#1,196 – Using Fluent-Style Syntax when Chaining Iterators

You can chain iterator code together using a fluent-style syntax if you define extension methods for the corresponding IEnumerable<T> type that you’re using.  In the code below, we chain several iterators together, progressively filtering an IEnumerable<Dog> collection.

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

            Console.ReadLine();
        }

        private static IEnumerable<Dog> AllMyDogs()
        {
            yield return new Dog("Kirby", Breed.BorderCollie, 14);
            yield return new Dog("Jack", Breed.JackRussell, 15);
            yield return new Dog("Ruby", Breed.Mutt, 4);
            yield return new Dog("Lassie", Breed.Collie, 19);
            yield return new Dog("Shep", Breed.Collie, 2);
            yield return new Dog("Foofoo", Breed.Sheltie, 8);
            yield return new Dog("Pongo", Breed.Dalmatian, 4);
            yield return new Dog("Rooster", Breed.WestHighlandTerrier, 1);
        }
    }

    static class DogFilters
    {
        public static IEnumerable<Dog> YoungDogs(this IEnumerable<Dog> dogs)
        {
            foreach (Dog d in dogs)
                if (d.Age < 10)
                    yield return d;
        }

        public static IEnumerable<Dog> HerdingDogs(this IEnumerable<Dog> dogs)
        {
            foreach (Dog d in dogs)
                if ((d.Breed == Breed.BorderCollie) ||
                    (d.Breed == Breed.Collie) ||
                    (d.Breed == Breed.Sheltie))
                    yield return d;
        }
    }

Here’s the output:
1196-001

#1,194 – Chaining Iterators Together

An iterator can work on an IEnumerable<T>, using one sequence as input and generating a second sequence from the first.

In the example below, we use the output of one iterator as the input for another iterator.

        static void Main(string[] args)
        {
            IEnumerable<Dog> dogs1 = AllMyDogs();

            Console.WriteLine("============");
            foreach (Dog d in dogs1)
                Console.WriteLine(d);

            IEnumerable<Dog> dogs2 = YoungDogs(dogs1);

            Console.WriteLine("============");
            foreach (Dog d in dogs2)
                Console.WriteLine(d);

            IEnumerable<Breed> breeds = BreedsByNamePattern(dogs2, "oo");

            Console.WriteLine("============");
            foreach (Breed b in breeds)
                Console.WriteLine(b);

            Console.ReadLine();
        }

        private static IEnumerable<Dog> AllMyDogs()
        {
            yield return new Dog("Kirby", Breed.BorderCollie, 14);
            yield return new Dog("Jack", Breed.JackRussell, 15);
            yield return new Dog("Ruby", Breed.Mutt, 4);
            yield return new Dog("Lassie", Breed.Collie, 12);
            yield return new Dog("Foofoo", Breed.Sheltie, 8);
            yield return new Dog("Pongo", Breed.Dalmatian, 4);
            yield return new Dog("Rooster", Breed.WestHighlandTerrier, 1);
        }

        private static IEnumerable<Dog> YoungDogs(IEnumerable<Dog> dogs)
        {
            foreach (Dog d in dogs)
                if (d.Age < 10)
                    yield return d;
        }

        private static IEnumerable<Breed> BreedsByNamePattern(IEnumerable<Dog> dogs, string pattern)
        {
            foreach (Dog d in dogs)
                if (d.Name.Contains(pattern))
                    yield return d.Breed;
        }

1194-001

#559 – An Iterator Can Return an Enumerable Type or an Enumerator

An iterator is a block of code that contains one or more yield statements.  It can return one of the following interfaces:

  • IEnumerable  (in System.Collections)
  • IEnumerable<T>  (in System.Collections.Generic)
  • IEnumerator  (in System.Collections)
  • IEnumerator<T>  (in System.Collections.Generic)

You’d return an enumerable type when you want provide client code with a collection of objects and you don’t want to write your own custom collection class.

You’d typically return an enumerator when you’re implementing the GetEnumerator method in a custom collection class.