Nested enumeration of one Container's Elements causes missing elements in outer enumerator

[ghemberg]

Consider the following code in a Visitor:

private List currentList;

public override VisitList(List list)
{
    this.currentList = list;
    base.VisitList(list);
}

public override VisitItem(Item item)
{
    Console.WriteLine(this.currentList.Elements.Count());
    base.VisitItem(item);
}

For a List containing 4 Item elements, you'd expect to see the following on the Console:

4
4
4
4

What you actually get is this:

4

[ghemberg]

This is caused by CachedEnumerableImpl<T>.GetEnumerator():

public IEnumerator<T> GetEnumerator()
{
    foreach (var item in cache)
    {
        yield return item;
    }

    if (enumerator == null)
        enumerator = enumerable.GetEnumerator();

    while (enumerator.MoveNext())
    {
        cache.Add(enumerator.Current);
        yield return enumerator.Current;
    }
}

The VisitList(List) enumerator gets to the second yield statement and calls VisitItem(Item) for the first Item it gets. VisitItem(Item) then re-uses the same enumerator instance and enumerates over the remaining elements. As VisitList(List)'s enumerating resumes, enumerator has moved to the end of the enumeration and enumerator.MoveNext() returns false (and the remaining Item elements are skipped)...