Iterator Block

ref: Iterators, iterator blocks and data pipelines Iterator block implementation details

LINQ是C#非常重大的一个feature，也是一个非常值得深入探讨的主题。然而LINQ并非C#在单个版本中发布的一个单独feature，而是由多个feature组合而成。

迭代器作为C#3.0发布的feature，由此开始窥见LINQ，可以算是LINQ的基石。

迭代器有两个接口，分别是 IEnumerator 以及 IEnumerable，由于从C#2.0开始进入泛型阶段，因此两个接口各自存在泛型及非泛型定义。

在C# in Depth 4ed原文中，作者是这样阐述这两个接口的关系及区别的：

An IEnumerable is a sequence that can be iterated over, whereas an IEnumerator is like a cursor within a sequence. Multiple IEnumerator instances can probably iterate over the same IEnumerable without changing its state at all.

If that didn’t make much sense, you might want to think about an IEnumerable as a book and an IEnumerator as a bookmark. There can be multiple bookmarks within a book at any one time. Moving a bookmark to the next page doesn’t change the book or any of the other bookmarks, but it does change that bookmark’s state: its position within the book.

我觉得非常形象。