diff --git a/src/AddIns/Debugger/Debugger.AddIn/NRefactory/ExpressionEvaluationVisitor.cs b/src/AddIns/Debugger/Debugger.AddIn/NRefactory/ExpressionEvaluationVisitor.cs
index 40db94eda9..30ea878d5c 100644
--- a/src/AddIns/Debugger/Debugger.AddIn/NRefactory/ExpressionEvaluationVisitor.cs
+++ b/src/AddIns/Debugger/Debugger.AddIn/NRefactory/ExpressionEvaluationVisitor.cs
@@ -29,6 +29,7 @@ using ICSharpCode.NRefactory.CSharp.Resolver;
 using ICSharpCode.NRefactory.Semantics;
 using ICSharpCode.NRefactory.TypeSystem;
 using ICSharpCode.NRefactory.Utils;
+using ICSharpCode.SharpDevelop;
 using ICSharpCode.SharpDevelop.Services;
 
 namespace Debugger.AddIn
@@ -239,7 +240,7 @@ namespace Debugger.AddIn
 			var val = resolver.ResolveUnaryOperator(operatorType, operand);
 			if (val.IsCompileTimeConstant)
 				return Convert(val);
-			throw new InvalidOperationException();
+			throw new GetValueException("Operator {0} is not supported for {1}!", operatorType, new CSharpAmbience().ConvertType(operand.Type));
 		}
 		
 		Value VisitBinaryOperator(OperatorResolveResult result, BinaryOperatorType operatorType, bool checkForOverflow = false)
@@ -256,14 +257,41 @@ namespace Debugger.AddIn
 			var val = resolver.ResolveBinaryOperator(operatorType, lhsRR, rhsRR);
 			if (val.IsCompileTimeConstant)
 				return Convert(val);
-			if (operatorType == BinaryOperatorType.Add &&
-			    (lhsRR.Type.IsKnownType(KnownTypeCode.String) || rhsRR.Type.IsKnownType(KnownTypeCode.String))) {
-				var method = debuggerTypeSystem.FindType(KnownTypeCode.String)
-					.GetMethods(m => m.Name == "Concat" && m.Parameters.Count == 2)
-					.Single(m => m.Parameters.All(p => p.Type.IsKnownType(KnownTypeCode.Object)));
-				return InvokeMethod(null, method, lhs, rhs);
+			switch (operatorType) {
+				case BinaryOperatorType.Equality:
+				case BinaryOperatorType.InEquality:
+					bool equality = (operatorType == BinaryOperatorType.Equality);
+					if (lhsRR.Type.IsKnownType(KnownTypeCode.String) && rhsRR.Type.IsKnownType(KnownTypeCode.String)) {
+						if (lhs.IsNull || rhs.IsNull) {
+							bool bothNull = lhs.IsNull && rhs.IsNull;
+							return Eval.CreateValue(evalThread, equality ? bothNull : !bothNull);
+						} else {
+							bool equal = (string)lhs.PrimitiveValue == (string)rhs.PrimitiveValue;
+							return Eval.CreateValue(evalThread, equality ? equal : !equal);
+						}
+					}
+					if (lhsRR.Type.IsReferenceType != false && rhsRR.Type.IsReferenceType != false) {
+						// Reference comparison
+						if (lhs.IsNull || rhs.IsNull) {
+							bool bothNull = lhs.IsNull && rhs.IsNull;
+							return Eval.CreateValue(evalThread, equality ? bothNull : !bothNull);
+						} else {
+							bool equal = lhs.Address == rhs.Address;
+							return Eval.CreateValue(evalThread, equality ? equal : !equal);
+						}
+					}
+					goto default;
+				case BinaryOperatorType.Add:
+					if (lhsRR.Type.IsKnownType(KnownTypeCode.String) || rhsRR.Type.IsKnownType(KnownTypeCode.String)) {
+						var method = debuggerTypeSystem.FindType(KnownTypeCode.String)
+							.GetMethods(m => m.Name == "Concat" && m.Parameters.Count == 2)
+							.Single(m => m.Parameters.All(p => p.Type.IsKnownType(KnownTypeCode.Object)));
+						return InvokeMethod(null, method, lhs, rhs);
+					}
+					goto default;
+				default:
+					throw new GetValueException("Operator {0} is not supported for {1} and {2}!", operatorType, new CSharpAmbience().ConvertType(lhsRR.Type), new CSharpAmbience().ConvertType(rhsRR.Type));
 			}
-			throw new InvalidOperationException();
 		}
 		
