Re: MustInherit in Window forms
From: Tom Dacon (tdacon_at_community.nospam)
Date: 07/24/04
- Next message: - HAL9000: "Re: Differance between ByVal and BeRef when handling objects"
- Previous message: Bill Rowell: "textbox text property"
- In reply to: Nick Hall: "Re: MustInherit in Window forms"
- Messages sorted by: [ date ] [ thread ]
Date: Fri, 23 Jul 2004 21:36:46 -0700
This is good.
Tom
"Nick Hall" <nickh@aslan.nospam.co.uk> wrote in message
news:eQunzILcEHA.3420@TK2MSFTNGP10.phx.gbl...
> Niklas,
>
> The way I've done this before is to use good old conditional compilation
> like so
>
> #If Debug then
> Public Class BaseClass
> #Else
> Public MustInherit BaseClass
> #End If
> ....
> #If Debug Then
> Protected Sub OverridableMethod()
> Throw New InvalidOperationException("Method not overriden!")
> End Sub
> #Else
> Protected MustOverride Sub OverridableMethod()
> #End if
> ....
>
> This means you get a runtime exception in debug builds if you call the
> method BUT you can use the Designer, or a compile time error when you
> compile in Release mode.
>
> Hope this helps,
>
> Nick Hall
>
> "Niklas" <Niklas@discussions.microsoft.com> wrote in message
> news:02641378-F4C8-4C9F-9932-CFA3AEA53DBD@microsoft.com...
> > 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
> > > >
> > > >
> > > >
> > >
> > >
> > >
>
>
- Next message: - HAL9000: "Re: Differance between ByVal and BeRef when handling objects"
- Previous message: Bill Rowell: "textbox text property"
- In reply to: Nick Hall: "Re: MustInherit in Window forms"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
