For C# codebases that have no documentation I use a script that has evolved over the years that generates a private documentation collection for myself.

It scans for static methods and data access classes using reflection and dumps their signature/return & parameter information into a documentation template. That way I can see what exits to leverage when adding new features - having spent way too much time over the years listening to 'why didn't you leverage this?' in code reviews.

I'm currently working on a new one that scans for stored procs in databases to see what's living in the dark down there.