Move string-based binding paths up to Avalonia.Markup. Make the LINQ Expression paths and raw ExpressionNodes (now public) the primarily supported syntax.

src/Avalonia.Base/Data/Core/AvaloniaPropertyAccessorNode.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using System.Reactive.Linq;
+using System.Text;
+
+namespace Avalonia.Data.Core
+{
+    public class AvaloniaPropertyAccessorNode : ExpressionNode, ISettableNode
+    {
+        private readonly bool _enableValidation;
+        private readonly AvaloniaProperty _property;
+
+        public AvaloniaPropertyAccessorNode(AvaloniaProperty property, bool enableValidation)
+        {
+            _property = property;
+            _enableValidation = enableValidation;
+        }
+
+        public override string Description => PropertyName;
+        public string PropertyName { get; }
+        public Type PropertyType => _property.PropertyType;
+
+        public bool SetTargetValue(object value, BindingPriority priority)
+        {
+            try
+            {
+                if (Target.IsAlive && Target.Target is IAvaloniaObject obj)
+                {
+                    obj.SetValue(_property, value, priority);
+                    return true;
+                }
+                return false;
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        protected override IObservable<object> StartListeningCore(WeakReference reference)
+        {
+            return (reference.Target as IAvaloniaObject)?.GetWeakObservable(_property) ?? Observable.Empty<object>();
+        }
+    }
+}

src/Avalonia.Base/Data/Core/EmptyExpressionNode.cs

@@ -6,7 +6,7 @@ using System.Reactive.Linq;
 
 namespace Avalonia.Data.Core
 {
-    internal class EmptyExpressionNode : ExpressionNode
+    public class EmptyExpressionNode : ExpressionNode
     {
         public override string Description => ".";
 

src/Avalonia.Base/Data/Core/ExpressionNode.cs

@@ -9,7 +9,7 @@ using Avalonia.Data;
 
 namespace Avalonia.Data.Core
 {
-    internal abstract class ExpressionNode : ISubject<object>
+    public abstract class ExpressionNode : ISubject<object>
     {
         protected static readonly WeakReference UnsetReference = 
             new WeakReference(AvaloniaProperty.UnsetValue);

src/Avalonia.Base/Data/Core/ExpressionNodeBuilder.cs

@@ -1,38 +0,0 @@
-// Copyright (c) The Avalonia Project. All rights reserved.
-// Licensed under the MIT license. See licence.md file in the project root for full license information.
-
-using System;
-using Avalonia.Data.Core.Parsers;
-using System.Linq.Expressions;
-
-namespace Avalonia.Data.Core
-{
-    internal static class ExpressionNodeBuilder
-    {
-        public static ExpressionNode Build(string expression, bool enableValidation = false)
-        {
-            if (string.IsNullOrWhiteSpace(expression))
-            {
-                throw new ArgumentException("'expression' may not be empty.");
-            }
-
-            var reader = new Reader(expression);
-            var parser = new ExpressionParser(enableValidation);
-            var node = parser.Parse(reader);
-
-            if (!reader.End)
-            {
-                throw new ExpressionParseException(reader.Position, "Expected end of expression.");
-            }
-
-            return node;
-        }
-
-        public static ExpressionNode Build(LambdaExpression expression, bool enableValidation = false)
-        {
-            var parser = new ExpressionTreeParser(enableValidation);
-
-            return parser.Parse(expression);
-        }
-    }
-}

src/Avalonia.Base/Data/Core/ExpressionObserver.cs

@@ -9,6 +9,7 @@ using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Data;
+using Avalonia.Data.Core.Parsers;
 using Avalonia.Data.Core.Plugins;
 
 namespace Avalonia.Data.Core
@@ -63,23 +64,11 @@ namespace Avalonia.Data.Core
         /// Initializes a new instance of the <see cref="ExpressionObserver"/> class.
         /// </summary>
         /// <param name="root">The root object.</param>
-        /// <param name="expression">The expression.</param>
-        /// <param name="enableDataValidation">Whether data validation should be enabled.</param>
+        /// <param name="node">The expression.</param>
         /// <param name="description">
-        /// A description of the expression. If null, <paramref name="expression"/> will be used.
+        /// A description of the expression. If null, <paramref name="node"/> will be used.
         /// </param>
         public ExpressionObserver(
-            object root,
-            string expression,
-            bool enableDataValidation = false,
-            string description = null)
-            : this(root, Parse(expression, enableDataValidation), description ?? expression)
-        {
-            Contract.Requires<ArgumentNullException>(expression != null);
-            Expression = expression;
-        }
-
-        private ExpressionObserver(
             object root,
             ExpressionNode node,
             string description = null)
@@ -107,25 +96,11 @@ namespace Avalonia.Data.Core
         /// Initializes a new instance of the <see cref="ExpressionObserver"/> class.
         /// </summary>
         /// <param name="rootObservable">An observable which provides the root object.</param>
-        /// <param name="expression">The expression.</param>
-        /// <param name="enableDataValidation">Whether data validation should be enabled.</param>
+        /// <param name="node">The expression.</param>
         /// <param name="description">
-        /// A description of the expression. If null, <paramref name="expression"/> will be used.
+        /// A description of the expression. If null, <paramref name="node"/> will be used.
         /// </param>
         public ExpressionObserver(
-            IObservable<object> rootObservable,
-            string expression,
-            bool enableDataValidation = false,
-            string description = null)
-            : this(rootObservable, Parse(expression, enableDataValidation), description ?? expression)
-        {
-            Contract.Requires<ArgumentNullException>(rootObservable != null);
-            Contract.Requires<ArgumentNullException>(expression != null);
-
-            Expression = expression;
-        }
-
-        private ExpressionObserver(
             IObservable<object> rootObservable,
             ExpressionNode node,
             string description)
@@ -137,7 +112,7 @@ namespace Avalonia.Data.Core
             _root = rootObservable;
             _finished = new Subject<Unit>();
         }
-        
+
         public static ExpressionObserver CreateFromExpression<T, U>(
             IObservable<T> rootObservable,
             Expression<Func<T, U>> expression,
@@ -155,26 +130,12 @@ namespace Avalonia.Data.Core
         /// Initializes a new instance of the <see cref="ExpressionObserver"/> class.
         /// </summary>
         /// <param name="rootGetter">A function which gets the root object.</param>
-        /// <param name="expression">The expression.</param>
+        /// <param name="node">The expression.</param>
         /// <param name="update">An observable which triggers a re-read of the getter.</param>
-        /// <param name="enableDataValidation">Whether data validation should be enabled.</param>
         /// <param name="description">
-        /// A description of the expression. If null, <paramref name="expression"/> will be used.
+        /// A description of the expression. If null, <paramref name="node"/> will be used.
         /// </param>
         public ExpressionObserver(
-            Func<object> rootGetter,
-            string expression,
-            IObservable<Unit> update,
-            bool enableDataValidation = false,
-            string description = null)
-            : this(rootGetter, Parse(expression, enableDataValidation), update, description ?? expression)
-        {
-            Contract.Requires<ArgumentNullException>(expression != null);
-
-            Expression = expression;
-        }
-
-        private ExpressionObserver(
             Func<object> rootGetter,
             ExpressionNode node,
             IObservable<Unit> update,
@@ -189,8 +150,8 @@ namespace Avalonia.Data.Core
             _node.Target = new WeakReference(rootGetter());
             _root = update.Select(x => rootGetter());
         }
-        
-        public static ExpressionObserver CreateFromExpression<T, U>(
+
+        public static ExpressionObserver Create<T, U>(
             Func<T> rootGetter,
             Expression<Func<T, U>> expression,
             IObservable<Unit> update,
@@ -291,21 +252,10 @@ namespace Avalonia.Data.Core
             return _result.Subscribe(observer);
         }
 
