#560 – Returning an Enumerator from an Iterator

You can return an enumerator from an iterator block.  This is something that you’d typically do when implementing your own collection class.

Below, we implement the DogsAndCows collection class, which manages a group of Dog and Cow objects.  It implements the IEnumerable interface, which means that it implements the GetEnumerator method, allowing iterating through the entire collection.

GetEnumerator uses an iterator block to return the enumerator.

    // Collection of dogs and cows
    public class DogsAndCows : IEnumerable
    {
        private List<Dog> theDogs = new List<Dog>();
        private List<Cow> theCows = new List<Cow>();

        public void AddDog(Dog d)
        {
            theDogs.Add(d);
        }

        public void AddCow(Cow c)
        {
            theCows.Add(c);
        }

        // Return first the dogs and then the cows
        public IEnumerator GetEnumerator()
        {
            foreach (Dog d in theDogs)
                yield return d;

            foreach (Cow c in theCows)
                yield return c;
        }
    }

We can iterate through this collection with foreach.

            foreach (IAnimal animal in oddPack)
                Console.WriteLine(animal.Name);