C# is an impressive language...but it seems to have one big limitation
that, from a C++ background, seems unacceptable.
Here's the problem:
I have a third-party Document class. (This means I can't change the
Document class.) I want to extend this (inherit from Document) as
MyDocument, adding new events and application-specific methods and
properties.
I submit that this can't be done in C#.
Consider this example:
class MyDocument : public Document
This doesn't work because the only way to open an existing Document is
using the static method Document.Load(s tring FileName), which returns a
Document object. There doesn't seem to be any way to convert or cast a
Document object to a MyDocument object. There is therefore no way to
"Load" a MyDocument object!
>From reading LOTS of posts on this group, the standard answers are:
1. "Wrap" the object, as follows:
class MyDocument
{
protected Document TheDocument;
}
But...this means that I have to wrap EACH of the hundreds of methods
and properties needed to manipulate the Document object. This is
clearly a case for inheritance, to allow the base class properties and
methods to be automatically available, with enhancements from the
derived class.
2. In MyDocument.Load , call Document.Load, and then COPY all of
Document's members to MyDocument.
BUT, this is prohibitive because COPYING all of Document's members is
prohibitive in terms of run time, and also in terms of development
effort (lines of code, and therefore, potential bugs).
In C++, I would do something like this:
class MyDocument : public Document
{
public static MyDocument Load(string FileName)
{
return (MyDocument)Doc ument.Load(File Name);
}
}
So my conclusion is that C# inheritance is broken (in practice) because
of its strict type-checking, and no allowance for such a common
scenario.
Did I miss something?
Jan 26 '07
64  3451 
Mark,
Yes, reinterpret_cas t is indeed unsafe. But if there are no added
members, it works, as in CRect : RECT.
But this strays from what I'm hoping to accomplish.
How is it that serialization can magically create a Document object
through casting an "object" to "Document," but I'm not allowed to use
the same trick to create a MyDocument instead? The Serialization
classes don't know about Document while deserialization is occuring,
but it still lets you cast the return value of Deserialize into a
Document type. What is the trick they are using?
Tony
On Jan 26, 2:30 pm, "Mark Wilden" <mwil...@commun itymtm.comwrote :
<gro...@isaacso ft.comwrote in messagenews:11* *************** ******@k78g2000 cwa.googlegroup s.com...
As I pointed out before, C++
does indeed allow you to write such a "helper" class; you can use
reinterpret_cas t to transform Document into a MyDocument.Is this quite true? My understanding is that <reinterpret_ca stsimply
performs an explicitly unsafe case of a pointer. It doesn't transform
anything. In this case, if a Document is created, it can never be
"transforme d" into another type.
///ark
On Jan 26, 12:53 pm, gro...@isaacsof t.com wrote:
Mark,
Yes, reinterpret_cas t is indeed unsafe. But if there are no added
members, it works, as in CRect : RECT.
But this strays from what I'm hoping to accomplish.
How is it that serialization can magically create a Document object
through casting an "object" to "Document," but I'm not allowed to use
the same trick to create a MyDocument instead?
Because .NET doesn't trust you. :-)
One of the goals of .NET was to produce code that was immune to the
sort of run-time memory hacking that is rampant in C and C++. If the
runtime lets you play tricks like reinterpret_cas t, then the code can't
be verifiably safe, can it?
As I said in my other post: different design goals. One goal of C++ was
to give the programmer total power to do whatever he wants, and the
compiler will trust him to get it right (by generating code no matter
what it might do at runtime). One goal of C# is to produce only code
that can be verified to not clobber memory / reinterpret memory
patterns as that which they are not / etc.
The two design goals are, obviously, incompatible.
If you want to do this sort of thing in C#, you have to use the
"unsafe" keyword to tell the compiler / runtime to lay off, but even
then I'm not sure of just how far you can go with that, because I've
never used it.
Hi,
<gr****@isaacso ft.comwrote in message
news:11******** **************@ s48g2000cws.goo glegroups.com.. .
| >You build it of course ;-)
|
| Hmmm...I'm not sure how that would happen. I'm still stuck with the
| fact that the third-party component's Load method returns a Document,
| which can't be converted to type MyDocument, even in DocumentLoader.
