well to do anything new in a C# project one typically ends up making a class inside a new namespace, if I just want to write a solver with no state it's quite jarring and distracts from otherwise a pretty good language that I like working in. So 2 new nouns and a handful of keywords.
That's it. No namespaces (which were never required anyway, not even in C# 1.0), no classes, no functions even. If you want to define a function, you can just do so in global scope, and if it's a single expression you don't even need the braces:
int Fib(int n) => (n <= 1) ? n : Fib(n - 1) + Fib(n - 2);
I have no idea what you are talking about. Care to present a single example?