Basically, I just ran a scan of my computer with Malwarebytes (updated the definitions before running), and it said my \"helloworld\" program written in C# has a trojan.
The problem could be that the Backdoor.MSIL.PGen Trojan is typically called 'hello.exe'. The name of your executable is presumably 'hello.exe' or 'helloworld.exe'.
Just rename your project or change the output executable to something not containing 'hello', and it should stop detecting it.
This answer is somewhat speculative, but given the name of your project, and a history of over-aggressive detection of this malware (see here), it seems a reasonable stab.