How to fix "Unexpected error (32801)" when compiling VB 6 program?

Question

I have to maintain an old VB 6 ActiveX DLL called by another third-party program for which I have no sources. This DLL works and compiles fine against the API of said program for about 6 years and 3 major versions.

But now when I try to compile the DLL against a new major version the mentioned error occurs. It seems the error occurs before "my" code is called so there´s no use debugging or logging. The only remedy was to compile w/o binary compatibility which is no real option. My Google search turned up quite some people with the same problem but no solution.

Does anybody here know how to fix this issue ?

Answer 1

I finally figure out how to diagnose VB6 error 32801 in a systemic way.

My theory is When the VB6 compiler is creating a project or binary compatible library, the compiler decompiles the type information from the referenced library. Error 32801 occurs the source code's type information is not the same as the referenced library.

There is a tool called OLEView. This tool can decompile the COM type information into an IDL text. What I do is decompile the referenced library in to IDL and take the last good build of the failing library. Most times it is a build server version but the build does not work on a developer workstation. Decompile the last good build. Use a text comparison tool, like WINMerge, and find the differences between the type libraries. The differences make it easy to track down the problem.

Depending on the difference will determine how to correct. Mitigation can be done by either correcting the reference DLL, or by source code correction, or source code references.

Answer 2

It sounds like one of the types in the interfaces defined in your new DLL is different from one in the previous DLL. I'm deducing you use types defined in the third party program in your public interfaces of your DLL. It sounds to me like the third party has changed the definition of one of the types but kept the name and GUIDs the same. You could use something like OLE/COM Object viewer to check whether that's true. If it is true then you can complain to the publisher of the 3rd party program. Do you have enough political power to succeed?

Bruce McKinney, the guru who wrote Hardcore Visual Basic 6, ran into the same issue with a structure in a type library, where he changed some of the member types. The only fix he could find was (essentially) to break binary compatibility - and that's after some correspondence with the VB6 compiler team, who he knew fairly well. I don't think anyone else could do better.