Very simple implementation of remembering expanded nodes - just made ObjectGraphControl.Expanded static. It should be probably remembered separately per method (different variables in different methods have different meaning. Cleared on project close?).

src/AddIns/Debugger/Debugger.AddIn/Visualizers/Graph/ObjectGraph/ObjectGraphBuilder.cs

@@ -242,7 +242,7 @@ namespace Debugger.AddIn.Visualizers.Graph
 				propertyList.Add(new ObjectGraphProperty
 				                 { Name = memberProp.Name,
 				                 	Expression = propExpression, Value = "",
-				                 	PropInfo = memberProp, IsAtomic = true, TargetNode = null });
+				                 	MemberInfo = memberProp, IsAtomic = true, TargetNode = null });
 				
 			}
 			return propertyList.Sorted(ObjectPropertyComparer.Instance);

src/AddIns/Debugger/Debugger.AddIn/Visualizers/Graph/ObjectGraphControl.xaml.cs

@@ -27,19 +27,19 @@ namespace Debugger.AddIn.Visualizers.Graph
 	/// </summary>
 	public partial class ObjectGraphControl : UserControl
 	{
-		private WindowsDebugger debuggerService;
-		private EnumViewModel<LayoutDirection> layoutViewModel;
-		private ObjectGraph objectGraph;
-		private ObjectGraphBuilder objectGraphBuilder;
+		WindowsDebugger debuggerService;
+		EnumViewModel<LayoutDirection> layoutViewModel;
+		ObjectGraph objectGraph;
+		ObjectGraphBuilder objectGraphBuilder;
 		
-		private PositionedGraph oldPosGraph;
-		private PositionedGraph currentPosGraph;
-		private GraphDrawer graphDrawer;
+		PositionedGraph oldPosGraph;
+		PositionedGraph currentPosGraph;
+		GraphDrawer graphDrawer;
 		
 		static double mouseWheelZoomSpeed = 0.05;
 		
 		/// <summary> Long-lived map telling which graph nodes and content nodes the user expanded. </summary>
-		private Expanded expanded = new Expanded();
+		static Expanded expanded = new Expanded();
 
 		public ObjectGraphControl()
 		{
@@ -136,7 +136,7 @@ namespace Debugger.AddIn.Visualizers.Graph
 		{
 			this.objectGraphBuilder = new ObjectGraphBuilder(debuggerService);
 			Log.Debug("Debugger visualizer: Building graph for expression: " + txtExpression.Text);
-			return this.objectGraphBuilder.BuildGraphForExpression(expression, this.expanded.Expressions);
+			return this.objectGraphBuilder.BuildGraphForExpression(expression, expanded.Expressions);
 		}
 		
 		void LayoutGraph(ObjectGraph graph)
@@ -150,7 +150,7 @@ namespace Debugger.AddIn.Visualizers.Graph
 			this.oldPosGraph = this.currentPosGraph;
 			Log.Debug("Debugger visualizer: Calculating graph layout");
 			var layoutDirection = layoutViewModel.SelectedEnumValue;
-			this.currentPosGraph = new TreeLayout(layoutDirection).CalculateLayout(graph, this.expanded);
+			this.currentPosGraph = new TreeLayout(layoutDirection).CalculateLayout(graph, expanded);
 			Log.Debug("Debugger visualizer: Graph layout done");
 			RegisterExpandCollapseEvents(this.currentPosGraph);
 			

src/AddIns/Debugger/Debugger.AddIn/Visualizers/Graph/ObjectGraphWindow.xaml.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Linq;
 using System.Text;
 using System.Windows;
 using System.Windows.Controls;
@@ -13,7 +14,6 @@ using System.Windows.Input;
 using System.Windows.Media;
 using System.Windows.Media.Imaging;
 using System.Windows.Shapes;
-
 using Debugger.AddIn.Visualizers.Graph.Layout;
 using ICSharpCode.SharpDevelop.Debugging;
 using ICSharpCode.SharpDevelop.Services;
@@ -27,73 +27,74 @@ namespace Debugger.AddIn.Visualizers.Graph
 	{
 		private WindowsDebugger debuggerService;
 		
