Take me back!

After iterating the tree down, its time do go the opposite way: up!

public static IEnumerable<T> ToRoot<T>(
     T item, Func<T, T> selector)
{
  if (Equals(item, default(T))) yield break;
    yield return item;
 
  foreach (var x in ToRoot(selector(item), selector))
    yield return x;
}

And how to use it:

 
static IEnumerable<int> CreateList(ProductDataSet.ProductRow row)
{
  return ToRoot(row, x => x.ProductRowParent).Select(x => x.ProductId);
}