[nem-en] Multiple project solutions

[vc]

> Our assumption from the beginning was that macros will be used by
> advanced programmers. I guess the special API and markers are a way to
> go instead of limiting features at all.

It's rationally. But, Murphy's laws wins. :)

> Is it because of some macro adding code? Probably all macros (maybe
> even those running from attributes) adding new stuff should check if
> they are not duplicating anything. The problem can be observed when
> compiling methods with such expression level macros, which contain
> some error - compiler tries to compile again in error mode and this
> yield duplicate members.
> Currently the adviced solution is to check if compiler is in ErrorMode
> before generating new classes/ members.

It's bad way. ErrorMode may be caused by error in code.

> Define*  are such methods. They could check if we are already in
> method bodies compilationg stage AND in intellisense mode.

+1 

But in this case we have different behavior.