-		public ObjectGraphWindow()
+		private ObjectGraphWindow()
 		{
 			InitializeComponent();
 			
 			debuggerService = DebuggerService.CurrentDebugger as WindowsDebugger;
-			if (debuggerService == null)
-				throw new ApplicationException("Only windows debugger is currently supported");
-			
-			registerEvents();
-			instance = this;
+			if (debuggerService == null) throw new DebuggerVisualizerException("Only windows debugger is currently supported");
 		}
 		
 		protected override void OnClosed(EventArgs e)
 		{
 			base.OnClosed(e);
-			unregisterEvents();
+			UnregisterDebuggerEvents();
 			this.objectGraphControl.ClearUIControlCache();
-			instance = null;
+			ObjectGraphWindow.Instance = null;			// allow release
 		}
 		
 		public ICSharpCode.NRefactory.Ast.Expression ShownExpression
 		{
-			get {
-				return this.objectGraphControl.ShownExpression;
-			}
-			set {
-				this.objectGraphControl.ShownExpression = value;
-			}
+			get { return this.objectGraphControl.ShownExpression; }
+			set { this.objectGraphControl.ShownExpression = value; }
 		}
 		
-		static ObjectGraphWindow instance;
 		/// <summary> When Window is visible, returns reference to the Window. Otherwise returns null. </summary>
-		public static ObjectGraphWindow Instance
-		{
-			get { return instance; }
-		}
+		public static ObjectGraphWindow Instance { get; private set; }
 		
+		/// <summary>
+		/// Shows the singleton instance of ObjectGraphWindow and also returns it.
+		/// </summary>
+		/// <returns></returns>
 		public static ObjectGraphWindow EnsureShown()
 		{
-			var window = ObjectGraphWindow.Instance ?? new ObjectGraphWindow();
-			window.Topmost = true;
-			window.Show();
-			return window;
+			if (ObjectGraphWindow.Instance == null) {
+				ObjectGraphWindow.Instance = new ObjectGraphWindow();
+				ObjectGraphWindow.Instance.Topmost = true;
+			}
+			ObjectGraphWindow.Instance.RegisterDebuggerEvents();
+			ObjectGraphWindow.Instance.Show();
+			return ObjectGraphWindow.Instance;
 		}
 		
-		private void registerEvents()
+		static bool isDebuggerEventsRegistered = false;
+		
+		void RegisterDebuggerEvents()
 		{
-			debuggerService.IsProcessRunningChanged += new EventHandler(debuggerService_IsProcessRunningChanged);
-			debuggerService.DebugStopped += new EventHandler(debuggerService_DebugStopped);
+			if (!isDebuggerEventsRegistered) {
+				debuggerService.IsProcessRunningChanged += DebuggerService_IsProcessRunningChanged;
+				debuggerService.DebugStopped += DebuggerService_DebugStopped;
+				// cannot use debuggerService.IsProcessRunningChanged.GetInvocationList() from outside
+				isDebuggerEventsRegistered = true;
+			}
 		}
 		
-		private void unregisterEvents()
+		void UnregisterDebuggerEvents()
 		{
-			debuggerService.IsProcessRunningChanged -= new EventHandler(debuggerService_IsProcessRunningChanged);
-			debuggerService.DebugStopped -= new EventHandler(debuggerService_DebugStopped);
+			debuggerService.IsProcessRunningChanged -= DebuggerService_IsProcessRunningChanged;
+			debuggerService.DebugStopped -= DebuggerService_DebugStopped;
+			isDebuggerEventsRegistered = false;
 		}
 		
-		public void debuggerService_IsProcessRunningChanged(object sender, EventArgs e)
+		void DebuggerService_IsProcessRunningChanged(object sender, EventArgs e)
 		{
 			// on step or breakpoint hit
-			if (!debuggerService.IsProcessRunning)
-			{
+			if (!debuggerService.IsProcessRunning) {
 				this.objectGraphControl.Refresh();
 			}
 		}
 		
-		public void debuggerService_DebugStopped(object sender, EventArgs e)
+		void DebuggerService_DebugStopped(object sender, EventArgs e)
 		{
 			this.Close();
 		}