Olostan

sealed class Dumper
{
  static public string Dump(Node root)
  {
    var sb = new StringBuilder();
    Action<Node, string> action = null;
    action = (node, indent) =>
    {
      // Precondition: indentation and prefix has already been output
      // Precondition: indent is correct for node's *children*
      sb.AppendLine(node.Text);
      for (int i = 0; i < node.Children.Count; ++i)
      {
        bool last = i == node.Children.Count - 1;
        var child = node.Children[i];
        sb.Append(indent);
        sb.Append(last ? '└' : '├');
        sb.Append('─');
        action(child, indent + (last ? "  " : "│ "));
      }
    };

    action(root, "");

    return sb.ToString();
  }
}