Is there a built-in way to convert IEnumerator to IEnumerable

Is there a built-in way to convert IEnumerator<T> to IEnumerable<T>?

Solution 1:

The easiest way of converting I can think of is via the yield statement

public static IEnumerable<T> ToIEnumerable<T>(this IEnumerator<T> enumerator) {
  while ( enumerator.MoveNext() ) {
    yield return enumerator.Current;
  }
}

compared to the list version this has the advantage of not enumerating the entire list before returning an IEnumerable. using the yield statement you'd only iterate over the items you need, whereas using the list version, you'd first iterate over all items in the list and then all the items you need.

for a little more fun you could change it to

public static IEnumerable<K> Select<K,T>(this IEnumerator<T> e, 
                                         Func<K,T> selector) {
      while ( e.MoveNext() ) {
        yield return selector(e.Current);
      }
    }

you'd then be able to use linq on your enumerator like:

IEnumerator<T> enumerator;
var someList = from item in enumerator
               select new classThatTakesTInConstructor(item);

Solution 2:

You could use the following which will kinda work.

public class FakeEnumerable<T> : IEnumerable<T> {
  private IEnumerator<T> m_enumerator;
  public FakeEnumerable(IEnumerator<T> e) {
    m_enumerator = e;
  }
  public IEnumerator<T> GetEnumerator() { 
    return m_enumerator;
  }
  // Rest omitted 
}

This will get you into trouble though when people expect successive calls to GetEnumerator to return different enumerators vs. the same one. But if it's a one time only use in a very constrained scenario, this could unblock you.

I do suggest though you try and not do this because I think eventually it will come back to haunt you.

A safer option is along the lines Jonathan suggested. You can expend the enumerator and create a List<T> of the remaining items.

public static List<T> SaveRest<T>(this IEnumerator<T> e) {
  var list = new List<T>();
  while ( e.MoveNext() ) {
    list.Add(e.Current);
  }
  return list;
}