Hello world in C# today looks like this:

  System.Console.WriteLine("Hello world!");

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);