When using another assembly, I prefer adding it to the References
section of the project that uses it (thus adding the appropriate /FU
option to the compiler command line) over an explicit #using directive
in the source code. So far, this has worked without any problems.

Recently, however, I tried to reference an assembly for which this just
doesn't work. I have added it to References, the emitted command line
contains the /FU option with the correct path, and yet the compiler does
not recognize any of its types. Once I add a #using directive of
identical content to the source file to be compiled - normally a
redundant step -, the problem goes away.

Has anyone ever experienced anything like this and can provide an
explanantion or at least a clue? Does it make a difference whether the
assembly to be used was written in C# or C++?