#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)

        public void AddCow(Cow 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)

#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.


#555 – Enumerable Objects and Enumerators

An enumerable object is a type that represents a sequence of elements.  The type implements either the IEnumerable or IEnumerable<T> interface, which means that it implements a GetEnumerator method, which is called to get the enumerator for the enumerable class.

An enumerator is an object that knows how to move through a sequence of elements.  It implements either the IEnumerator or IEnumerator<T> interface, which means that it exposes a Current property that points to the current element in the sequence and a MoveNext method that moves to the next element.

You can think of an enumerator as a sort of pointer into a sequence of elements.

Enumerators, used on enumerable objects, are the internal mechanism that allows using a foreach statement to iterate through a collection of items.