If you've ever poked around the .NET class libraries in Reflector, you probably would have noticed that the generic collection classes all have implementations of their IEnumerator as a struct rather than a class. As you will see, this design decision has some rather unfortunate side effects...

The "side-effects" that are talked about here are the natural effects of dealing with a value-type. The same behavior would happen if he was wrapping an Int32.

The 99% use-case of a generic enumerator is in a foreach block. It absolutely makes sense to have an enumerator be a struct to cover this case and see real benefit.

All this is is a case of having to do a little be of research to correctly use a feature a little bit outside of it's intended use-case. We all use enumerators outside of foreach blocks but I'm willing to bet that foreach or LINQ statements cover 99% of whatever real coding you do.