#1,197 – Iterator Can Generate Enumerable of Infinite Length

An iterator can be written to generate an enumerable sequence of infinite length.  The iterator will execute for as long as client code continues to request the next element in the sequence.

Below is a simple iterator that generates prime numbers.  Note that we can still use the iterator, although its implementation includes an infinite loop.  The loop will execute only as many times as needed.

        static void Main(string[] args)
        {
            Console.Write("Enter limit:");
            int limit = int.Parse(Console.ReadLine());

            foreach (int p in AllPrimes())
            {
                if (p > limit)
                    break;
                Console.WriteLine(p);
            }

            Console.ReadLine();
        }

        private static IEnumerable<int> AllPrimes()
        {
            List<int> primesSoFar = new List<int>();
            primesSoFar.Add(2);
            yield return 2;
            primesSoFar.Add(3);
            yield return 3;

            int testPrime = 5;
            while (true)
            {
                bool isPrime = true;
                foreach (int n in primesSoFar)
                {
                    if (testPrime % n == 0)
                    {
                        isPrime = false;
                        break;
                    }
                }

                if (isPrime)
                {
                    primesSoFar.Add(testPrime);
                    yield return testPrime;
                }

                testPrime += 2;
            }
        }