How Can I Use IEnumerator.Reset()?

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

public class PeopleEnum : IEnumerator
{
    public Person[] _people;

    // Enumerators are positioned before the first element 
    // until the first MoveNext() call. 
    int position = -1;

    public PeopleEnum(Person[] list)
    {
        _people = list;
    }

    public bool MoveNext()
    {
        position++;
        return (position < _people.Length);
    }

    public void Reset()
    {
        position = -1;
    }

    object IEnumerator.Current
    {
        get
        {
            return Current;
        }
    }

    public Person Current
    {
        get
        {
            try
            {
                return _people[position];
            }
            catch (IndexOutOfRangeException)
            {
                throw new InvalidOperationException();
            }
        }
    }
}

Answer 2

I recommend not using it. A lot of modern IEnumerable implementations will just throw an exception.

Getting enumerators is hardly ever "expensive". It is enumerating them all (fully) that can be expensive.

Answer 3

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.