they all use the base message class to do the operations -
i.e:

The base class method =
Public MustOverride Function MessageReceived (ByRef objMessage As
CMessage) As Boolean

My derived version
Public Overrides Function MessageReceived (ByRef objMessage As
CEmailMessage) As Boolean
' Do something
End Function.

The overridden version complains - stating that I still need to
override MessageReceived (ByRef objMessage CMessage) As Boolean.
The derived class must contain a procedure with the same signature. In

your case, the signature is different because the type of the argument is
CEmailMessage, not CMessage as in the base class.

In C++, you can do this - i.e. it resolves the virtual table
correctly.

For example - check these c++ files...(apolog ies if they don't look
nice, I knocked them up quickly to prove the point)

--
Armin

http://www.plig.net/nnq/nquote.html
http://www.netmeister.org/news/learn2quote.html

[Jay B. Harlow [MVP - Outlook]]

Richard,
..NET is not C++! Remember C++ also supports Multiple Inheritance where as
..NET does not.

C++ allows for covariant parameters, however .NET (the CLR itself) does not
allow for covariant parameters.

I understand that you can use Eiffel for .NET http://www.eiffel.com/ and
have covariant parameters, however you will not be able to use that code
with C# or VB.NET, as it is not CLS complient...

Hope this helps
Jay

"Richard Tappenden" <ri******@mailk ey.com> wrote in message
news:ui******** ******@TK2MSFTN GP11.phx.gbl...

Hi Armin.

Thanks for the reply - I understand what you are saying, however the
CEmailMessage class is derived from CMessage - so it should work (at least
it does work that way in C++)

<<snip>>

[Armin Zingler]

"Richard Tappenden" <ri******@mailk ey.com> schrieb

Thanks for the reply - I understand what you are saying, however
the CEmailMessage class is derived from CMessage - so it should work
(at least it does work that way in C++)

In .NET, the signatures must match exactly. I haven't thought about why they
have to (yet).
--
Armin

http://www.plig.net/nnq/nquote.html
http://www.netmeister.org/news/learn2quote.html

[Armin Zingler]

"Armin Zingler" <az*******@free net.de> schrieb

"Richard Tappenden" <ri******@mailk ey.com> schrieb

Thanks for the reply - I understand what you are saying, however
the CEmailMessage class is derived from CMessage - so it should
work (at least it does work that way in C++)

In .NET, the signatures must match exactly. I haven't thought about
why they have to (yet).

Now I did. ;-)

If I am the author of the base class, I want to force the author(s) of the
derived class(es) to accept *any* CMessage object. If you change the
signature to CEMailMessage, you break this enforcement because not any
CMessage object can be passed anymore.
--
Armin

http://www.plig.net/nnq/nquote.html
http://www.netmeister.org/news/learn2quote.html

[Richard Tappenden]

That really defeats the point of OO programming - remember the CEmailMessage
is "a kind of" CMessage object - therefore I should be able to pass it into
the base class method - and it will do anything with it that it could with a
CMessage.

If I then implement a more complete, specific kind of message object and a
manager class that deals with them as well - it should be able to take
advantage of the new improved class, and yet still be "a kind of" base
object.

I must say, I'm pretty disappointed that this hasn't been put into .NET, and
I really think it will make a few things difficult when moving code over
from C++ to C#.

Incidently, in C++ I think you can enforce types by using the Explicit
keyword, which would restrict the types as per your idea below.

If I am the author of the base class, I want to force the author(s) of the
derived class(es) to accept *any* CMessage object. If you change the
signature to CEMailMessage, you break this enforcement because not any
CMessage object can be passed anymore.