-        private static ExpressionNode Parse(string expression, bool enableDataValidation)
-        {
-            if (!string.IsNullOrWhiteSpace(expression))
-            {
-                return ExpressionNodeBuilder.Build(expression, enableDataValidation);
-            }
-            else
-            {
-                return new EmptyExpressionNode();
-            }
-        }
-
         private static ExpressionNode Parse(LambdaExpression expression, bool enableDataValidation)
         {
-            return ExpressionNodeBuilder.Build(expression, enableDataValidation);
+            var parser = new ExpressionTreeParser(enableDataValidation);
+            return parser.Parse(expression);
         }
 
         private static object ToWeakReference(object o)

src/Avalonia.Base/Data/Core/IndexerNodeBase.cs

@@ -13,7 +13,7 @@ using Avalonia.Utilities;
 
 namespace Avalonia.Data.Core
 {
-    abstract class IndexerNodeBase : ExpressionNode, ISettableNode
+    public abstract class IndexerNodeBase : ExpressionNode, ISettableNode
     {
         protected override IObservable<object> StartListeningCore(WeakReference reference)
         {

src/Avalonia.Base/Data/Core/LogicalNotNode.cs

@@ -7,7 +7,7 @@ using Avalonia.Data;
 
 namespace Avalonia.Data.Core
 {
-    internal class LogicalNotNode : ExpressionNode, ITransformNode
+    public class LogicalNotNode : ExpressionNode, ITransformNode
     {
         public override string Description => "!";
 

src/Avalonia.Base/Data/Core/Parsers/ExpressionVisitorNodeBuilder.cs

@@ -11,12 +11,14 @@ namespace Avalonia.Data.Core.Parsers
     class ExpressionVisitorNodeBuilder : ExpressionVisitor
     {
         private static PropertyInfo AvaloniaObjectIndexer;
+        private static MethodInfo CreateDelegateMethod;
 
         private readonly bool enableDataValidation;
 
         static ExpressionVisitorNodeBuilder()
         {
             AvaloniaObjectIndexer = typeof(AvaloniaObject).GetProperty("Item", new[] { typeof(AvaloniaProperty) });
+            CreateDelegateMethod = typeof(MethodInfo).GetMethod("CreateDelegate", new[] { typeof(Type), typeof(object) });
         }
 
         public List<ExpressionNode> Nodes { get; }
@@ -70,7 +72,7 @@ namespace Avalonia.Data.Core.Parsers
             if (node.Indexer == AvaloniaObjectIndexer)
             {
                 var property = GetArgumentExpressionValue<AvaloniaProperty>(node.Arguments[0]);
-                Nodes.Add(new PropertyAccessorNode($"{property.OwnerType.Name}.{property.Name}", enableDataValidation));
+                Nodes.Add(new AvaloniaPropertyAccessorNode(property, enableDataValidation));
             }
             else
             {
@@ -166,6 +168,13 @@ namespace Avalonia.Data.Core.Parsers
                 return Visit(Expression.MakeIndex(node.Object, property, node.Arguments));
             }
 
+            if (node.Method == CreateDelegateMethod)
+            {
+                var visited = Visit(node.Arguments[1]);
+                Nodes.Add(new PropertyAccessorNode(GetArgumentExpressionValue<MethodInfo>(node.Object).Name, enableDataValidation));
+                return visited;
+            }
+
             throw new ExpressionParseException(0, $"Invalid expression type in binding expression: {node.NodeType}.");
         }
 

src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs

@@ -10,7 +10,7 @@ using Avalonia.Data.Core.Plugins;
 
 namespace Avalonia.Data.Core
 {
-    internal class PropertyAccessorNode : ExpressionNode, ISettableNode
+    public class PropertyAccessorNode : ExpressionNode, ISettableNode
     {
         private readonly bool _enableValidation;
         private IPropertyAccessor _accessor;

src/Avalonia.Base/Data/Core/StreamNode.cs

@@ -8,7 +8,7 @@ using System.Reactive.Linq;
 
 namespace Avalonia.Data.Core
 {
-    internal class StreamNode : ExpressionNode
+    public class StreamNode : ExpressionNode
     {
         public override string Description => "^";
 

src/Markup/Avalonia.Markup.Xaml/Converters/SelectorTypeConverter.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Markup.Xaml.Converters
 
         public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
         {
-            var parser = new SelectorParser((t, ns) => context.ResolveType(ns, t));
+            var parser = new SelectorParser(context.ResolveType);
 
             return parser.Parse((string)value);
         }

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/BindingExtension.cs

@@ -37,6 +37,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
             return new Binding
             {
+                TypeResolver = descriptorContext.ResolveType,
                 Converter = Converter,
                 ConverterParameter = ConverterParameter,
                 ElementName = pathInfo.ElementName ?? ElementName,

src/Markup/Avalonia.Markup.Xaml/Templates/MemberSelector.cs

@@ -4,6 +4,7 @@
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using System;
 using System.Reactive.Linq;
 
@@ -37,7 +38,7 @@ namespace Avalonia.Markup.Xaml.Templates
                 return o;
             }
 
-            var expression = new ExpressionObserver(o, MemberName);
+            var expression = ExpressionObserverBuilder.Build(o, MemberName);
             object result = AvaloniaProperty.UnsetValue;
 
             expression.Subscribe(x => result = x);

src/Markup/Avalonia.Markup.Xaml/Templates/TreeDataTemplate.cs

@@ -8,6 +8,7 @@ using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Data.Core;
 using Avalonia.Markup.Data;
+using Avalonia.Markup.Parsers;
 using Avalonia.Metadata;
 
 namespace Avalonia.Markup.Xaml.Templates
@@ -41,7 +42,7 @@ namespace Avalonia.Markup.Xaml.Templates
         {
             if (ItemsSource != null)
             {
-                var obs = new ExpressionObserver(item, ItemsSource.Path);
+                var obs = ExpressionObserverBuilder.Build(item, ItemsSource.Path);
                 return InstancedBinding.OneWay(obs, BindingPriority.Style);
             }
 

src/Markup/Avalonia.Markup/Data/Binding.cs

@@ -10,6 +10,7 @@ using Avalonia.Controls;
 using Avalonia.Data.Converters;
 using Avalonia.Data.Core;
 using Avalonia.LogicalTree;
+using Avalonia.Markup.Parsers;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Data
@@ -86,6 +87,11 @@ namespace Avalonia.Data
 
