Re: MustInherit in Window forms

From: Nick Hall (nickh_at_aslan.nospam.co.uk)
Date: 07/23/04 

Date: Fri, 23 Jul 2004 13:39:08 +0100

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
> > >
> > >
> > >
> >
> >
> >

Relevant Pages

  • Re: MustInherit in Window forms
    ... > #If Debug then ... > Public MustInherit BaseClass ... > compile in Release mode. ... > the Base form to implement DisconnectFromAbrServer. ...
    (microsoft.public.dotnet.languages.vb)
  • Re: Debug Vs Release
    ... > If I compile that code in debug mode then dissassemble the EXE, ... > private static void Main(stringargs) ...
    (microsoft.public.dotnet.framework.aspnet)
  • Re: Getting a stack trace from a running application
    ... Unfortunately, the only infromation they can provide is a memory address, which is pretty useless as it doesn't tell me if my main exe is the culprit or if it's some dll or ocx or whatever. ... If you compile the the program with debug information, the debugger will show the responsible line of your code that caused the error. ...
    (comp.lang.basic.visual.misc)
  • Re: copymemory basic question
    ... I have used that technique before; that is, put a MessageBox in a VC program ... workspace and then debug the program without using the MessageBox or MsgBox. ... > - Compile and save. ... not able to show anything due to the optimization. ...
    (microsoft.public.vb.winapi)