You simply have a crappy component :)
Use other
--
Ignacio Machin
machin AT laceupsolutions com
Hi,
"Michael D. Ober" <obermd.@.alum. mit.edu.nospamw rote in message
news:45******** *************@n ews.qwest.net.. .
| --
| Ignacio Machin
| machin AT laceupsolutions com
| >
| This isn't a C# issue. It is a dotNet issue as I have run into the same
| issue of inheriting a base class (in my case, VB 2005 and
StringCollectio n)
| and providing a method for the base class object to become the object at
the
| heart of the derived class.
Not at all, A base class cannot become a derived class, A derived class can
be treated as a base class without any problem.
Think about, what would hppen if the derived object add a new property,
clearly the base does not has it , so what can the compiler do in this
case??
| This is a design limitation in the framework
No, it's simply bad design of the base class, or trying to made a bad use of
the class.
On Jan 26, 11:36 am, "Michael D. Ober" <obermd.@.alum. mit.edu.nospam>
wrote:
"Ignacio Machin ( .NET/ C# MVP )" <machin TA laceupsolutions .comwrote in
messagenews:%2* **************@ TK2MSFTNGP06.ph x.gbl...
Hi,
"Mythran" <kip_pot...@hot mail.comwrote in message
news:e8******** ******@TK2MSFTN GP04.phx.gbl...
|
|
| <gro...@isaacso ft.comwrote in message
|news:11******* *************** @v45g2000cwv.go oglegroups.com. ..
| Ignacio,
| >
| Thank you for your thoughts.
| >
| The Document class does have a public default constructor, and it is
| not sealed. I have no problem creating a "new" Document object, or a
| "new" MyDocument object, for that matter. What I can't do is
"convert"
| an existing Document object (such as the one returned by
Document.Load)
| into a MyDocument.
As I said in another post, giving the struct you are presenting you would
have the same issue no matter what language you use. You cannot treat a
base
class as a derived class, you can do the opposite though.
In your case you will have to implement your own Load method. I would
first
check though if Document provide a Load method instance. or maybe a Clone
method
--
Ignacio Machin
machin AT laceupsolutions comThis isn't a C# issue. It is a dotNet issue as I have run into the same
issue of inheriting a base class (in my case, VB 2005 and StringCollectio n)
and providing a method for the base class object to become the object at the
heart of the derived class. This is a design limitation in the framework
itself. From a business logic perspective, it should be doable.
For the record, no language that I know of lets you do this. C++ allows
you to pretend to do it via an unsafe cast, but you're not really
getting a derived object, just playing with a base class object as
though it were a derived object, which is fodder for disaster unless
you keep careful track of what you're doing.
You can't use a base class instance as though it were a derived class
instance because the latter may have more fields than the former, and
so the base class instance doesn't have enough space to store the state
of the derived class.
There _are_ various strategies for using a base class instance as a
model for creating a derived class. Most of them, however, require some
coding. There's no "automatic" way of copying state from one instance
to another, which might be a nice addition.
On 26 Jan 2007 12:26:15 -0800, gr****@isaacsof t.com wrote:
>Here is some more relevant information that I've just learned:
The Document.Load static method relies on System.Runtime. Serialization
to create a new Document object. Formatter.Deser ialize returns an
object that is a Document, and this Document object can't be cast to a
MyDocument. Deserialization can't occur in a constructor, because the
Document object already exists, and you can't replace "this" with the
new Document object.
So it would seem that the limitation is there because of the way
serializatio n works, not because of a bad design on the part of the
author of Document.
Tony
Taking a completely different tack, how easy would it be for you to
parse through the Serialized Document file and change it to whatever
the equivalent Serialized MyDocument file would look like?
Given that you are trying to upcast the derived class, which goes
against the design goals of C#, this might be an easier way to work
round your problem.
rossum
No, it's simply bad design of the base class, or trying to made a bad use of
the class.
OK, please enlighten me. What would be the proper design of the base
class? I see no way to inherit from such a base class, regardless of
the base class design!
Tony
On Jan 26, 4:44 pm, "Ignacio Machin \( .NET/ C# MVP \)" <machin TA
laceupsolutions .comwrote:
Hi,
"Michael D. Ober" <obermd.@.alum. mit.edu.nospamw rote in messagenews:45* *************** *****@news.qwes t.net...
| --
| Ignacio Machin
| machin AT laceupsolutions com
| >
| This isn't a C# issue. It is a dotNet issue as I have run into the same
| issue of inheriting a base class (in my case, VB 2005 and
StringCollectio n)
| and providing a method for the base class object to become the object at
the
| heart of the derived class.
