C# Fibonacci Generator

fib.cs

using System;
using System.Collections;

namespace CSharpFib
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            foreach (ulong x in Fibber(5)) {
                Console.WriteLine(x);
            }
        }

        public static IEnumerable Fibber(int c)
        {
            ulong a, b, o;
            a = o = 0;
            b = 1;
            do {
                yield return a;
                o = a;
                a = b;
                b = o + b;
            } while (--c > 0);
        }


    }
}