Code

private delegate void ProcessLinkDelegate(DiagramLink link);

private void miGraphDepthSearch_Click(object sender, EventArgs e)
{
	linkCounter = 0;

	DiagramNode start = diagram.ActiveItem as DiagramNode;
	if (start != null)
		DepthSearch(start, ProcessLink, new Dictionary<DiagramNode, bool>());
}

private void miGraphBreadthSearch_Click(object sender, EventArgs e)
{
	linkCounter = 0;

	DiagramNode start = diagram.ActiveItem as DiagramNode;
	if (start != null)
		BreadthSearch(start, ProcessLink);
}

void DepthSearch(DiagramNode start, ProcessLinkDelegate processLink,
	Dictionary<DiagramNode, bool> visitedNodes)
{
	visitedNodes[start] = true;

	foreach (DiagramLink link in start.OutgoingLinks)
	{
		processLink(link);
		if (visitedNodes.ContainsKey(link.Destination))
			continue;
		DepthSearch(link.Destination, processLink, visitedNodes);
	}
}

void BreadthSearch(DiagramNode start, ProcessLinkDelegate processLink)
{
	Dictionary<DiagramNode, bool> visitedNodes = new Dictionary<DiagramNode, bool>();
	Queue<DiagramNode> queue = new Queue<DiagramNode>();

	queue.Enqueue(start);
	visitedNodes[start] = true;

	while(queue.Count > 0)
	{
		DiagramNode node = queue.Dequeue();
		foreach (DiagramLink link in node.OutgoingLinks)
		{
			processLink(link);
			if (visitedNodes.ContainsKey(link.Destination))
				continue;
			queue.Enqueue(link.Destination);
			visitedNodes[link.Destination] = true;
		}
	}
}

private void ProcessLink(DiagramLink link)
{
	link.Text = linkCounter.ToString();
	linkCounter++;
}

private int linkCounter = 0;