#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;
            }
        }

#1,195 – Iterator Produces Only as Many Elements as Are Needed

When implementing an iterator, you write code that generates a sequence of elements using the yield return statement to return each consecutive element.

The full body of the iterator code may not execute.  Only the code required to return the elements iterated upon is executed.  In the example below, we iterate over a sequence produced by an iterator, using foreach.

Executing this code, only a portion of the AllMyDogs() method is executed, since we exit early out of  the foreach loop.

            static void Main(string[] args)
            {
                foreach (Dog d in AllMyDogs())
                {
                    Console.WriteLine(d);
                    if (d.Breed == Breed.JackRussell)
                        break;
                }

                Console.ReadLine();
            }

            private static IEnumerable<Dog> AllMyDogs()
            {
                Console.WriteLine("* returning Kirby");
                yield return new Dog("Kirby", Breed.BorderCollie, 14);
                Console.WriteLine("* returning Jack");
                yield return new Dog("Jack", Breed.JackRussell, 15);
                Console.WriteLine("* returning Ruby");
                yield return new Dog("Ruby", Breed.Mutt, 4);
                Console.WriteLine("* returning Lassie");
                yield return new Dog("Lassie", Breed.Collie, 12);
                Console.WriteLine("* returning Foofoo");
                yield return new Dog("Foofoo", Breed.Sheltie, 8);
            }

#562 – What an Iterator Looks Like Under the Covers

When you use an iterator to return an enumerable sequence of items, the C# compiler converts your iterator block (containing one or more yield return statements) into a new class that performs the actual enumeration.  The new class actually implements all four interfaces that an iterator can return–IEnumerable, IEnumerable<T>, IEnumerator and IEnumerator<T>.

Below is a simple iterator block that returns a sequence of two elements.

        private static IEnumerable<int> MyIterator()
        {
            yield return 100;
            yield return 200;
        }

If you use a disassembler to look at the IL that the compiler generates for this code, you’ll see a new class that implements the four interfaces.

You’ll notice that the class implements GetEnumerator, which just returns the current instance of the class.  (Code shown is from Reflector).

The MoveNext method returns the next element in the sequence.