Iterators are at the core of .NET programming. Only rarely do developers actually work against indexed data, preferring to use for-each loops for most tasks. But is this inherently sequential access method appropriate as we turn more to multi-threaded applications?
If you consider an iterator primary task of getting the next element, then look at how it does it, you should observe a major design flaw… the operation is not atomic. An iterator’s implementation is IEnumerator or IEnumerator(Of T), and IEnumerator require you to first call MoveNext then read the Current property. If you allow multiple threads to use the same IEnumerator, then you could get a call sequence such as MoveNext, MoveNext, Current, Current, which would skip one item and repeat the next one. So IEnumerator is not well designed for threading. This is a known design limitation, but rather than address that, languages like C# have implemented their iterators to be a different iterator per thread. That is they not only recognise the limitation but they also enforce it.
Bill continues with notes about how a new IEnumerator would have to work in order to properly support multiple threads.