Securing source code when distributing debug .NET .dll files?

Question

I've got a piece of web software which has a private component (a computational .DLL) that is consumed by a jointly developed website (ASP.NET). I develop the private .DLL and wanted to make sure that even when compiled as a debug .dll, it doesn't display/leak any of the private source code when the web-site developers are debugging the site and then step into calls that are inside the private .dll.

1. Are there any special compile flags/settings or techniques to protect the resulting .dll ? Preferably something I can setup within Visual Studio 2010 without expensive add-ons.

2. Not knowing the structure of windows binaries/dlls, is it even possible for the source to be embedded as meta data in the resulting binaries (.dll or .pdb) ? If it's not veen possible for the source to be embedded into the debug .dll/pdb then I guess I can drop my paranoia levels ... (FYI, some embedded systems concat the source and symbols at the end of the debug binary image)

Any other tips and points on securing IP inside DLLs is also appreciated.