RosettaCodeData/Task/Fibonacci-sequence/C-sharp/fibonacci-sequence-3.cs

public static ulong Fib(uint x) {
    if (x == 0) return 0;

    ulong prev = 0;
    ulong next = 1;
    for (int i = 1; i < x; i++)
    {
        ulong sum = prev + next;
        prev = next;
        next = sum;
    }
    return next;
}