I tried to look for an answer regarding this question but could not find a topic discussing it comprehensively.
For me, it is a bit unclear how the licensing goes when interfacing proprietary software with the GPL licensed WinSCP.exe through an MPL licensed wrapper library. In the set up, the interfaces communicate as below:
Proprietary module <=> winscpnet.dll (MPL) <=> WinSCP.exe (GPL)
When looking at the GPL terms, the latter part section 5 of GPL in my opinion states that if you distribute the combination of winscpnet.dll and WinSCP.exe somewhere, the both need to be licensed as GPL. https://www.gnu.org/licenses/gpl.html
"A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate."
As winscpnet.dll is an extension to WinSCP.exe, the combination of the two cannot be considered as an aggregate. Therefore 5 c) applies and winscpnet.dll is also GPL.
"c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it."
Whether GPL would now affect the proprietary module in the description above is in my opinion the question of whether we consider it as an extension to WinSCP or not. In my case the answer is not.
However, I am wondering what was the original purpose of licensing the winscpnet.dll under a separate MPL license? Can you shed light on that?