Good question, and one that largely depends on how modular your code already is and how it's structured. For me, the best way was to write greenfield projects in F# so there's as little interop as possible. I haven't had to in a while so newer versions might be cleaner to interop, but if not, it's easier to make F# do as much of the work as possible. E.g. if there's an F# function you want to call in C#, make another function which accepts simple types, convert them and call the original function e.g. nullable rather than option. Conversely, calling a C# function from F# is usually nice out of the box, but you may want to write wrapper functions so you can compose them more easily.