         public WeakReference DefaultAnchor { get; set; }
 
+        /// <summary>
+        /// Gets or sets a function used to resolve types from names in the binding path.
+        /// </summary>
+        public Func<string, string, Type> TypeResolver { get; set; }
+
         /// <inheritdoc/>
         public InstancedBinding Initiate(
             IAvaloniaObject target,
@@ -193,20 +199,22 @@ namespace Avalonia.Data
                 var update = target.GetObservable(StyledElement.DataContextProperty)
                     .Skip(1)
                     .Select(_ => Unit.Default);
-                var result = new ExpressionObserver(
+                var result = ExpressionObserverBuilder.Build(
                     () => target.GetValue(StyledElement.DataContextProperty),
                     path,
                     update,
-                    enableDataValidation);
+                    enableDataValidation,
+                    typeResolver: TypeResolver);
 
                 return result;
             }
             else
             {
-                return new ExpressionObserver(
+                return ExpressionObserverBuilder.Build(
                     GetParentDataContext(target),
                     path,
-                    enableDataValidation);
+                    enableDataValidation,
+                    typeResolver: TypeResolver);
             }
         }
 
@@ -219,11 +227,12 @@ namespace Avalonia.Data
             Contract.Requires<ArgumentNullException>(target != null);
 
             var description = $"#{elementName}.{path}";
-            var result = new ExpressionObserver(
+            var result = ExpressionObserverBuilder.Build(
                 ControlLocator.Track(target, elementName),
                 path,
                 enableDataValidation,
-                description);
+                description,
+                typeResolver: TypeResolver);
             return result;
         }
 
@@ -255,10 +264,11 @@ namespace Avalonia.Data
                     throw new InvalidOperationException("Invalid tree to traverse.");
             }
 
-            return new ExpressionObserver(
+            return ExpressionObserverBuilder.Build(
                 controlLocator,
                 path,
-                enableDataValidation);
+                enableDataValidation,
+                typeResolver: TypeResolver);
         }
 
         private ExpressionObserver CreateSourceObserver(
@@ -268,7 +278,7 @@ namespace Avalonia.Data
         {
             Contract.Requires<ArgumentNullException>(source != null);
 
-            return new ExpressionObserver(source, path, enableDataValidation);
+            return ExpressionObserverBuilder.Build(source, path, enableDataValidation, typeResolver: TypeResolver);
         }
 
         private ExpressionObserver CreateTemplatedParentObserver(
@@ -282,11 +292,12 @@ namespace Avalonia.Data
                 .Skip(1)
                 .Select(_ => Unit.Default);
 
-            var result = new ExpressionObserver(
+            var result = ExpressionObserverBuilder.Build(
                 () => target.GetValue(StyledElement.TemplatedParentProperty),
                 path,
                 update,
-                enableDataValidation);
+                enableDataValidation,
+                typeResolver: TypeResolver);
 
             return result;
         }

src/Avalonia.Base/Data/Core/Parsers/ArgumentListParser.cs → src/Markup/Avalonia.Markup/Markup/Parsers/ArgumentListParser.cs

@@ -1,11 +1,12 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using Avalonia.Data.Core;
 using System;
 using System.Collections.Generic;
 using System.Text;
 
-namespace Avalonia.Data.Core.Parsers
+namespace Avalonia.Markup.Parsers
 {
     internal static class ArgumentListParser
     {

src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionObserverBuilder.cs

@@ -0,0 +1,75 @@
+using Avalonia.Data.Core;
+using System;
+using System.Collections.Generic;
+using System.Reactive;
+using System.Text;
+
+namespace Avalonia.Markup.Parsers
+{
+    public class ExpressionObserverBuilder
+    {
+        internal static ExpressionNode Parse(string expression, bool enableValidation = false, Func<string, string, Type> typeResolver = null)
+        {
+            if (string.IsNullOrWhiteSpace(expression))
+            {
+                return new EmptyExpressionNode();
+            }
+
+            var reader = new Reader(expression);
+            var parser = new ExpressionParser(enableValidation, typeResolver);
+            var node = parser.Parse(reader);
+
+            if (!reader.End)
+            {
+                throw new ExpressionParseException(reader.Position, "Expected end of expression.");
+            }
+
+            return node;
+        }
+
+        public static ExpressionObserver Build(
+            object root,
+            string expression,
+            bool enableDataValidation = false,
+            string description = null,
+            Func<string, string, Type> typeResolver = null)
+        {
+            return new ExpressionObserver(
+                root,
+                Parse(expression, enableDataValidation, typeResolver),
+                description ?? expression);
+        }
+
+        public static ExpressionObserver Build(
+            IObservable<object> rootObservable,
+            string expression,
+            bool enableDataValidation = false,
+            string description = null,
+            Func<string, string, Type> typeResolver = null)
+        {
+            Contract.Requires<ArgumentNullException>(rootObservable != null);
+            return new ExpressionObserver(
+                rootObservable,
+                Parse(expression, enableDataValidation, typeResolver),
+                description ?? expression);
+        }
+
+
+        public static ExpressionObserver Build(
+            Func<object> rootGetter,
+            string expression,
+            IObservable<Unit> update,
+            bool enableDataValidation = false,
+            string description = null,
+            Func<string, string, Type> typeResolver = null)
+        {
+            Contract.Requires<ArgumentNullException>(rootGetter != null);
+
+            return new ExpressionObserver(
+                () => rootGetter(),
+                Parse(expression, enableDataValidation, typeResolver),
+                update,
+                description ?? expression);
+        }
+    }
+}

src/Avalonia.Base/Data/Core/Parsers/ExpressionParser.cs → src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs

@@ -1,18 +1,22 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers.Nodes;
 using System;
 using System.Collections.Generic;
 using System.Linq;
 
-namespace Avalonia.Data.Core.Parsers
+namespace Avalonia.Markup.Parsers
 {
     internal class ExpressionParser
     {
-        private bool _enableValidation;
+        private readonly bool _enableValidation;
+        private readonly Func<string, string, Type> _typeResolver;
 
-        public ExpressionParser(bool enableValidation)
+        public ExpressionParser(bool enableValidation, Func<string, string, Type> typeResolver)
         {
+            _typeResolver = typeResolver;
             _enableValidation = enableValidation;
         }
 
@@ -130,7 +134,19 @@ namespace Avalonia.Data.Core.Parsers
 
         private State ParseAttachedProperty(Reader r, List<ExpressionNode> nodes)
         {
-            var owner = IdentifierParser.Parse(r);
+            string ns = string.Empty;
+            string owner;
+            var ownerOrNamespace = IdentifierParser.Parse(r);
+
+            if (r.TakeIf(':'))
+            {
+                ns = ownerOrNamespace;
+                owner = IdentifierParser.Parse(r);
+            }
+            else
+            {
+                owner = ownerOrNamespace;
+            }
 
             if (r.End || !r.TakeIf('.'))
             {
@@ -144,7 +160,9 @@ namespace Avalonia.Data.Core.Parsers
                 throw new ExpressionParseException(r.Position, "Expected ')'.");
             }
 
