Yep, it was definitely our being relative noob's to C# (as I suspected) that was the problem.
Great!
But I don't think it's our noobishness (is that a word?) that's the main problem... It strikes me as being a problem with the API. Why mix COM and .NET like that?
Well, I know why, it's because Autocad is primarily legacy code and they're sticking a .NET wrapper around the thing, but still, this strikes me as hole in the wrapper. As such, it would be nice to have it documented, rather than expect all .NET programmers to be fluent in COM...
I agree, it's something that we probably would've eventually figured out. I already suspected this, because I had looked at various Breaklines collections in the debugger and noticed that they were COM objects, but I had no idea how to deal with it in C#. But hopefully, now that we're hammering Autodesk with complaints, they'll either document the usage, or better yet, fix the wrapper...
Still, now I can get back to re-writing Land Desktop's "Breakline By Points" routine for Civil-3D, since Autodesk decided we didn't need that functionality any more... (Yeah, I know it's relatively easy to draw 3D-Polylines and then add them to a surface, but it's annoying trying to keep track of which polylines have been turned into breaklines and which ones haven't, make sure things are on the right layer, yadda yadda.)