Code

diagram.CommandBindings.Add(
	new CommandBinding(EditingCommands.TabForward, OnTabForward));
diagram.InputBindings.Add(
	new InputBinding(EditingCommands.TabForward, new KeyGesture(Key.Tab)));

private void OnTabForward(object sender, ExecutedRoutedEventArgs e)
{
	int numNodes = diagram.Nodes.Count;
	if (numNodes == 0)
		return;

	var currentNode = diagram.ActiveItem as DiagramNode;
	if (currentNode == null)
	{
		diagram.ActiveItem = diagram.Nodes[0];
		return;
	}

	int next = (diagram.Nodes.IndexOf(currentNode) + 1) % numNodes;
	diagram.Selection.Change(diagram.Nodes[next]);
}