-            nodes.Add(new PropertyAccessorNode(owner + '.' + name, _enableValidation));
+            var property = AvaloniaPropertyRegistry.Instance.FindRegistered(_typeResolver(ns, owner), name);
+
+            nodes.Add(new AvaloniaPropertyAccessorNode(property, _enableValidation));
             return State.AfterMember;
         }
 
@@ -157,7 +175,7 @@ namespace Avalonia.Data.Core.Parsers
                 throw new ExpressionParseException(r.Position, "Indexer may not be empty.");
             }
 
-            nodes.Add(new IndexerNode(args));
+            nodes.Add(new StringIndexerNode(args));
             return State.AfterMember;
         }
         

src/Avalonia.Base/Data/Core/Parsers/IdentifierParser.cs → src/Markup/Avalonia.Markup/Markup/Parsers/IdentifierParser.cs

@@ -4,7 +4,7 @@
 using System.Globalization;
 using System.Text;
 
-namespace Avalonia.Data.Core.Parsers
+namespace Avalonia.Markup.Parsers
 {
     internal static class IdentifierParser
     {

src/Avalonia.Base/Data/Core/IndexerNode.cs → src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs

@@ -12,12 +12,13 @@ using System.Linq;
 using System.Reflection;
 using System.Reactive.Linq;
 using Avalonia.Data;
+using Avalonia.Data.Core;
 
-namespace Avalonia.Data.Core
+namespace Avalonia.Markup.Parsers.Nodes
 {
-    internal class IndexerNode : IndexerNodeBase
+    internal class StringIndexerNode : IndexerNodeBase
     {
-        public IndexerNode(IList<string> arguments)
+        public StringIndexerNode(IList<string> arguments)
         {
             Arguments = arguments;
         }

src/Avalonia.Base/Data/Core/Parsers/Reader.cs → src/Markup/Avalonia.Markup/Markup/Parsers/Reader.cs

@@ -3,7 +3,7 @@
 
 using System;
 
-namespace Avalonia.Data.Core.Parsers
+namespace Avalonia.Markup.Parsers
 {
     internal class Reader
     {

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs

@@ -52,11 +52,11 @@ namespace Avalonia.Markup.Parsers
 
                 if (ofType != null)
                 {
-                    result = result.OfType(_typeResolver(ofType.TypeName, ofType.Xmlns));
+                    result = result.OfType(_typeResolver(ofType.Xmlns, ofType.TypeName));
                 }
                 if (@is != null)
                 {
-                    result = result.Is(_typeResolver(@is.TypeName, @is.Xmlns));
+                    result = result.Is(_typeResolver(@is.Xmlns, @is.TypeName));
                 }
                 else if (@class != null)
                 {

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.cs

@@ -10,6 +10,7 @@ using System.Threading.Tasks;
 using Avalonia.Data;
 using Avalonia.Data.Converters;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Avalonia.UnitTests;
 using Moq;
 using Xunit;
@@ -22,7 +23,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_Value()
         {
             var data = new Class1 { StringValue = "foo" };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(string));
             var result = await target.Take(1);
 
             Assert.Equal("foo", result);
@@ -34,7 +35,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Set_Simple_Property_Value()
         {
             var data = new Class1 { StringValue = "foo" };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(string));
 
             target.OnNext("bar");
 
@@ -47,7 +48,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Set_Indexed_Value()
         {
             var data = new { Foo = new[] { "foo" } };
-            var target = new BindingExpression(new ExpressionObserver(data, "Foo[0]"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "Foo[0]"), typeof(string));
 
             target.OnNext("bar");
 
@@ -60,7 +61,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Convert_Get_String_To_Double()
         {
             var data = new Class1 { StringValue = $"{5.6}" };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(double));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(double));
             var result = await target.Take(1);
 
             Assert.Equal(5.6, result);
@@ -72,7 +73,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Getting_Invalid_Double_String_Should_Return_BindingError()
         {
             var data = new Class1 { StringValue = "foo" };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(double));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(double));
             var result = await target.Take(1);
 
             Assert.IsType<BindingNotification>(result);
@@ -84,7 +85,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Coerce_Get_Null_Double_String_To_UnsetValue()
         {
             var data = new Class1 { StringValue = null };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(double));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(double));
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -96,7 +97,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Convert_Set_String_To_Double()
         {
             var data = new Class1 { StringValue = $"{5.6}" };
-            var target = new BindingExpression(new ExpressionObserver(data, "StringValue"), typeof(double));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "StringValue"), typeof(double));
 
             target.OnNext(6.7);
 
@@ -109,7 +110,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Convert_Get_Double_To_String()
         {
             var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue"), typeof(string));
             var result = await target.Take(1);
 
             Assert.Equal($"{5.6}", result);
@@ -121,7 +122,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Convert_Set_Double_To_String()
         {
             var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue"), typeof(string));
 
             target.OnNext($"{6.7}");
 
@@ -135,7 +136,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { StringValue = "foo" };
             var target = new BindingExpression(
-                new ExpressionObserver(data, "StringValue"),
+                ExpressionObserverBuilder.Build(data, "StringValue"),
                 typeof(int),
                 42,
                 DefaultValueConverter.Instance);
@@ -156,7 +157,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { StringValue = "foo" };
             var target = new BindingExpression(
-                new ExpressionObserver(data, "StringValue", true),
+                ExpressionObserverBuilder.Build(data, "StringValue", true),
                 typeof(int),
                 42,
                 DefaultValueConverter.Instance);
@@ -177,7 +178,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { StringValue = "foo" };
             var target = new BindingExpression(
-                new ExpressionObserver(data, "StringValue"),
+                ExpressionObserverBuilder.Build(data, "StringValue"),
                 typeof(int),
                 "bar",
                 DefaultValueConverter.Instance);
@@ -199,7 +200,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { StringValue = "foo" };
             var target = new BindingExpression(
-                new ExpressionObserver(data, "StringValue", true),
+                ExpressionObserverBuilder.Build(data, "StringValue", true),
                 typeof(int),
                 "bar",
                 DefaultValueConverter.Instance);
@@ -220,7 +221,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Setting_Invalid_Double_String_Should_Not_Change_Target()
         {
             var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue"), typeof(string));
 
             target.OnNext("foo");
 
@@ -234,7 +235,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { DoubleValue = 5.6 };
             var target = new BindingExpression(
-                new ExpressionObserver(data, "DoubleValue"),
+                ExpressionObserverBuilder.Build(data, "DoubleValue"),
                 typeof(string),
                 "9.8",
                 DefaultValueConverter.Instance);
@@ -250,7 +251,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Coerce_Setting_Null_Double_To_Default_Value()
         {
             var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue"), typeof(string));
 
             target.OnNext(null);
 
@@ -263,7 +264,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Coerce_Setting_UnsetValue_Double_To_Default_Value()
         {
             var data = new Class1 { DoubleValue = 5.6 };
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue"), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue"), typeof(string));
 
             target.OnNext(AvaloniaProperty.UnsetValue);
 
