SYMPTOMS
Unresolved external errors may occur when you compile with a default calling convention of __fastcall (/Gr) or __stdcall (/Gz) and call ConvertStringToBSTR or ConvertBSTRToString.
CAUSE
The prototypes are missing the __cdecl calling convention, which is the calling convention of these functions in comsupp.lib.
RESOLUTION
Build with __cdecl as the default calling convention. Do not build with __fastcall or __stdcall as the default calling convention.