Let’s say I have a directory A, and has a project file x.csproj and sln in that directory.
I also have resource file x.resources in it.
How can I set the resource file without the absolute path C:\A\x.resources, but relative path .\x.resources? I tried with x.resources but it doesn’t work.

Just ran into the same problem. My co-worker didn’t even try to fix it in the UI but just hacked the CSPROJ file in a text editor:
CSPROJ Path: C:\foo\bar\SolutionNm\ProjectNm\ProjectNm.csproj
Res Path: C:\foo\bar\Icons.res
Original:
Modified:
As long as you don’t try to modify the value in the UI again, it won’t complain
Cheers,
Aaron