@@ -279,7 +280,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var converter = new Mock<IValueConverter>();
 
             var target = new BindingExpression(
-                new ExpressionObserver(data, "DoubleValue"),
+                ExpressionObserverBuilder.Build(data, "DoubleValue"),
                 typeof(string),
                 converter.Object,
                 converterParameter: "foo");
@@ -297,7 +298,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var data = new Class1 { DoubleValue = 5.6 };
             var converter = new Mock<IValueConverter>();
             var target = new BindingExpression(
-                new ExpressionObserver(data, "DoubleValue"),
+                ExpressionObserverBuilder.Build(data, "DoubleValue"),
                 typeof(string),
                 converter.Object,
                 converterParameter: "foo");
@@ -314,7 +315,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { DoubleValue = 5.6 };
             var converter = new Mock<IValueConverter>();
-            var target = new BindingExpression(new ExpressionObserver(data, "DoubleValue", true), typeof(string));
+            var target = new BindingExpression(ExpressionObserverBuilder.Build(data, "DoubleValue", true), typeof(string));
             var result = new List<object>();
 
             target.Subscribe(x => result.Add(x));

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_AttachedProperty.cs

@@ -8,21 +8,25 @@ using System.Threading.Tasks;
 using Avalonia.Diagnostics;
 using Avalonia.Data.Core;
 using Xunit;
+using Avalonia.Markup.Parsers;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
     public class ExpressionObserverTests_AttachedProperty
     {
+        private readonly Func<string, string, Type> _typeResolver;
+
         public ExpressionObserverTests_AttachedProperty()
         {
             var foo = Owner.FooProperty;
+            _typeResolver = (_, name) => name == "Owner" ? typeof(Owner) : null;
         }
 
         [Fact]
         public async Task Should_Get_Attached_Property_Value()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "(Owner.Foo)");
+            var target = ExpressionObserverBuilder.Build(data, "(Owner.Foo)", typeResolver: _typeResolver);
             var result = await target.Take(1);
 
             Assert.Equal("foo", result);
@@ -30,6 +34,19 @@ namespace Avalonia.Base.UnitTests.Data.Core
             Assert.Null(((IAvaloniaObjectDebug)data).GetPropertyChangedSubscribers());
         }
 
+        [Fact]
+        public async Task Should_Get_Attached_Property_Value_With_Namespace()
+        {
+            var data = new Class1();
+            var target = ExpressionObserverBuilder.Build(
+                data,
+                "(NS:Owner.Foo)",
+                typeResolver: (ns, name) => ns == "NS" && name == "Owner" ? typeof(Owner) : null);
+            var result = await target.Take(1);
+            Assert.Equal("foo", result);
+            Assert.Null(((IAvaloniaObjectDebug)data).GetPropertyChangedSubscribers());
+        }
+
         [Fact]
         public async Task Should_Get_Chained_Attached_Property_Value()
         {
@@ -41,7 +58,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 }
             };
 
-            var target = new ExpressionObserver(data, "Next.(Owner.Foo)");
+            var target = ExpressionObserverBuilder.Build(data, "Next.(Owner.Foo)", typeResolver: _typeResolver);
             var result = await target.Take(1);
 
             Assert.Equal("bar", result);
@@ -53,7 +70,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_Simple_Attached_Value()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "(Owner.Foo)");
+            var target = ExpressionObserverBuilder.Build(data, "(Owner.Foo)", typeResolver: _typeResolver);
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -77,7 +94,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 }
             };
 
-            var target = new ExpressionObserver(data, "Next.(Owner.Foo)");
+            var target = ExpressionObserverBuilder.Build(data, "Next.(Owner.Foo)", typeResolver: _typeResolver);
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -96,7 +113,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             Func<Tuple<ExpressionObserver, WeakReference>> run = () =>
             {
                 var source = new Class1();
-                var target = new ExpressionObserver(source, "(Owner.Foo)");
+                var target = ExpressionObserverBuilder.Build(source, "(Owner.Foo)", typeResolver: _typeResolver);
                 return Tuple.Create(target, new WeakReference(source));
             };
 
@@ -113,7 +130,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1();
 
-            Assert.Throws<ExpressionParseException>(() => new ExpressionObserver(data, "(Owner)"));
+            Assert.Throws<ExpressionParseException>(() => ExpressionObserverBuilder.Build(data, "(Owner)", typeResolver: _typeResolver));
         }
 
         [Fact]
@@ -121,7 +138,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1();
 
-            Assert.Throws<ExpressionParseException>(() => new ExpressionObserver(data, "(Owner.Foo.Bar)"));
+            Assert.Throws<ExpressionParseException>(() => ExpressionObserverBuilder.Build(data, "(Owner.Foo.Bar)", typeResolver: _typeResolver));
         }
 
         private static class Owner

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_AvaloniaProperty.cs

@@ -8,6 +8,7 @@ using System.Threading.Tasks;
 using Avalonia.Diagnostics;
 using Avalonia.Data.Core;
 using Xunit;
+using Avalonia.Markup.Parsers;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
@@ -22,7 +23,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_Value()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = await target.Take(1);
 
             Assert.Equal("foo", result);
@@ -34,7 +35,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_ClrProperty_Value()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "ClrProperty");
+            var target = ExpressionObserverBuilder.Build(data, "ClrProperty");
             var result = await target.Take(1);
 
             Assert.Equal("clr-property", result);
@@ -44,7 +45,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_Simple_Property_Value()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -63,7 +64,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             Func<Tuple<ExpressionObserver, WeakReference>> run = () =>
             {
                 var source = new Class1();
-                var target = new ExpressionObserver(source, "Foo");
+                var target = ExpressionObserverBuilder.Build(source, "Foo");
                 return Tuple.Create(target, new WeakReference(source));
             };
 

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_DataValidation.cs

@@ -8,6 +8,7 @@ using System.Linq;
 using System.Reactive.Linq;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -19,7 +20,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Doesnt_Send_DataValidationError_When_DataValidatation_Not_Enabled()
         {
             var data = new ExceptionTest { MustBePositive = 5 };
-            var observer = new ExpressionObserver(data, nameof(data.MustBePositive), false);
+            var observer = ExpressionObserverBuilder.Build(data, nameof(data.MustBePositive), false);
             var validationMessageFound = false;
 
             observer.OfType<BindingNotification>()
@@ -36,7 +37,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Exception_Validation_Sends_DataValidationError()
         {
             var data = new ExceptionTest { MustBePositive = 5 };
-            var observer = new ExpressionObserver(data, nameof(data.MustBePositive), true);
+            var observer = ExpressionObserverBuilder.Build(data, nameof(data.MustBePositive), true);
             var validationMessageFound = false;
 
             observer.OfType<BindingNotification>()
@@ -53,7 +54,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Indei_Validation_Does_Not_Subscribe_When_DataValidatation_Not_Enabled()
         {
             var data = new IndeiTest { MustBePositive = 5 };
-            var observer = new ExpressionObserver(data, nameof(data.MustBePositive), false);
+            var observer = ExpressionObserverBuilder.Build(data, nameof(data.MustBePositive), false);
 
             observer.Subscribe(_ => { });
 