 		Value VisitConditionalOperator(OperatorResolveResult result, BinaryOperatorType bitwiseOperatorType)
@@ -357,7 +385,7 @@ namespace Debugger.AddIn
 				return val;
 			if (result.Conversion.IsNullLiteralConversion)
 				return val;
-			throw new NotImplementedException(string.Format("conversion '{0}' not implemented!", result.Conversion));
+			throw new GetValueException("conversion '{0}' not implemented!", result.Conversion);
 		}
 		
 		Value Visit(LocalResolveResult result)
@@ -455,12 +483,21 @@ namespace Debugger.AddIn
 	
 	public class ResolveResultPrettyPrinter
 	{
-		public ResolveResultPrettyPrinter()
+		ResolveResultPrettyPrinter()
 		{
-			
 		}
 		
-		public string Print(ResolveResult result)
+		public static string Print(ResolveResult result)
+		{
+			try {
+				return new ResolveResultPrettyPrinter().PrintInternal(result);
+			} catch (NotImplementedException ex) {
+				SD.Log.Warn(ex);
+				return "";
+			}
+		}
+		
+		string PrintInternal(ResolveResult result)
 		{
 			if (result == null)
 				return "";
@@ -486,7 +523,7 @@ namespace Debugger.AddIn
 		
 		string Visit(MemberResolveResult result)
 		{
-			string text = Print(result.TargetResult);
+			string text = PrintInternal(result.TargetResult);
 			if (!string.IsNullOrWhiteSpace(text))
 				text += ".";
 			return text + result.Member.Name;
@@ -553,7 +590,7 @@ namespace Debugger.AddIn
 		{
 			StringBuilder sb = new StringBuilder();
 			
-			sb.Append(Print(result.TargetResult));
+			sb.Append(PrintInternal(result.TargetResult));
 			sb.Append('.');
 			sb.Append(result.Member.Name);
 			
diff --git a/src/AddIns/Debugger/Debugger.AddIn/Pads/AutoCompleteTextBox.cs b/src/AddIns/Debugger/Debugger.AddIn/Pads/AutoCompleteTextBox.cs
index 41b39d6533..1510a5efc7 100644
--- a/src/AddIns/Debugger/Debugger.AddIn/Pads/AutoCompleteTextBox.cs
+++ b/src/AddIns/Debugger/Debugger.AddIn/Pads/AutoCompleteTextBox.cs
@@ -20,6 +20,7 @@ using System;
 using System.Text;
 using System.Windows;
 using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
 using System.Windows.Input;
 using System.Windows.Media;
 using System.Windows.Threading;
@@ -102,6 +103,7 @@ namespace Debugger.AddIn.Pads.Controls
 			this.editor.TextArea.TextEntered += editor_TextArea_TextEntered;
 			
 			this.Content = this.editor.TextArea;
+			this.messageView = new ToolTip { PlacementTarget = this, Placement = PlacementMode.Bottom, StaysOpen = true };
 			
 			HorizontalContentAlignment = HorizontalAlignment.Stretch;
 			VerticalContentAlignment = VerticalAlignment.Stretch;
@@ -119,22 +121,38 @@ namespace Debugger.AddIn.Pads.Controls
 			}
 		}
 		
