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? 64  3424 
How can something be broken when it functions as designed?
You also assume that a common scenario in C++ would be common in C#,
which is not a very good assumption either.
My answer would be create a static factory class which instantiates the
proper subclass. That is a pretty old pattern, predating C# or Java,
and you'd probably want to use it in C++ as well.
On Jan 26, 12:06 pm, gro...@isaacsof t.com wrote:
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?
Just a word of advice.
When posting on a newgroup of c#, don't come out with statements like 'c#
inheritence broken' just because you don't know the c# way of doing
something you are used to.
Better to say i can do this in c++ but can't seem to find a comparative
solution in c#, here is my problem etc etc. You'll find you get a much
better response.
<gr****@isaacso ft.comwrote in message
news:11******** *************@a 75g2000cwd.goog legroups.com...
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,
Thanks for your response. My assertion is that the DESIGN is broken!
But I'll settle for a work-around, if there is one.
If I understand your suggestion, I've already tried this. But...I
don't have any problem creating a new MyDocument object when I need it;
my problem is loading a MyDocument from a file. There is only one
method in Document that does this:
class Document
{
public static Document Load(string FileName);
}
I can certainly write a new Load method to override the base method, as
follows:
class MyDocument : Document
{
public static MyDocument Load(string FileName)
{
return Document.Load(F ileName);
}
}
The problem is, the above code won't compile, because it is not valid
to cast the Document returned by Document.Load to a MyDocument.
Does that make sense?
Tony
On Jan 26, 11:31 am, "Andy" <a...@med-associates.comw rote:
How can something be broken when it functions as designed?
You also assume that a common scenario in C++ would be common in C#,
which is not a very good assumption either.
My answer would be create a static factory class which instantiates the
proper subclass. That is a pretty old pattern, predating C# or Java,
and you'd probably want to use it in C++ as well.
On Jan 26, 12:06 pm, gro...@isaacsof t.com wrote:
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?- Hide quoted text -- Show quoted text -
Hi,
<gr****@isaacso ft.comwrote in message
news:11******** *************@a 75g2000cwd.goog legroups.com...
| C# is an impressive language...but it seems to have one big limitation
| that, from a C++ background, seems unacceptable.
Not a good start IMHO :)
| 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#.
Why not?
Now, if the class is declared as sealed or the only constructor it has is a
private default constructor, well then maybe the designer did not want that
the class to be inheritable in the first place :)
| 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),
This is an indication that the class is not creatable, and the default
constructor is not public
Then we have two options:
1- it's declared protected:
protected void Document() {}
2- It's declared private
private void Document() {}
In the first case you can derive from Document without any problem, in the
later then or the designer was trying to avoid what you want to do or
simply made a mistake :)
| >From reading LOTS of posts on this group, the standard answers are:
|
| 1. "Wrap" the object, as follows:
| class MyDocument
| {
| protected Document TheDocument;
| }
Honestly I haven't worked in C++ in a while but I could bet that a similar
situation can be created in C++.
| 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);
| }
| }
What if Document declare a private constructor? in this escenario there is
no way for MyDocyment to construct a Document and the compiler will complain
(the same escenario you have now in C# )
|
| 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?
Yes, and I hope that my explanation is clear enough to see what you missed.
Post back if still have questions
--
Ignacio Machin
machin AT laceupsolutions com
Hi,
"Andy" <an***@med-associates.comw rote in message
news:11******** **************@ v45g2000cwv.goo glegroups.com.. .
| How can something be broken when it functions as designed?
|
| You also assume that a common scenario in C++ would be common in C#,
| which is not a very good assumption either.
|
| My answer would be create a static factory class which instantiates the
| proper subclass. That is a pretty old pattern, predating C# or Java,
| and you'd probably want to use it in C++ as well.
I thikn that in the OP case either the writter of Document wrote it as a
"sealed" class or simply made an error. The class is clearly not creatable
and apparentely is also not derivable ( sealed )
It has nothing to do with C# capabilities.
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.
Tony
On Jan 26, 12:28 pm, "Ignacio Machin \( .NET/ C# MVP \)" <machin TA
laceupsolutions .comwrote:
Hi,
<gro...@isaacso ft.comwrote in messagenews:11* *************** *****@a75g2000c wd.googlegroups .com...
| C# is an impressive language...but it seems to have one big limitation
| that, from a C++ background, seems unacceptable.
Not a good start IMHO :)
| 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#.
Why not?
Now, if the class is declared as sealed or the only constructor it has is a
private default constructor, well then maybe the designer did not want that
the class to be inheritable in the first place :)
| 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),
This is an indication that the class is not creatable, and the default
constructor is not public
Then we have two options:
1- it's declared protected:
protected void Document() {}
2- It's declared private
private void Document() {}
In the first case you can derive from Document without any problem, in the
later then or the designer was trying to avoid what you want to do or
simply made a mistake :)
| >From reading LOTS of posts on this group, the standard answers are:
|
| 1. "Wrap" the object, as follows:
| class MyDocument
| {
| protected Document TheDocument;
| }
Honestly I haven't worked in C++ in a while but I could bet that a similar
situation can be created in C++.
| 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);
| }
| }
What if Document declare a private constructor? in this escenario there is
no way for MyDocyment to construct a Document and the compiler will complain
(the same escenario you have now in C# )
|
| 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?
Yes, and I hope that my explanation is clear enough to see what you missed.
Post back if still have questions
--
Ignacio Machin
machin AT laceupsolutions com
Hi,
<gr****@isaacso ft.comwrote in message
news:11******** **************@ q2g2000cwa.goog legroups.com...
| Andy,
|
| Thanks for your response. My assertion is that the DESIGN is broken!
There is nothing wrong with teh design of C#.
What is broken is the design of the class you are trying to use :)
| But I'll settle for a work-around, if there is one.
|
| If I understand your suggestion, I've already tried this. But...I
| don't have any problem creating a new MyDocument object when I need it;
| my problem is loading a MyDocument from a file. There is only one
| method in Document that does this:
|
| class Document
| {
| public static Document Load(string FileName);
| }
Bad Design maybe?
If you do this you are certainly limiting yourself in that no derived class
will be able to override Load()
| I can certainly write a new Load method to override the base method, as
| follows:
|
| class MyDocument : Document
| {
| public static MyDocument Load(string FileName)
| {
| return Document.Load(F ileName);
| }
| }
|
| The problem is, the above code won't compile, because it is not valid
| to cast the Document returned by Document.Load to a MyDocument.
|
| Does that make sense?
YES, and let me tell that again YES, IT DOES !
Not only that but it's the same IN ALL OTHER OOP LANGUAGES.
a MyDocument is a Document, the opposite (what you are trying to do) is not
true.
--
Ignacio Machin
machin AT laceupsolutions com
<gr****@isaacso ft.comwrote in message
news:11******** **************@ v45g2000cwv.goo glegroups.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.
Tony
Hi,
"Mythran" <ki********@hot mail.comwrote in message
news:e8******** ******@TK2MSFTN GP04.phx.gbl...
|
|
| <gr****@isaacso ft.comwrote in message
| news:11******** **************@ v45g2000cwv.goo glegroups.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 com
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