Code

public class MyDiagram : Diagram
{
	protected override AutomationPeer OnCreateAutomationPeer()
	{
		return new MyDiagramAutomationPeer(this);
	}

	class MyDiagramAutomationPeer :
		FrameworkElementAutomationPeer,
		ISelectionProvider
	{
		public MyDiagramAutomationPeer(FrameworkElement owner) :
			base(owner)
		{
			var diagram = (MyDiagram)owner;
			nodePeers = new Dictionary<DiagramNode, DiagramNodeAutomationPeer>();

			diagram.NodeCreated +=
				(s, e) => RaiseAutomationEvent(AutomationEvents.StructureChanged);
			diagram.NodeDeleted +=
				(s, e) => RaiseAutomationEvent(AutomationEvents.StructureChanged);
			diagram.NodeSelected +=
				(s, e) => RaiseAutomationEvent(AutomationEvents.SelectionItemPatternOnElementAddedToSelection);
			diagram.NodeDeselected +=
				(s, e) => RaiseAutomationEvent(AutomationEvents.SelectionItemPatternOnElementRemovedFromSelection);
		}

		protected override string GetClassNameCore()
		{
			return "MyDiagram";
		}

		protected override AutomationControlType GetAutomationControlTypeCore()
		{
			return AutomationControlType.List;
		}

		protected override List<AutomationPeer> GetChildrenCore()
		{
			var children = new List<AutomationPeer>();
			foreach (var node in Diagram.Nodes)
				children.Add(PeerForNode(node));
			return children;
		}

		DiagramNodeAutomationPeer PeerForNode(DiagramNode node)
		{
			if (nodePeers.ContainsKey(node))
				return nodePeers[node];

			var nodePeer = new DiagramNodeAutomationPeer(node, this);
			nodePeers[node] = nodePeer;
			return nodePeer;
		}

		public override object GetPattern(PatternInterface patternInterface)
		{
			if (patternInterface == PatternInterface.Selection)
				return this;
			return base.GetPattern(patternInterface);
		}

		#region ISelectionProvider Members

		public IRawElementProviderSimple[] GetSelection()
		{
			var selectionPeers = new List<IRawElementProviderSimple>();
			foreach (var node in Diagram.Selection.Nodes)
				selectionPeers.Add(ProviderFromPeer(PeerForNode(node)));
			return selectionPeers.ToArray();
		}

		public bool CanSelectMultiple
		{
			get { return true; }
		}

		public bool IsSelectionRequired
		{
			get { return false; }
		}

		#endregion

		MyDiagram Diagram
		{
			get { return (MyDiagram)Owner; }
		}

		Dictionary<DiagramNode, DiagramNodeAutomationPeer> nodePeers;
	}

	class DiagramNodeAutomationPeer :
		FrameworkElementAutomationPeer,
		ISelectionItemProvider
	{
		public DiagramNodeAutomationPeer(FrameworkElement owner, MyDiagramAutomationPeer parent) :
			base(owner)
		{
			this.parent = parent;
		}

		MyDiagramAutomationPeer parent;

		protected override string GetClassNameCore()
		{
			return "DiagramNode";
		}

		protected override AutomationControlType GetAutomationControlTypeCore()
		{
			return AutomationControlType.ListItem;
		}

		public override object GetPattern(PatternInterface patternInterface)
		{
			if (patternInterface == PatternInterface.SelectionItem)
				return this;

			return base.GetPattern(patternInterface);
		}

		#region ISelectionItemProvider Members

		public void AddToSelection()
		{
			Node.Selected = true;
		}

		public bool IsSelected
		{
			get { return Node.Selected; }
		}

		public void RemoveFromSelection()
		{
			Node.Selected = false;
		}

		public void Select()
		{
			Node.Selected = true;
		}

		public IRawElementProviderSimple SelectionContainer
		{
			get { return ProviderFromPeer(parent); }
		}

		#endregion

		DiagramNode Node
		{
			get { return (DiagramNode)Owner; }
		}
	}
}