@@ -64,7 +65,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Enabled_Indei_Validation_Subscribes()
         {
             var data = new IndeiTest { MustBePositive = 5 };
-            var observer = new ExpressionObserver(data, nameof(data.MustBePositive), true);
+            var observer = ExpressionObserverBuilder.Build(data, nameof(data.MustBePositive), true);
             var sub = observer.Subscribe(_ => { });
 
             Assert.Equal(1, data.ErrorsChangedSubscriptionCount);
@@ -76,7 +77,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Validation_Plugins_Send_Correct_Notifications()
         {
             var data = new IndeiTest();
-            var observer = new ExpressionObserver(data, nameof(data.MustBePositive), true);
+            var observer = ExpressionObserverBuilder.Build(data, nameof(data.MustBePositive), true);
             var result = new List<object>();
             
             var errmsg = string.Empty;
@@ -122,7 +123,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 Inner = new IndeiTest()
             };
 
-            var observer = new ExpressionObserver(
+            var observer = ExpressionObserverBuilder.Build(
                 data,
                 $"{nameof(Container.Inner)}.{nameof(IndeiTest.MustBePositive)}",
                 true);
@@ -142,7 +143,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var container = new Container();
             var inner = new IndeiTest();
 
-            var observer = new ExpressionObserver(
+            var observer = ExpressionObserverBuilder.Build(
                 container,
                 $"{nameof(Container.Inner)}.{nameof(IndeiTest.MustBePositive)}",
                 true);

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Indexer.cs

@@ -11,6 +11,7 @@ using Avalonia.Diagnostics;
 using Avalonia.Data.Core;
 using Avalonia.UnitTests;
 using Xunit;
+using Avalonia.Markup.Parsers;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
@@ -20,7 +21,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Array_Value()
         {
             var data = new { Foo = new [] { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
             var result = await target.Take(1);
 
             Assert.Equal("bar", result);
@@ -32,7 +33,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_UnsetValue_For_Invalid_Array_Index()
         {
             var data = new { Foo = new[] { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[invalid]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[invalid]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -44,7 +45,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_UnsetValue_For_Invalid_Dictionary_Index()
         {
             var data = new { Foo = new Dictionary<int, string> { { 1, "foo" } } };
-            var target = new ExpressionObserver(data, "Foo[invalid]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[invalid]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -56,7 +57,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_UnsetValue_For_Object_Without_Indexer()
         {
             var data = new { Foo = 5 };
-            var target = new ExpressionObserver(data, "Foo[noindexer]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[noindexer]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -68,7 +69,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_MultiDimensional_Array_Value()
         {
             var data = new { Foo = new[,] { { "foo", "bar" }, { "baz", "qux" } } };
-            var target = new ExpressionObserver(data, "Foo[1, 1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1, 1]");
             var result = await target.Take(1);
 
             Assert.Equal("qux", result);
@@ -80,7 +81,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Value_For_String_Indexer()
         {
             var data = new { Foo = new Dictionary<string, string> { { "foo", "bar" }, { "baz", "qux" } } };
-            var target = new ExpressionObserver(data, "Foo[foo]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[foo]");
             var result = await target.Take(1);
 
             Assert.Equal("bar", result);
@@ -92,7 +93,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Value_For_Non_String_Indexer()
         {
             var data = new { Foo = new Dictionary<double, string> { { 1.0, "bar" }, { 2.0, "qux" } } };
-            var target = new ExpressionObserver(data, "Foo[1.0]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1.0]");
             var result = await target.Take(1);
 
             Assert.Equal("bar", result);
@@ -104,7 +105,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Array_Out_Of_Bounds_Should_Return_UnsetValue()
         {
             var data = new { Foo = new[] { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[2]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[2]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -116,7 +117,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Array_With_Wrong_Dimensions_Should_Return_UnsetValue()
         {
             var data = new { Foo = new[] { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1,2]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1,2]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -128,7 +129,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task List_Out_Of_Bounds_Should_Return_UnsetValue()
         {
             var data = new { Foo = new List<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[2]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[2]");
             var result = await target.Take(1);
 
             Assert.Equal(AvaloniaProperty.UnsetValue, result);
@@ -140,7 +141,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_List_Value()
         {
             var data = new { Foo = new List<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
             var result = await target.Take(1);
 
             Assert.Equal("bar", result);
@@ -152,7 +153,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_INCC_Add()
         {
             var data = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[2]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[2]");
             var result = new List<object>();
 
             using (var sub = target.Subscribe(x => result.Add(x)))
@@ -170,7 +171,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_INCC_Remove()
         {
             var data = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[0]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[0]");
             var result = new List<object>();
 
             using (var sub = target.Subscribe(x => result.Add(x)))
@@ -188,7 +189,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_INCC_Replace()
         {
             var data = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
             var result = new List<object>();
 
             using (var sub = target.Subscribe(x => result.Add(x)))
@@ -209,7 +210,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             // method, but even if it did we need to test with ObservableCollection as well
             // as AvaloniaList as it implements PropertyChanged as an explicit interface event.
             var data = new { Foo = new ObservableCollection<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -225,7 +226,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_INCC_Reset()
         {
             var data = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -244,7 +245,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             data.Foo["foo"] = "bar";
             data.Foo["baz"] = "qux";
 
-            var target = new ExpressionObserver(data, "Foo[foo]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[foo]");
             var result = new List<object>();
 
             using (var sub = target.Subscribe(x => result.Add(x)))
@@ -263,7 +264,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_SetArrayIndex()
         {
             var data = new { Foo = new[] { "foo", "bar" } };
-            var target = new ExpressionObserver(data, "Foo[1]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[1]");
 
             using (target.Subscribe(_ => { }))
             {
@@ -286,7 +287,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 }
             };
             
-            var target = new ExpressionObserver(data, "Foo[foo]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[foo]");
             using (target.Subscribe(_ => { }))
             {
                 Assert.True(target.SetValue(4));
@@ -308,7 +309,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 }
             };
             
-            var target = new ExpressionObserver(data, "Foo[bar]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[bar]");
             using (target.Subscribe(_ => { }))
             {
                 Assert.True(target.SetValue(4));
@@ -326,7 +327,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             data.Foo["foo"] = "bar";
             data.Foo["baz"] = "qux";
 
-            var target = new ExpressionObserver(data, "Foo[foo]");
+            var target = ExpressionObserverBuilder.Build(data, "Foo[foo]");
 
             using (target.Subscribe(_ => { }))
             {
@@ -343,7 +344,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new[] { 1, 2, 3 };
 
-            var target = new ExpressionObserver(data, "[1]");
+            var target = ExpressionObserverBuilder.Build(data, "[1]");
 
             var value = await target.Take(1);
 

