C# preprocessor differentiate between operating systems

What you are asking for is possible but needs a bit of work.

1. Define a preprocessor variable in your csproj

   <PropertyGroup Condition=" '$(OS)' == 'Windows_NT' ">
     <DefineConstants>_WINDOWS</DefineConstants>
   </PropertyGroup>
   

2. Use that in your code

   #if _WINDOWS
     // your windows stuff
   #else
     // your *nix stuff
   #endif
   

I find this technique useful when you have constants that are dependent on the OS (for example native library names)