Sometimes you want to visit a full message hierarchy.
One such case is in the OpenPop.NET test application that downloads messages and shows you the message hierarchy of each email as seen below.



The test application will build this information in a TreeNode hierarchy using a TreeNodeBuilder that implements a IAnswerMessageTraverser.
The generic type of IAnswerMessageTraverser is the return type of the methods. Since we are building a TreeNode hierarchy, it is TreeNode.

using System;
using System.Windows.Forms;
using OpenPop.Mime;
using OpenPop.Mime.Traverse;
using Message = OpenPop.Mime.Message;

namespace OpenPop.TestApplication
{
    /// <summary>
    /// Builds up a <see cref="TreeNode"/> with the same hierarchy as
    /// a <see cref="Message"/>.
    /// 
    /// The root <see cref="TreeNode"/> has the subject as text and has one one child.
    /// The root has no Tag attribute set.
    /// 
    /// The children of the root has the MediaType of the <see cref="MessagePart"/> as text and the
    /// MessagePart's children as <see cref="TreeNode"/> children.
    /// Each <see cref="MessagePart"/> <see cref="TreeNode"/> has a Tag property set to that <see cref="MessagePart"/>
    /// </summary>
    internal class TreeNodeBuilder : IAnswerMessageTraverser<TreeNode>
    {
        public TreeNode VisitMessage(Message message)
        {
            if(message == null)
                throw new ArgumentNullException("message");

            // First build up the child TreeNode
            TreeNode child = VisitMessagePart(message.MessagePart);

            // Then create the topmost root node with the subject as text
            TreeNode topNode = new TreeNode(message.Headers.Subject, new[] { child });

            return topNode;
        }

        public TreeNode VisitMessagePart(MessagePart messagePart)
        {
            if(messagePart == null)
                throw new ArgumentNullException("messagePart");

            // Default is that this MessagePart TreeNode has no children
            TreeNode[] children = new TreeNode[0];

            if(messagePart.IsMultiPart)
            {
                // If the MessagePart has children, in which case it is a MultiPart, then
                // we create the child TreeNodes here
                children = new TreeNode[messagePart.MessageParts.Count];

                for(int i = 0; i<messagePart.MessageParts.Count; i++)
                {
                    children[i] = VisitMessagePart(messagePart.MessageParts[i]);
                }
            }

            // Create the current MessagePart TreeNode with the found children
            TreeNode currentNode = new TreeNode(messagePart.ContentType.MediaType, children);

            // Set the Tag attribute to point to the MessagePart.
            currentNode.Tag = messagePart;

            return currentNode;
        }
    }
}

As you might have noticed, it is an recursive traversal through the Message Hierarchy Tree. The root node needs to know its children before it is constructed, therefore it calls the VisistMessagePart with the Message.MessagePart property.
That method will then build the tree below it, and in that sense the tree is constructed from the bottom up.

To use this TreeNodeBuilder, you do just like the OpenPop.NET Test application does:

TreeNode root = new TreeNodeBuilder().VisitMessage(message);