It sounds like Hydra documentation will be forthcoming in a month or so (they shipped Hydra 3 last month, and they said they didn’t think the docs were worth delaying two months for, so the math works out to about a month left). In the meantime, importing interfaces using Hydra turns out to be easy.
In Delphi, there’s a “Hydra” menu in the menu bar (who’da thunk it?), and one of the menu items lets you browse to a .NET assembly and select which interfaces you want to import, at which point it creates a Win32 unit for you. If you do it again, it replaces the unit it generated the first time, which is good.
The IDE menu item is kind of nice to have, although I’d also want a command-line tool that I can tie into a rakefile or something, so I know the two sides don’t ever get out of sync. I’ve posted to their newsgroups asking if they have such a thing. If they don’t, I’ll probably bug them to write one. (grin)
Your .NET interface must descend from IHYCrossPlatformInterface, and must have a Guid attribute; otherwise the tool won’t import it. String return types seem to translate quite nicely, and I assume they work; they look about right. There are rumors that you can marshal complicated types like records if you write the MarshalAs stuff yourself; I’m not sure how that works.
There’s also a similar menu item in Visual Studio, which will read Delphi code and generate a corresponding .NET interface. I haven’t tried it yet, but it’s in the Tools > Hydra submenu.