Recurse.cs

public static IEnumerable<T> Recurse<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> childSelector, bool depthFirst = false)
{
    List<T> queue = new List<T>(source); ;

    while (queue.Count > 0)
    {
        var item = queue[0];
        queue.RemoveAt(0);

        var children = childSelector(item);

        if (depthFirst)
        {
            queue.InsertRange(0, children);
        }
        else
        {
            queue.AddRange(children);
        }

        yield return item;
    }
}

public static IEnumerable<T> Recurse<T>(this T source, Func<T, T> childSelector, bool depthFirst = false)
{
    var list = new List<T>() { source };
    var childListSelector = new Func<T, IEnumerable<T>>(item =>
    {
        var child = childSelector(item);
        if (child == null)
        {
            return new List<T>();
        }
        else
        {
            return new List<T>() { child };
        }
    });

    foreach (var result in Recurse(list, childListSelector, depthFirst))
    {
        yield return result;
    }
}