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Lifetime.cs

@@ -9,6 +9,7 @@ using System.Reactive.Subjects;
 using Microsoft.Reactive.Testing;
 using Avalonia.Data.Core;
 using Xunit;
+using Avalonia.Markup.Parsers;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
@@ -18,7 +19,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Complete_When_Source_Observable_Completes()
         {
             var source = new BehaviorSubject<object>(1);
-            var target = new ExpressionObserver(source, "Foo");
+            var target = ExpressionObserverBuilder.Build(source, "Foo");
             var completed = false;
 
             target.Subscribe(_ => { }, () => completed = true);
@@ -31,7 +32,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Complete_When_Source_Observable_Errors()
         {
             var source = new BehaviorSubject<object>(1);
-            var target = new ExpressionObserver(source, "Foo");
+            var target = ExpressionObserverBuilder.Build(source, "Foo");
             var completed = false;
 
             target.Subscribe(_ => { }, () => completed = true);
@@ -44,7 +45,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Complete_When_Update_Observable_Completes()
         {
             var update = new Subject<Unit>();
-            var target = new ExpressionObserver(() => 1, "Foo", update);
+            var target = ExpressionObserverBuilder.Build(() => 1, "Foo", update);
             var completed = false;
 
             target.Subscribe(_ => { }, () => completed = true);
@@ -57,7 +58,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Complete_When_Update_Observable_Errors()
         {
             var update = new Subject<Unit>();
-            var target = new ExpressionObserver(() => 1, "Foo", update);
+            var target = ExpressionObserverBuilder.Build(() => 1, "Foo", update);
             var completed = false;
 
             target.Subscribe(_ => { }, () => completed = true);
@@ -72,7 +73,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var scheduler = new TestScheduler();
             var source = scheduler.CreateColdObservable(
                 OnNext(1, new { Foo = "foo" }));
-            var target = new ExpressionObserver(source, "Foo");
+            var target = ExpressionObserverBuilder.Build(source, "Foo");
             var result = new List<object>();
 
             using (target.Subscribe(x => result.Add(x)))
@@ -91,7 +92,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var scheduler = new TestScheduler();
             var update = scheduler.CreateColdObservable<Unit>();
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(() => data, "Foo", update);
+            var target = ExpressionObserverBuilder.Build(() => data, "Foo", update);
             var result = new List<object>();
 
             using (target.Subscribe(x => result.Add(x)))

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Method.cs

@@ -1,5 +1,6 @@
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -30,7 +31,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Method()
         {
             var data = new TestObject();
-            var observer = new ExpressionObserver(data, nameof(TestObject.MethodWithoutReturn));
+            var observer = ExpressionObserverBuilder.Build(data, nameof(TestObject.MethodWithoutReturn));
             var result = await observer.Take(1);
 
             Assert.NotNull(result);
@@ -46,7 +47,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Method_WithCorrectDelegateType(string methodName, Type expectedType)
         {
             var data = new TestObject();
-            var observer = new ExpressionObserver(data, methodName);
+            var observer = ExpressionObserverBuilder.Build(data, methodName);
             var result = await observer.Take(1);
 
             Assert.IsType(expectedType, result);
@@ -58,7 +59,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Can_Call_Method_Returned_From_Observer()
         {
             var data = new TestObject();
-            var observer = new ExpressionObserver(data, nameof(TestObject.MethodWithReturnAndParameters));
+            var observer = ExpressionObserverBuilder.Build(data, nameof(TestObject.MethodWithReturnAndParameters));
             var result = await observer.Take(1);
 
             var callback = (Func<int, int>)result;
@@ -74,7 +75,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_Error_Notification_If_Too_Many_Parameters(string methodName)
         {
             var data = new TestObject();
-            var observer = new ExpressionObserver(data, methodName);
+            var observer = ExpressionObserverBuilder.Build(data, methodName);
             var result = await observer.Take(1);
 
             Assert.IsType<BindingNotification>(result);

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Negation.cs

@@ -6,6 +6,7 @@ using System.Reactive.Linq;
 using System.Threading.Tasks;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests.Data.Core
@@ -16,7 +17,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Negate_Boolean_Value()
         {
             var data = new { Foo = true };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.False((bool)result);
@@ -28,7 +29,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Negate_0()
         {
             var data = new { Foo = 0 };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.True((bool)result);
@@ -40,7 +41,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Negate_1()
         {
             var data = new { Foo = 1 };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.False((bool)result);
@@ -52,7 +53,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Negate_False_String()
         {
             var data = new { Foo = "false" };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.True((bool)result);
@@ -64,7 +65,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Negate_True_String()
         {
             var data = new { Foo = "True" };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.False((bool)result);
@@ -76,7 +77,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_For_String_Not_Convertible_To_Boolean()
         {
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -92,7 +93,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_For_Value_Not_Convertible_To_Boolean()
         {
             var data = new { Foo = new object() };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -108,7 +109,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Return_False_For_Invalid_Value()
         {
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             target.Subscribe(_ => { });
 
             Assert.False(target.SetValue("bar"));
@@ -120,7 +121,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Can_SetValue_For_Valid_Value()
         {
             var data = new Test { Foo = true };
-            var target = new ExpressionObserver(data, "!Foo");
+            var target = ExpressionObserverBuilder.Build(data, "!Foo");
             target.Subscribe(_ => { });
 
             Assert.True(target.SetValue(true));

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Observable.cs

@@ -7,6 +7,7 @@ using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -21,7 +22,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var source = new BehaviorSubject<string>("foo");
                 var data = new { Foo = source };
-                var target = new ExpressionObserver(data, "Foo");
+                var target = ExpressionObserverBuilder.Build(data, "Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -41,7 +42,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var source = new BehaviorSubject<string>("foo");
                 var data = new { Foo = source };
-                var target = new ExpressionObserver(data, "Foo^");
+                var target = ExpressionObserverBuilder.Build(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -60,7 +61,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new Class1();
-                var target = new ExpressionObserver(data, "Next^.Foo");
+                var target = ExpressionObserverBuilder.Build(data, "Next^.Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -83,7 +84,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var source = new BehaviorSubject<string>("foo");
                 var data = new { Foo = source };
-                var target = new ExpressionObserver(data, "Foo^", true);
+                var target = ExpressionObserverBuilder.Build(data, "Foo^", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -105,7 +106,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var data1 = new Class1();
                 var data2 = new Class2("foo");
-                var target = new ExpressionObserver(data1, "Next^.Foo", true);
+                var target = ExpressionObserverBuilder.Build(data1, "Next^.Foo", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -128,7 +129,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new Class2("foo");
-                var target = new ExpressionObserver(data, "Foo^", true);
+                var target = ExpressionObserverBuilder.Build(data, "Foo^", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Property.cs

@@ -12,6 +12,7 @@ using Avalonia.Data.Core;
 using Avalonia.UnitTests;
 using Xunit;
 using System.Threading.Tasks;
+using Avalonia.Markup.Parsers;
 
 namespace Avalonia.Base.UnitTests.Data.Core
 {
@@ -21,7 +22,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_Value()
         {
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = await target.Take(1);
 
             Assert.Equal("foo", result);
@@ -33,7 +34,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Get_Simple_Property_Value_Type()
         {
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
 
             target.Subscribe(_ => { });
 
@@ -46,7 +47,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_Value_Null()
         {
             var data = new { Foo = (string)null };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = await target.Take(1);
 
             Assert.Null(result);
@@ -58,7 +59,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_From_Base_Class()
         {
             var data = new Class3 { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = await target.Take(1);
 
