Just an offhand guess, but is the F# (.net 4.0) trying to run against a .NET 2.0 runtime? Which mscorlib is it referencing? Check the arguments being sent to the f# compiler (fsc.exe I think) - if you ...

F# 8, a new edition of Microsoft’s open-source, “functional-first” programming language, now is generally available, emphasizing simpler, more uniform, and more performant F# programming. F# 8 has ...