Question

I would like to use an IEnumerable to generate a sequence of values — specifically, a list of Excel-like column headers.

private IEnumerable<string> EnumerateSymbolNames()
{
  foreach (var sym in _symbols)
  {
    yield return sym;
  }

  foreach (var sym1 in _symbols)
  {
    foreach (var sym2 in _symbols)
    {
      yield return sym1 + sym2;
    }
  }

  yield break;
}
private readonly string[] _symbols = new string[] { "A", "B", "C", ...};

This works fine if I fetch the values from a foreach loop. But what I want is to use the iterator block as a state machine and fetch the next available column header in response to a user action. And this — consuming the generated values — is where I’ve run into trouble.

So far I’ve tried

return EnumerateSymbolNames().Take(1).FirstOrDefault();

return EnumerateSymbolNames().Take(1).SingleOrDefault();

return EnumerateSymbolNames().FirstOrDefault();

var enumerator = EnumerateSymbolNames().GetEnumerator();
enumerator.MoveNext();
return enumerator.Current;

… but none of these have worked. (All repeatedly return “A”.)
Based on the responses to this question, I’m wondering what I want is even possible — although several of the responses to that post suggest techniques similar to my last one.

And no, this is not a homework assignment 🙂

Answer 1

When you use GetEnumerator, you need to use the same enumerator for each iteration. If you call GetEnumerator a second time, it will start over at the beginning of the collection.

If you want to use Take, you must first Skip the number of records that have already been processed.