How to add a member field to an existing class in runtime?

[Mannie]

How can I add a member field to an already existing class during runtime, or
rather: add some hidden value to an object dynamically during runtime?

I need this because I'm creating a simple OO database which needs references
to objects when updating. The value that I want to add on the fly is the
object id.

I know there are many examples using Reflection.Emit showing how to
dynamically create a new class and adding things to it, but that's not what
I want. I really need to use existing classes.

Thanks!
Manfred Suttorp

 

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

If adding members dynamically would even be possible, it's far from a
good idea. It's my personal opinion that reflection should be used as
little as possible, and the idea of altering an existing class goes far
beyond that. It breaks the fundamental principles of object oriented
programming.

Can't use inheritance to expand on the class?

public class MyClass : ExistingClass {

private int id;

public MyClass(int id) : base() {
this.id = id;
}

public int Id { get { return this.id; } set { this.id = value; } }

}

 

[Mannie]

Thanks for your reply Göran.
Whilst I do agree that one should obey the fundamental principles of OOP, I
want the trick in this case anyway.
Deriving from existing classes is simply not acceptable here: what if some
sealed class must be persisted in my OO database? I would have to resort to
a wrapper class. It creates horrible code.
Also, the name of <ExistingClass> is not known to my OO database in advance,
so any code using the database must use a wrapper (be it a derived class or
a container, it's the same in this case) for every possible Type that I wish
to persist. I simply don't want that. The nitty-gritty details of the
persistence mechanism should be irrelevant to the consuming code and so it
should not be bothered with things like its object id if it's not essential
to its reason of existence. Also, one could argue that subclassing for the
sake of facilitating some low-level technical requirement, having nothing to
do with specialization is not very good OO practice either.

More suggestions are very welcome. One other thing though - and no offence
meant to Göran - I've seen a lot of good and bad practices, patterns and
antipatterns in my career. I'm not really interested in more lectures about
them. All I want is a good answer to my question.

Regards,
Manfred

 

[Ben Voigt]

How can I add a member field to an already existing class during runtime,
or rather: add some hidden value to an object dynamically during runtime?

try one of the dynamic languages JScript.NET or IronPython?

 

[Damien]

Thanks for your reply Göran.
Whilst I do agree that one should obey the fundamental principles of OOP,I
want the trick in this case anyway.
Deriving from existing classes is simply not acceptable here: what if some
sealed class must be persisted in my OO database? I would have to resort to
a wrapper class. It creates horrible code.
Also, the name of <ExistingClass> is not known to my OO database in advance,
so any code using the database must use a wrapper (be it a derived class or
a container, it's the same in this case) for every possible Type that I wish
to persist. I simply don't want that. The nitty-gritty details of the
persistence mechanism should be irrelevant to the consuming code and so it
should not be bothered with things like its object id if it's not essential
to its reason of existence. Also, one could argue that subclassing for the
sake of facilitating some low-level technical requirement, having nothingto
do with specialization is not very good OO practice either.

More suggestions are very welcome. One other thing though - and no offence
meant to Göran - I've seen a lot of good and bad practices, patterns and
antipatterns in my career. I'm not really interested in more lectures about
them. All I want is a good answer to my question.

Regards,
Manfred

Hi Mannie,

It sounds like you're looking for something along the lines of aspect
oriented programming, or at least I've read articles on these topics
which deal with some interesting concepts (like providing proxies which
act like the instances of a class but have additional behaviour bolted
on). The typical examples used with AOP tend to be adding logging
transparently, but I think other things (like adding additional
members) may also be possible.

If you've already explored this area, apologies for wasting your time.
If you're interested in this area then... Sorry, haven't done any
myself, just read the articles. I'd suggest googling for aspect
oriented programming and seeing where you can go from there.

HTH

Damien.