Question

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

Answer 1

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;
}