Should Class Helpers be used in developing new cod

Question

Delphi 8 introduced Class Helpers for the purposes of mapping the VCL/RTL to the .NET object hierarchy. They allow injecting methods into an existing class without overriding the the class or modifying the original. Later versions of Delphi found class helpers improved and they were ported to Win32.

In the help it says "they should not be viewed as a design tool to be used when developing new code."

Class Helpers violate traditional OOP, but I don't think that makes them a bad thing. Is this warning warranted?

Should class helpers be used when developing new code?

Do you use them when developing new code?

Why or why not?

Per Malcolm's comments: New code means daily application development, where you have some 3rd party libraries, some existing code, and then code you are writing.

Answer 1

Maybe a good aproach you can use is (as I use it):

1. Always give preference to inheritance over class helpers, use them only when inheritance is not an option.
2. Give preference to Class helpers over bare global methods.
3. If you're going to need the extendend functionality in more than a Unit, try something else (like class wrappers).

.Net Extensions methods are way too similar and where created and supported for the exactly same reason: Make an Extention of the base classes (rather than an upgrade wich in Delphi.Net was not an option in order to try to make Delphi native code kind of "compatible" with .Net code - IMHO this was too ambitious)

Anyway, Delphi Class helpers are still quite a tool in some situations.

Answer 2

These sound like C# extension methods. I would say that while extension methods like these are useful when you don't have the ability to modify a class that you need to extend with functionality, they are a poor way to design your own code. When designing your own code, you'd like all the functionality to be located in the same code file as much as possible rather than spread across different classes. I'd say use them for what they were intended for -- basically as decorators to add new functionality to closed classes -- and don't use them in designing your own code.

Answer 3

Depends what you mean by "new code".

They aren't really relevant for classes you are newly developing, so in that case, no, they probably shouldn't be used.

But even in a brand new project, you may still need to modify an existing class that you can't change in other ways (vcl class, third-party class, etc). In this case, sure, I'd say go ahead.

They're not evil in and of themselves. Like most other things, you just need to understand how they work and use them in an appropriate context.

Answer 4

I use them a lot. I use Remote Objects and the objects there are created by the RO engine so you cannot add to them without descending from them and then other bits of messing around. Class Helpers mean I can treat them like any other object. and while a class can only have one helper, you can descend helper classes so you get the inherited behaviour.

Answer 5

Microsoft based LINQ heavily around their Extension Methods. In that light you should use Class Helpers in new code if that improves your code. See What are good uses for class helpers? for some good uses.

Answer 6

Sorry, can't help but be Captain Obvious for a moment: If the internal Delphi people themselves state "they should not be viewed as a design tool to be used when developing new code" then by definition they shouldn't be used. They are there for extending the VCL for their own purposes only. Who else is going to give you a better reason than the people that wrote it?