target state: Porting VB6 Code into C#, undertake the whole project with all conceivable processes that are involved.
What would be your approach if you do not have knowledge about the domain?
There is nearly any documentation, just legacy code (up to 100.000 - 300.000 lines of code and comments vb6 files that contain up to 14.000 lines of code) written in VB6.
Disclaimer: I work for Great Migrations
We rewrite large VB6/ASP/COM applications to .NET (primarily C#) for a living and we have developed a software analysis and reengineering tool to help us do it. This tool is essentially like a VB6/ASP/COM compiler and a decompiler that authors .NET codes. Of course since the VB6 platform is very different from .NET, a direct compile/decompile is not desirable or viable, so our tool has an "analyzer" that implements various code reengineering algorithms to deal with VB6-C# incompatibilities. There is also a programmable "author" that allows the migration team to prescribe rules for setting up .NET code files, restructuring the code, and doing things like replacing COM APIs and ActiveX controls with .NET classes -- depending on what the team needs or wants.
As a by product of compiling and analyzing the code our tool produces a model of the entire VB6/ASP/COM system being upgraded. This model can be used to produce extremely detailed reports about the internal structure of the system. These models can be used to help reverse-engineer the code -- if you know the right questions to ask and you would need to understand the problem domain to do a good job.
Of course once you have build-complete .NET, you can use the various analytics and code review tools that work off assemblies. Some versions of Visual Studio have these tools and there are open source tools such as FxCop, NDepends). There are also some fantastic dynamic analysis tools (EQUATEC Tracer) that I have used.
In the end though migration teams are going to very hard pressed to verify any unknown system. Even if you are staying on the same platform, you would unable to prove an application it is "correct" if you do not know how to run it and how to setup/enter expected inputs and find/verify expected outputs. We normally leave this to the customer!
If we are doing verification for the customer, we rely heavily on side-by-side testing to validate the new version of your system -- assuming we know how to run the legacy application we assume that given the same sets of inputs and usecases it should exhibit the same behaviors and produce the same results. I have heard this Approval Tests in unit testing circles.
I admit we also rely heavily on the knowledge that the VB6/COM code is a complete, detailed, formal and production tested description of the data structures and logic of the system and that we are putting this information through a tested and retested systematic transformation. We have been developing compilers since 1977 and we have worked very hard on this VB6/ASP compiler to make sure the .NET codes that we generate preserves the semantics of the original VB6. It is not 100% every time - but it is getting closer all the time. Then again doing things by hand does not guarantee 100% correct code on the first try either...
mark's answer about Great Migrations is excellent. Do be aware there are competitor automatic tools, which also have a very good reputation.