C# – Distinction between iterator and enumerator


An interview question for a .NET 3.5 job is "What is the difference between an iterator and an enumerator"?

This is a core distinction to make, what with LINQ, etc.

Anyway, what is the difference? I can't seem to find a solid definition on the net. Make no mistake, I can find the meaning of the two terms but I get slightly different answers. What would be the best answer for an interview?

IMO an iterator "iterates" over a collection, and an enumerator provides the functionality to iterate, but this has to be called.

Also, using the yield keyword is said to save state. What exactly is this state? Is there an example of this benefit occurring?

Best Solution

Iterating means repeating some steps, while enumerating means going through all values in a collection of values. So enumerating usually requires some form of iteration.

In that way, enumerating is a special case of iterating where the step is getting a value from a collection.

Note the "usually" – enumerating may also be performed recursively, but recursion and iteration are so closely related that I would not care about this small difference.

You may also enumerate values you do not explicitly store in a collection. For example, you can enumerate the natural number, primes, or whatever but you would calculate these values during the enumeration and not retrieve them from a physical collection. You understand this case as enumerating a virtual collection with its values defined by some logic.

I assume Reed Copsey got the point. In C# there are two major ways to enumerate something.

  1. Implement Enumerable and a class implementing IEnumerator
  2. Implement an iterator with the yield statement

The first way is harder to implement and uses objects for enumerating. The second way is easier to implement and uses continuations.