Yes, classes are just POCOs (sometimes with some sealed subclasses, to hack sum types) and business logic is static functions in separate classes. Like I said, C# makes this somewhat easier, with things like readonly fields, get-only auto-properties, System.Collections.Immutable, readonly structs, pattern matching, etc. With Java you have to take more care to encapsulate things properly, though I’m excited for records coming into the language.

The most salient problem is the fact that the languages do not distinguish between references and values (aka “mutability by default”) which means that the compiler will not maintain those invariants for you unless you annotate things specifically, and the authors of the libraries you use do the same.

My overall preference is for F#, which is ironically a better object oriented language than C#, by virtue of using ML-like syntax instead of C-like. Nonetheless, C# is a passable functional programming language these days, and the language designers keep adding more functional-inspired features with every release.