Recursion in C# iterator

Question

Is it possible to use recursion in an iterator implementing System.Collections.IEnumerable? I have a tree structure declared roughly like this:

public class Node
{
    public Node Sibling;
    public Node Child;
}

I would like to iterate over the nodes in a tree. I would like to do something like this (pseudocode, I guess this won't compile):

public class NodeIterator : System.Collections.IEnumerable
{
    Node m_root;

    public System.Collections.IEnumerator GetEnumerator()
    {
        recursiveYield(m_root);
    }

    System.Collections.IEnumeraton recursiveYield(Node node)
    {
        yield return node;
        if (node.Child)
        {
            recursiveYield(node.Child);
        }
        if (node.Sibling)
        {
            recursiveYield(node.Sibling);
        }
    }
}

Is this somehow possible? I realise this can be solved without recursion using a Node deque in the GetEnumerator function.

Answer 1

Yes, all you need is to iterate the return value from the call site. Like so:

IEnumerable<T> Recursive(Node node)
{
    yield return node;
    foreach (var siblingNode in Recursive(node.Sibling))
    {
        yield return siblingNode;
    }
    foreach (var childNode in Recursive(node.Child))
    {
        yield return childNode;
    }
}

For the record, this isn't better than using a queue to achieve e.g. breadth-first traversal. The memory requirement for something like this is identical in the worst case.

Answer 2

No because the recursiveYield(Node node) function would return a collection and you can only yield an item