+		ToolTip messageView;
+		
 		void editor_TextArea_TextEntered(object sender, TextCompositionEventArgs e)
 		{
-			if (e.Text != ".") return;
-			DebuggerCompletionContext context = null;
-			StackFrame frame = WindowsDebugger.CurrentStackFrame;
-			if (frame == null) {
-				if (DebugContext != null) {
-					context = DebugContext;
+			if (e.Text == ".") {
+				DebuggerCompletionContext context = null;
+				StackFrame frame = WindowsDebugger.CurrentStackFrame;
+				if (frame == null) {
+					if (DebugContext != null) {
+						context = DebugContext;
+					}
+				} else {
+					context = new DebuggerCompletionContext(frame);
 				}
+				if (context == null) return;
+				var binding = DebuggerDotCompletion.PrepareDotCompletion(editor.Text.Substring(0, editor.CaretOffset), context);
+				if (binding == null) return;
+				binding.HandleKeyPressed(editorAdapter, '.');
 			} else {
-				context = new DebuggerCompletionContext(frame);
+				// TODO : implement automated error checking CSharpParser.ParseExpression does not report useful error messages.
+//				Error[] errors;
+//				if (!DebuggerDotCompletion.CheckSyntax(Text, out errors)) {
+//					StringBuilder output = new StringBuilder();
+//					foreach (var error in errors) {
+//						output.AppendLine(error.Message + " at " + error.Region.Begin);
+//					}
+//					messageView.Content = output.ToString();
+//					messageView.IsOpen = true;
+//				} else {
+//					messageView.IsOpen = false;
+//				}
 			}
-			if (context == null) return;
-			var binding = DebuggerDotCompletion.PrepareDotCompletion(editor.Text.Substring(0, editor.CaretOffset), context);
-			if (binding == null) return;
-			binding.HandleKeyPressed(editorAdapter, '.');
 		}
 		
 		public void FocusEditor()
diff --git a/src/AddIns/Debugger/Debugger.AddIn/Pads/DebuggerDotCompletion.cs b/src/AddIns/Debugger/Debugger.AddIn/Pads/DebuggerDotCompletion.cs
index 9ab75a4063..7ef10201d3 100644
--- a/src/AddIns/Debugger/Debugger.AddIn/Pads/DebuggerDotCompletion.cs
+++ b/src/AddIns/Debugger/Debugger.AddIn/Pads/DebuggerDotCompletion.cs
@@ -33,11 +33,12 @@ namespace Debugger.AddIn.Pads.Controls
 {
 	static class DebuggerDotCompletion
 	{
-		public static bool CheckSyntax(string expression)
+		public static bool CheckSyntax(string expression, out Error[] errors)
 		{
 			var p = new CSharpParser();
 			p.ParseExpression(expression);
-			return !p.Errors.Any();
+			errors = p.Errors.ToArray();
+			return !errors.Any();
 		}
 		
 		public static ICodeCompletionBinding PrepareDotCompletion(string expressionToComplete, DebuggerCompletionContext context)
diff --git a/src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs b/src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs
index 0e486fe9f6..a9df381344 100644
--- a/src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs
+++ b/src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs
@@ -629,12 +629,7 @@ namespace ICSharpCode.SharpDevelop.Services
 			if (resolveResult == null)
 				return;
 			if (resolveResult is LocalResolveResult || resolveResult is MemberResolveResult) {
-				string text = string.Empty;
-				try {
-					text = new ResolveResultPrettyPrinter().Print(resolveResult);
-				} catch (NotImplementedException ex) {
-					SD.Log.Warn(ex);
-				}
+				string text = ResolveResultPrettyPrinter.Print(resolveResult);
 				Func<Value> getValue = delegate {
 					ExpressionEvaluationVisitor eval = new ExpressionEvaluationVisitor(CurrentStackFrame, EvalThread, CurrentStackFrame.AppDomain.Compilation);
 					return eval.Convert(resolveResult);