Not at all, A base class cannot become a derived class, A derived class can
be treated as a base class without any problem.
Think about, what would hppen if the derived object add a new property,
clearly the base does not has it , so what can the compiler do in this
case??
| This is a design limitation in the framework
No, it's simply bad design of the base class, or trying to made a bad use of
the class.
"Bruce Wood" <br*******@cana da.comwrote in message
news:11******** **************@ a75g2000cwd.goo glegroups.com.. .
>
On Jan 26, 11:36 am, "Michael D. Ober" <obermd.@.alum. mit.edu.nospam>
wrote:
>"Ignacio Machin ( .NET/ C# MVP )" <machin TA laceupsolutions .comwrote
in
messagenews:%2 *************** @TK2MSFTNGP06.p hx.gbl...
Hi,
"Mythran" <kip_pot...@hot mail.comwrote in message
news:e8******* *******@TK2MSFT NGP04.phx.gbl.. .
|
|
| <gro...@isaacso ft.comwrote in message
|news:11******* *************** @v45g2000cwv.go oglegroups.com. ..
| Ignacio,
| >
| Thank you for your thoughts.
| >
| The Document class does have a public default constructor, and it
is
| not sealed. I have no problem creating a "new" Document object, or
a
| "new" MyDocument object, for that matter. What I can't do is
"convert"
| an existing Document object (such as the one returned by
Document.Load)
| into a MyDocument.
As I said in another post, giving the struct you are presenting you
would
have the same issue no matter what language you use. You cannot treat a
base
class as a derived class, you can do the opposite though.
In your case you will have to implement your own Load method. I would
first
check though if Document provide a Load method instance. or maybe a
Clone
method
--
Ignacio Machin
machin AT laceupsolutions comThis isn't a C# issue. It is a dotNet
issue as I have run into the same
issue of inheriting a base class (in my case, VB 2005 and
StringCollecti on)
and providing a method for the base class object to become the object at
the
heart of the derived class. This is a design limitation in the framework
itself. From a business logic perspective, it should be doable.
For the record, no language that I know of lets you do this. C++ allows
you to pretend to do it via an unsafe cast, but you're not really
getting a derived object, just playing with a base class object as
though it were a derived object, which is fodder for disaster unless
you keep careful track of what you're doing.
You can't use a base class instance as though it were a derived class
instance because the latter may have more fields than the former, and
so the base class instance doesn't have enough space to store the state
of the derived class.
There _are_ various strategies for using a base class instance as a
model for creating a derived class. Most of them, however, require some
coding. There's no "automatic" way of copying state from one instance
to another, which might be a nice addition.
A generalized method of converting a base class into a derived class with
the knowledge that the derived class may still require some initialization
would definitely be useful, even if you had to do it via a constructor that
takes an object of the base class as it's argument. The missing syntax, in
vb is
mybase = objBaseClass
In C# I think it would be
base = objBaseClass
In either case, since this is occurring in a constructor in place of the
base.New() or mybase.New statement, the programmer knows he still has to
instantiate and initialize all other private and protected objects in the
derived class.
Mike Ober.
<gr****@isaacso ft.comwrote:
No, it's simply bad design of the base class, or trying to made a bad use of
the class.
OK, please enlighten me. What would be the proper design of the base
class? I see no way to inherit from such a base class, regardless of
the base class design!
Instead of having a static method which returns a Document, create an
*instance* method which loads data into an existing instance. That's
the way XmlDocumentLoad works, for example.
--
Jon Skeet - <sk***@pobox.co m> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too
What is wrong with this approach?
class MyDocument : Document
{
public void CopyFromDocumen t(Document Original)
{
foreach (System.Reflect ion.PropertyInf o Prop in
Original.GetTyp e().GetProperti es())
{
if (Prop.CanWrite)
{
Prop.SetValue(t his, Prop.GetValue(O riginal, null),
null);
}
}
foreach (System.Reflect ion.FieldInfo Field in
Original.GetTyp e().GetFields() )
{
Field.SetValue( this, Field.GetValue( Original));
}
}
}
Something similar can be implemented in the base class and would be
inherited by your class. There may be a performance issue with
Reflection though.
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