c# - Why the Reset() method on Enumerator class must throw a NotSupportedException()?

Question

From what I saw on http://csharpindepth.com/Articles/Chapter6/IteratorBlockImplementation.aspx, and article by Jon Skeet, the c# specification itself says that. What would be the reason?

Answer 1

It is impossible to support properly in all sequences; many are once only (network streams, etc). And if you can't rely on it all the time, it is useless, as the abstraction is broken. Sure you could have an IResettableEnumerator, but Reset() on IEnumerator doesn't work. Frankly, it was a mistake (IMO).

I suspect it would also have made iterator blocks even more complicated (they are currently one of the two most complex parts of the compiler; I can't remember which is "top"; them, or anonymous methods / captured variables).