21. Method parameters: ref, out, in, params, optional, named

void Swap(ref int a, ref int b) { (a, b) = (b, a); }   // caller's variables change
bool TryParseAge(string s, out int age) => int.TryParse(s, out age); // must assign
double Length(in Vector3 v) => Math.Sqrt(v.X * v.X);  // read-only by reference (avoids copy of big struct)
int Sum(params int[] xs) => xs.Sum();                  // Sum(1,2,3)
void Send(string to, string subject = "Hi", bool urgent = false) { }

int x = 1, y = 2;
Swap(ref x, ref y);
Send("a@b.com", urgent: true);   // named argument skips optional ones