[littlecranky67]

> That means all .NET EXEs and DLLs will be cross-compatible with Ubuntu since 5.0

Well, if compiled for the correct target framework, then yes. See https://learn.microsoft.com/en-us/dotnet/standard/frameworks

Note that .NET nowadays is cross-plattform, that means still you cannot just execute any dll/exe that uses plattform dependent code. This is the same as any other C/C++/Java program, once you program calls into OS library/kernel APIs, it is no longer cross-platform and those APIs have to exist on your target. So no, if you use any Win32 feature using P/Invoke, the tool will not just automagically run on Linux.