flatten enumerable extension

flatten.cs

internal static class Extensions
{
  public static IEnumerable<T> Flatten<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> childSelector)
  {
    foreach (var item in source)
    {
      yield return item;

      foreach (var d in childSelector(item).Flatten(childSelector))
      {
        yield return d;
      }
    }
  }

  public static IEnumerable<T> Flatten<T>(this T source, Func<T, IEnumerable<T>> childSelector)
  {
    yield return source;

    foreach (var d in childSelector(source).Flatten(childSelector))
    {
      yield return d;
    }
  }
}