Re: MustInherit in Window forms

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From: Niklas (Niklas_at_discussions.microsoft.com)
Date: 07/23/04 

Date: Thu, 22 Jul 2004 23:41:01 -0700

Hi
Thank you. I understand the concepts and with the knowledge I have, I wanted to create a base form which forced the developers which derives from the Base form to implement DisconnectFromAbrServer(). My way of doing it created problems in Visual Studio, see old message below. When I try to open the derived form in VS I get the error message "...The desigenr need to create a instance of type 'WindowApplication1.frmBaseMustInherit' but it can not because the type is declared as abstract". If I run the code the derived form is displayed. It seems to me that Visual Studio 2003 do not allow forms to be MustInherit! Can I force developers to not to forget to implement the method DisconnectFromAbrServer() in another way, now when MustInherit and MustOverride do not work in VS?
Regards
/Niklas

"Tom Dacon" wrote:

> Oops...
>
> .... so you can choose to code a generic DisconnectFromAbrServer() method
> there, and it'll be run if the SUBCLASS implementor chooses not to implement
> a custom version of the method. ...
>
> Sorry 'bout that.
>
> Tom Dacon
> Dacon Software Consulting
>
> "Tom Dacon" <tdacon@community.nospam> wrote in message
> news:OlCdImAcEHA.2388@TK2MSFTNGP11.phx.gbl...
> > The essence of a MustInherit form is that it's an abstract form: it can't
> > itself be instantiated. Think of an abstract Automobile class, marked
> > MustInherit, with subclasses such as Ford Explorer, Mazda Miata, etc. You
> > can't just walk into an automobile dealership and buy an Automobile. You
> > have to buy an instance of an instantiable subclass such as a Ford
> Explorer.
> > An abstract class can be thought of as a template or a design pattern,
> sort
> > of, and is most useful as a mechanism for polymorphism: that is, you can
> > downcast any instance of a subclass to the base class and invoke one of a
> > known list of members that are available for all subclasses of the base
> > class, while actually running the code that was written for the subclass
> > (although for completeness' sake I must note that you can force the base
> > class's method be be invoked, if you choose).
> >
> > Consequently, much depends on whether your base class is usable itself:
> that
> > is, do you want applications to be able to just create and use an instance
> > of the base class as a 'concrete class', or must they subclass it with
> > custom logic in all cases?
> >
> > If you don't want your base class to be instantiated, mark it MustInherit.
> > Even in a MustInherit class, you can supply default implementations of
> > properties and methods, so you can choose to code a generic
> > DisconnectFromAbrServer() method there, and it'll be run if the base class
> > implementor chooses not to implement a custom version of the method. You
> can
> > force it to be run in all cases, or you can allow it to be overridden in
> the
> > subclass through the overridable keyword.
> >
> > If you are OK with having your base class instantiated, you don't mark it
> > MustInherit. If a generic implementation of DisconnectFromAbrServer()
> method
> > will suffice for all subclasses if no other is available, you can
> implement
> > it in the base class. If you are OK with subclasses supplying their own
> > implementation, mark the method Overridable. If you REQUIRE that they
> > implement their own (i.e., a generic implemenation cannot be used), mark
> the
> > method MustOverride and don't supply any code in the base class.
> >
> > And document, document, document...
> >
> > This whole inheritance business, what with new, shadows, overrides,
> > overridable, mustoverride, mustinherit, etc., is pretty complex. I'd
> suggest
> > taking the time to write a testcase that exercises the various
> combinations
> > so that you can study the nuances. I've done this for myself in C#, but
> > haven't yet taken the time to do the same for VB, otherwise I'd post the
> > testcase.
> >
> > HTH,
> > Tom Dacon
> > Dacon Software Consulting
> >
> >
> > "Niklas" <Niklas@discussions.microsoft.com> wrote in message
> > news:7DF800BC-EDD9-470E-BA70-643888034AE7@microsoft.com...
> > > Hi
> > > I have made a base class and to be sure that developers do not forget to
> > implement DisconnectFromAbrServer I have done the following:
> > >
> > > Friend MustInherit Class frmBaseAbrForm
> > > Inherits System.Windows.Forms.Form
> > > Implements IAbrServerEnabled
> > > ...
> > > Friend MustOverride Sub DisconnectFromAbrServer() Implements
> > IAbrServerEnabled.DisconnectFromAbrServer
> > >
> > > In the derived class:
> > >
> > > Friend Class frmNewTocc
> > > Inherits frmBaseAbrForm
> > > ...
> > > Friend Overrides Sub DisconnectFromAbrServer()
> > > 'TODO: Add code.
> > > End Sub
> > >
> > > The problem is that the VS designer can not create an instance of
> > frmBaseAbrForm because it is declared as abstract and an exception is
> thrown
> > in design time. What can I do? Is it not posible to create Forms as
> > MustInherit?
> > >
> > > Regards
> > > /Niklas
> >
> >
> >
>
>
>

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