Designer can't display inherited form

[Phil]

VS 2008
I have a windows form that was created in the normal way using the form
designer. I do not wish to use this form directly though, I want to use it
as a basis for some other forms. I have added a MustOverride Sub and have
added MustInherit to the class definition. I then created a second form,
this time by adding a class module, and specifying that it inherits from my
first form.
It all works fine, but when I double-click on the second form in Solution
Explorer it tries to open it using the Form designer which displays an error
message: "The designer must create an instance of type 'MyProject.Form1' but
it cannot because the type is declared as abstract.".
Is there any way to either fix this error, so that it displays correctly in
the designer, or to stop the IDE from trying to use the designer and take me
straight into the code view? The first form (the abstract one) displays in
the designer with no problems.
TIA
Phil.

[Tom Shelton]

On 2008-09-09, Phil <N/Awrote:

VS 2008
I have a windows form that was created in the normal way using the form
designer. I do not wish to use this form directly though, I want to use it
as a basis for some other forms. I have added a MustOverride Sub and have
added MustInherit to the class definition. I then created a second form,
this time by adding a class module, and specifying that it inherits from my
first form.
It all works fine, but when I double-click on the second form in Solution
Explorer it tries to open it using the Form designer which displays an error
message: "The designer must create an instance of type 'MyProject.Form1' but
it cannot because the type is declared as abstract.".
Is there any way to either fix this error, so that it displays correctly in
the designer, or to stop the IDE from trying to use the designer and take me
straight into the code view? The first form (the abstract one) displays in
the designer with no problems.
TIA
Phil.

The error makes some sense, as the designer is trying to create an instance
of a MustInherit class (abstract class) - which is not possible. The way I
usually overcome this, is to remove the MustInherit and change the
MustOverride to Overridable (virtual) in the base class - then I will usually
throw a System.NotImplemented exception (you can create a custom exception
here if you want, but I don't usually bother) from the base class versions of the
methods. This will allow the designer to create the instance of the form, and
still force derived classes to implement the base class methods - unless they
want the exception :) To be clear, here is what a base form might look like:

Public Class BaseForm
Inherits System.Windows.Forms.Form

Public Sub New ()
InitializeComponent()
End Sub

Public Overridable Sub MakeMeDoCoolStuff ()
Throw New System.NotImplementedException("You didn't override me!)
End Sub
End Class

--
Tom Shelton

[Jack Jackson]

On Tue, 9 Sep 2008 16:28:44 +0100, "Phil" <N/Awrote:

>VS 2008
I have a windows form that was created in the normal way using the form
designer. I do not wish to use this form directly though, I want to use it
as a basis for some other forms. I have added a MustOverride Sub and have
added MustInherit to the class definition. I then created a second form,
this time by adding a class module, and specifying that it inherits from my
first form.
It all works fine, but when I double-click on the second form in Solution
Explorer it tries to open it using the Form designer which displays an error
message: "The designer must create an instance of type 'MyProject.Form1' but
it cannot because the type is declared as abstract.".
Is there any way to either fix this error, so that it displays correctly in
the designer, or to stop the IDE from trying to use the designer and take me
straight into the code view? The first form (the abstract one) displays in
the designer with no problems.
TIA
Phil.

Instead of double-clicking, right-click and select View Code.

[Phil]

The error makes some sense, as the designer is trying to create an

instance
of a MustInherit class (abstract class) - which is not possible. The way
I
usually overcome this, is to remove the MustInherit and change the
MustOverride to Overridable (virtual) in the base class - then I will
usually
throw a System.NotImplemented exception (you can create a custom exception
here if you want, but I don't usually bother) from the base class versions
of the
methods. This will allow the designer to create the instance of the form,
and
still force derived classes to implement the base class methods - unless
they
want the exception :)

Thanks. I had thought of just including a blank method and marking it
overridable, but I hadn't thought of raising an exception, that sounds like
the best option if there is no attribute I can add to tell the IDE not to
use the designer.

Another solution I have found is to add another class definition in the same
source code file. I have some other classes that inherit from control
classes. If I put one of these in the same source file, before the source
for the form, the designer will not try to work with the form and will
display a handy link to switch to code view.

Cheers,
Phil.

[Olaf Rabbachin]

Hi,

Tom Shelton wrote:

Public Overridable Sub MakeMeDoCoolStuff ()
Throw New System.NotImplementedException("You didn't override me!)
End Sub

that sure is a neat approach to the problem that I didn't think of when I
had the exact same problem a year ago (damn!). As a result, I gave all
methods and properties that needed to be overrided a common prefix (and I
still hate it).

Back then I also posted a question in the NGs about this. The answer I got
was to build a separate DLL with the base form and reference that in the
project where I'm creating inherited forms. In my case though the base form
itself needed access to some of the application's data layer classes, hence
I didn't try it out - it would've caused too much work to completely change
the application and I never found the time to actually give it a shot in
some test-project.

Robert, maybe you want to try that out ..? I'd sure be interested in the
results.

Cheers,
Olaf