C# inheritance broken?

[groups]

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?

[Andy]

The code was:

class Employee { }
class VP extends Employee { }

Employee emp;
VP vp;

emp = new Employee();

vp = (VP)emp;

The book was Teach yourself Java 2 SDK 1.5 in 21 days from Sams and it
indicated code similar to the above would compile just fine.

Is that not the case? Or is there some detail that they didn't
mention at that point in the book?

On Jan 28, 12:17 pm, "Mike Schilling" <a...@newsgroup .nospamwrote:

"Andy" <ajj3...@alum.r it.eduwrote in messagenews:11* *************** *****@a75g2000c wd.googlegroups .com...

On Jan 26, 7:16 pm, "Bruce Wood" <brucew...@cana da.comwrote:

>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.

Actually, I was reading a Java book this weekend, and it seems that
you can do that in Java, likely with the same problems you would
encounter in C++.Casts are safe in Java, just as they are in C#. (That should be stated the

other way around, of course.)

[Jon Skeet [C# MVP]]

<gr****@isaacso ft.comwrote:

Yes, this makes sense. But System.Serializ ation doesn't allow this.
You cann't deserialize a stream into an already existing object.
Deserialization always creates a new object, and you can't assign the
newly deserialized object to 'this'.

Then you'll need to have something in the base class which allows you
to copy the data from an existing instance to a new one.

Think of serialization/deserialization as like dehydrating and
rehydrating vegetables - you wouldn't expect to dry out a carrot, then
rehydrate it and get a cauliflower!

--
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

[Jon Skeet [C# MVP]]

Andy <an***@med-associates.comw rote:

The code was:

class Employee { }
class VP extends Employee { }

Employee emp;
VP vp;

emp = new Employee();

vp = (VP)emp;

The book was Teach yourself Java 2 SDK 1.5 in 21 days from Sams and it
indicated code similar to the above would compile just fine.

Yes, it will compile fine, because the compiler doesn't look carefully
enough to see that emp is bound to just be an Employee, not a VP.

However, it will fail at runtime with a ClassCastExcept ion.

--
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

[Mike Schilling]

"Jon Skeet [C# MVP]" <sk***@pobox.co mwrote in message
news:MP******** *************** *@msnews.micros oft.com...

Andy <an***@med-associates.comw rote:

>The code was:

class Employee { }
class VP extends Employee { }

Employee emp;
VP vp;

emp = new Employee();

vp = (VP)emp;

The book was Teach yourself Java 2 SDK 1.5 in 21 days from Sams and it
indicated code similar to the above would compile just fine.

Yes, it will compile fine, because the compiler doesn't look carefully
enough to see that emp is bound to just be an Employee, not a VP.

However, it will fail at runtime with a ClassCastExcept ion.

And notice that exactly the same is true in C# (mod the name of the
exception).

[Ravichandran J.V.]

The Virtual constructor technique would allow this in C++, which you can
substitute with the Factory pattern in C#. There are some good examples
of the Factory pattern in C# on the web.

Or, you can also use a technique called Covariance in C# with the help
of delegates. More can be posted here if you are able to follow this far
and interested in using delegates in your code.

with regards,
J.V.Ravichandra n
- http://www.geocities.com/
jvravichandran
- Or, just search on "J.V.Ravichandr an"
at http://www.Google.com

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[Jon Skeet [C# MVP]]

Ravichandran J.V. <jv************ @yahoo.comwrote :

The Virtual constructor technique would allow this in C++, which you can
substitute with the Factory pattern in C#. There are some good examples
of the Factory pattern in C# on the web.

Or, you can also use a technique called Covariance in C# with the help
of delegates. More can be posted here if you are able to follow this far
and interested in using delegates in your code.

I fail to see how delegates are relevant here...

--
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

[Ravichandran J.V.]

"Covariance permits a method with a derived return type to be used as
the delegate,..." (msdn)

"When a delegate method has a return type that is more derived than the
delegate signature, it is said to be covariant. " (msdn)

It is just a technique that can substitute the virtual constructor
technique. It is not as if delegates should not be suggested if the
topic is on inheritance, is it? In this context, delegates help in
achieving the kind of covariance that is desired.

with regards,
J.V.Ravichandra n
- http://www.geocities.com/
jvravichandran
- Or, just search on "J.V.Ravichandr an"
at http://www.Google.com

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[Jon Skeet [C# MVP]]

On Jan 30, 9:18 am, Ravichandran J.V. <jvravichand... @yahoo.com>
wrote:

"Covariance permits a method with a derived return type to be used as
the delegate,..." (msdn)
"When a delegate method has a return type that is more derived than the
delegate signature, it is said to be covariant. " (msdn)

Indeed. I know all this... but it's not relevant.

It is just a technique that can substitute the virtual constructor
technique. It is not as if delegates should not be suggested if the
topic is on inheritance, is it? In this context, delegates help in
achieving the kind of covariance that is desired.

The OP wants to use an instance of a base class as if it were an
instance of a derived class. I still fail to see how delegate
covariance is relevant - all it means is that if you could use a
delegate which specifies that it returns an instance of MyDocument as
if it's an instance of a delegate which specifies that it returns an
instance of Document. That's not at all the same as what the OP's
trying to do.

In particular, it in no way allows you to convert an object of one
type to another type.

Jon

[Michael D. Ober]

Tom,

Do you have a an example of how to do this with reflection?

Mike Ober.

"Tom Shelton" <to*********@co mcastXXXXXXX.ne twrote in message
news:Xq******** *************** *******@comcast .com...

On 2007-01-27, Bruce Wood <br*******@cana da.comwrote:

>>A generalized method of converting a base class into a derived class
with
the knowledge that the derived class may still require some
initializatio n
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.

Agreed. That would be nice. I wouldn't need it often, but it would get
one out of some tricky corners. My only point was that the whole "I
have a base class instance and I want to treat it as though it were a
derived class instance" is completely unsafe and therefore runs
against the grain of what .NET is supposed to be all about. Being able
to copy state, though, would be handy.

Reflection :)

--
Tom Shelton

[groups]

Jake Stevenson's posted an example earlier on this thread.

That's all fine, but it does not get around the cost of *copying* the
object.

I suppose that what I really want here is a sort of "reference"
constructor, something like the "Attach" method in ATL's CComPtr
class. Come to think of it, CComPtr actually "wraps" the interface
being used, but its wrapper is generated automatically by a
preprocessor. Too bad C# doesn't have an automatic wrapper generator
similar to the #import directive in C++!

Tony

On Jan 30, 7:55 am, "Michael D. Ober" <obermd.@.alum. mit.edu.nospam>
wrote:

Tom,

Do you have a an example of how to do this with reflection?

Mike Ober.

"Tom Shelton" <tom_shel...@co mcastXXXXXXX.ne twrote in messagenews:Xq* *************** **************@ comcast.com...

On 2007-01-27, Bruce Wood <brucew...@cana da.comwrote:

>A generalized method of converting a base class into a derived class
with
the knowledge that the derived class may still require some
initializati on
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.

Agreed. That would be nice. I wouldn't need it often, but it would get
one out of some tricky corners. My only point was that the whole "I
have a base class instance and I want to treat it as though it were a
derived class instance" is completely unsafe and therefore runs
against the grain of what .NET is supposed to be all about. Being able
to copy state, though, would be handy.

Reflection :)

--
Tom Shelton- Hide quoted text -- Show quoted text -