Question

How exactly is the right way to call IEnumerator.Reset?

The documentation says:

The Reset method is provided for COM interoperability. It does not necessarily need to be implemented; instead, the implementer can simply throw a NotSupportedException.

Okay, so does that mean I’m not supposed to ever call it?

It’s so tempting to use exceptions for flow control:

using (enumerator = GetSomeExpensiveEnumerator())
{
    while (enumerator.MoveNext()) { ... }

    try { enumerator.Reset(); } //Try an inexpensive method
    catch (NotSupportedException)
    { enumerator = GetSomeExpensiveEnumerator(); } //Fine, get another one

    while (enumerator.MoveNext()) { ... }
}

Is that how we’re supposed to use it? Or are we not meant to use it from managed code at all?

Answer 1

never; ultimately this was a mistake. The correct way to iterate a sequence more than once is to call .GetEnumerator() again – i.e. use foreach again. If your data is non-repeatable (or expensive to repeat), buffer it via .ToList() or similar.

It is a formal requirement in the language spec that iterator blocks throw exceptions for this method. As such, you cannot rely on it working. Ever.