How can I list the properties of an object programmatically in VB6?

Question

A friend of mine is working on a legacy VB6 project. I haven't touched that language in ten years, so I'm pretty rusty. Anyway, is there any kind of reflection API for VB6? Specifically, he needs a way to iterate the properties (and types) of a user-created Class. (In other words, not an external COM object, but an internal "Class Module" as it's called.)

How can this be done?

Answer 1

Is this the sort of thing you're looking for:

Visual Basic: Inspect COM Components Using the TypeLib Information Object Library

It's discussed on this thread here on SO: Self Inspection of VB6 UDTs

I've never tried this stuff myself.

Answer 2

Jay's answer is the way to go if your project is an ActiveX (ActiveX EXE, DLL, or OCX, as opposed to Standard EXE), and if the classes are public.

However, you mentioned that your friend wants to do this with "internal class modules". I'm not sure if you are referring to private .cls files (classes), or .bas files (modules), but either way, you can only use the TypeLib Information Object Library to reflect on public classes, user-defined types, constants, or enumerations.

You cannot use the library to reflect modules, private classes, or anything else that is declared private.

As a general rule of thumb, you can only use reflection on the things that you can see in the Object Browser when viewing your project's contents from another project. That is, if you compile your project, create a new project, add the first project as a reference, and then view the first project's contents in the Object Browser, anything you can see in the Object Browser can be accessed via the TypeLib Information Object Library. If something is not listed in the Object Browser, then you won't be able to use the TypeLib Information Object Library to reflect it.

VB6 doesn't have any built-in support for run-time reflection or introspection. Using the TypeLib Information Object Library for reflection works for ActiveX VB6 projects because ActiveX projects are compiled in COM components with embedded type libraries, but as mentioned you can only access the data types that are publicly exposed in the compiled component's type library. Using the Object Browser is a quick way to determine what is in the type library because the Object Browser actually inspects the component's type library to populate what you see in the Object Browser, as long as you are viewing the component's contents from a separate project (if you view the component from its own VB6 project, it will display public and private data structures, i.e. everything that is visible in the IDE).