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 interoperabi

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

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.

Answer 3

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.