Iterate down a tree

Very often I see code like:

var list = new List<foo>();
AddRecursive(list, foo);
return list;

Nothing wrong with that except that there will be a lot of duplicate code after a while.

After some testing I was able to create an enumerator walking the tree.

public static IEnumerable<T> Recursive<T>(
    T node, Func<T, IEnumerable<T>> selector)
{
  yield return node;
  var children = selector(node);
  if (children == null) yield break;

  foreach (var child in children.SelectMany(x => Recursive(x, selector)))
  {
    yield return child;
  }
}

Let say you have this class:

public class Node {
  public int Id;
  public Node[] Children;
}

You can now find a given node using this statement

var node = Recursive(root, x => x.Children)
             .FirstOrDefault(x => x.Id == 10);