Enumerators, iterators, IEnumerable - a bit confused - asp.net-mvc-2

Question

I've read through the Iterators/Enumerators section of C# 3.0 in a Nutshell several times, but I'm still having a hard time grasping it. From their names, my initial thought is that an Iterator would iterate over a group of Enumerable objects. Am I on the right track? If so, then what about common generic collections, like a List<T>? Does List create/make use of an Iterator during some of its operations? Is T automatically Enumerable? Is there a cast? Does one even have anything to do with the other?
On a somewhat related note, while learning MVC, I've seen code like so:
public Article GetArticle(int id)
{
return _siteDB.Articles.SingleOrDefault(a => a.ArticleID == id);
}
public IEnumerable<Article> GetArticle(string title)
{
return _siteDB.Articles.Where(a => a.Title.StartsWith(title)).AsEnumerable<Article>();
}
What does having a return type of IEnumerable<T> give me?
EDIT: Okay, I think I'm beginning to get it. My confusion remains with Iterators. The book describes them as producers of Enumerators. I don't see where that actually happens with yield return. Does each yield create a new Enumerator?

Answer 1

The type IEnumerable gives you IEnumerable :)
It is an interface defining method GetEnumerator(), which returns IEnumerator. You can call the method as many times as you like and you always obtain new instance of IEnumerator.
The IEnumerator have property Current and methods MoveNext() and Reset() allowing the collection enumeration. The enumeration itself can be done by calling MoveNext() and reading the Current property if previous call of MoveNext() returns true.
Good example of implementation and usage is in documentation: http://msdn.microsoft.com/en-us/library/system.collections.ienumerator.aspx

Answer 2

From their names, my initial thought is that an Iterator would iterate over a group of Enumerable objects. Am I on the right track? If so, then what about common generic collections, like a List? Does List create/make use of an Iterator during some of its operations?
An iterator is a way to implement a method returning an IEnumerable<T> - it's really an implementation detail. Instead of having to create some custom class to enumerate through a collection, you can use an iterator to implement it, and the compiler does the hard work for you.
What does having a return type of IEnumerable give me?
This basically allows you to enumerate over the type. For example, the second method (which I would personally call GetArticles), would allow you to write:
foreach(var article in GetArticle(theTitle))
{
// Do something with article
}

Answer 3

Lets see if I can answer this.
So a List implements IEnumerable, and its not the T that has to implement it, anything in a list can be iterated, using a forech or for loop, for example. If you use a generic list like List, this means that what you have is a list of T where T is the object type defined by you, that means you don't have to cast it, but in the other hand you can only put objects of that same type in that list.
The code you have up there, can mean a series of thing:
1 - Any variable that implements IEnumerable will be able to hold the result of GetArticle,
Ex:
List yourList = GetArticle("test");
Queue myQueue = GetArticle("test");
2 - The actual processing of the query will be delayed. If you call Count for example, after you call the method, thats when it actually get executed/resolved.
3 - Because of number 2, if you are doing that on a DB as the code suggests, then you connection has to remain open till the time where you make use of the list, otherwise you will get an exception.
I think I have answered the questions.