             Assert.Equal("foo", result);
@@ -70,7 +71,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_Error_For_Root_Null()
         {
             var data = new Class3 { Foo = "foo" };
-            var target = new ExpressionObserver(default(object), "Foo");
+            var target = ExpressionObserverBuilder.Build(default(object), "Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -87,7 +88,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_Error_For_Root_UnsetValue()
         {
             var data = new Class3 { Foo = "foo" };
-            var target = new ExpressionObserver(AvaloniaProperty.UnsetValue, "Foo");
+            var target = ExpressionObserverBuilder.Build(AvaloniaProperty.UnsetValue, "Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -104,7 +105,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_Error_For_Observable_Root_Null()
         {
             var data = new Class3 { Foo = "foo" };
-            var target = new ExpressionObserver(Observable.Return(default(object)), "Foo");
+            var target = ExpressionObserverBuilder.Build(Observable.Return(default(object)), "Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -121,7 +122,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async void Should_Return_BindingNotification_Error_For_Observable_Root_UnsetValue()
         {
             var data = new Class3 { Foo = "foo" };
-            var target = new ExpressionObserver(Observable.Return(AvaloniaProperty.UnsetValue), "Foo");
+            var target = ExpressionObserverBuilder.Build(Observable.Return(AvaloniaProperty.UnsetValue), "Foo");
             var result = await target.Take(1);
 
             Assert.Equal(
@@ -138,7 +139,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Get_Simple_Property_Chain()
         {
             var data = new { Foo = new { Bar = new { Baz = "baz" } } };
-            var target = new ExpressionObserver(data, "Foo.Bar.Baz");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar.Baz");
             var result = await target.Take(1);
 
             Assert.Equal("baz", result);
@@ -150,7 +151,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Get_Simple_Property_Chain_Type()
         {
             var data = new { Foo = new { Bar = new { Baz = "baz" } } };
-            var target = new ExpressionObserver(data, "Foo.Bar.Baz");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar.Baz");
 
             target.Subscribe(_ => { });
 
@@ -163,7 +164,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public async Task Should_Return_BindingNotification_Error_For_Broken_Chain()
         {
             var data = new { Foo = new { Bar = 1 } };
-            var target = new ExpressionObserver(data, "Foo.Bar.Baz");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar.Baz");
             var result = await target.Take(1);
 
             Assert.IsType<BindingNotification>(result);
@@ -180,7 +181,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Return_BindingNotification_Error_For_Chain_With_Null_Value()
         {
             var data = new { Foo = default(object) };
-            var target = new ExpressionObserver(data, "Foo.Bar.Baz");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar.Baz");
             var result = new List<object>();
 
             target.Subscribe(x => result.Add(x));
@@ -202,7 +203,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Have_Null_ResultType_For_Broken_Chain()
         {
             var data = new { Foo = new { Bar = 1 } };
-            var target = new ExpressionObserver(data, "Foo.Bar.Baz");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar.Baz");
 
             Assert.Null(target.ResultType);
 
@@ -213,7 +214,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_Simple_Property_Value()
         {
             var data = new Class1 { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -232,7 +233,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Trigger_PropertyChanged_On_Null_Or_Empty_String()
         {
             var data = new Class1 { Bar = "foo" };
-            var target = new ExpressionObserver(data, "Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Bar");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -262,7 +263,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_End_Of_Property_Chain_Changing()
         {
             var data = new Class1 { Next = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -283,7 +284,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_Property_Chain_Changing()
         {
             var data = new Class1 { Next = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -316,7 +317,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
                 }
             };
 
-            var target = new ExpressionObserver(data, "Next.Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Next.Bar");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -349,7 +350,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Track_Property_Chain_Breaking_With_Missing_Member_Then_Mending()
         {
             var data = new Class1 { Next = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
             var result = new List<object>();
 
             var sub = target.Subscribe(x => result.Add(x));
@@ -384,7 +385,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { Foo = "foo" };
             var update = new Subject<Unit>();
-            var target = new ExpressionObserver(() => data.Foo, "", update);
+            var target = ExpressionObserverBuilder.Build(() => data.Foo, "", update);
             var result = new List<object>();
 
             target.Subscribe(x => result.Add(x));
@@ -404,7 +405,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var source = scheduler.CreateColdObservable(
                 OnNext(1, new Class1 { Foo = "foo" }),
                 OnNext(2, new Class1 { Foo = "bar" }));
-            var target = new ExpressionObserver(source, "Foo");
+            var target = ExpressionObserverBuilder.Build(source, "Foo");
             var result = new List<object>();
 
             using (target.Subscribe(x => result.Add(x)))
@@ -420,7 +421,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Subscribing_Multiple_Times_Should_Return_Values_To_All()
         {
             var data = new Class1 { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result1 = new List<object>();
             var result2 = new List<object>();
             var result3 = new List<object>();
@@ -443,7 +444,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Subscribing_Multiple_Times_Should_Only_Add_PropertyChanged_Handlers_Once()
         {
             var data = new Class1 { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
 
             var sub1 = target.Subscribe(x => { });
             var sub2 = target.Subscribe(x => { });
@@ -462,7 +463,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Set_Simple_Property_Value()
         {
             var data = new Class1 { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
 
             using (target.Subscribe(_ => { }))
             {
@@ -478,7 +479,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Set_Property_At_The_End_Of_Chain()
         {
             var data = new Class1 { Next = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
 
             using (target.Subscribe(_ => { }))
             {
@@ -494,7 +495,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Return_False_For_Missing_Property()
         {
             var data = new Class1 { Next = new WithoutBar() };
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
 
             using (target.Subscribe(_ => { }))
             {
@@ -508,7 +509,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Notify_New_Value_With_Inpc()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
             var result = new List<object>();
 
             target.Subscribe(x => result.Add(x));
@@ -523,7 +524,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Notify_New_Value_Without_Inpc()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Bar");
             var result = new List<object>();
 
             target.Subscribe(x => result.Add(x));
@@ -538,7 +539,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void SetValue_Should_Return_False_For_Missing_Object()
         {
             var data = new Class1();
-            var target = new ExpressionObserver(data, "Next.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Next.Bar");
 
             using (target.Subscribe(_ => { }))
             {
@@ -555,7 +556,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             var second = new Class1 { Foo = "bar" };
             var root = first;
             var update = new Subject<Unit>();
-            var target = new ExpressionObserver(() => root, "Foo", update);
+            var target = ExpressionObserverBuilder.Build(() => root, "Foo", update);
             var result = new List<object>();
             var sub = target.Subscribe(x => result.Add(x));
 
@@ -589,7 +590,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             Func<Tuple<ExpressionObserver, WeakReference>> run = () =>
             {
                 var source = new Class1 { Foo = "foo" };
-                var target = new ExpressionObserver(source, "Foo");
+                var target = ExpressionObserverBuilder.Build(source, "Foo");
                 return Tuple.Create(target, new WeakReference(source));
             };
 

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_SetValue.cs

@@ -5,6 +5,7 @@ using System;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -16,7 +17,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Set_Simple_Property_Value()
         {
             var data = new { Foo = "foo" };
-            var target = new ExpressionObserver(data, "Foo");
+            var target = ExpressionObserverBuilder.Build(data, "Foo");
 
             using (target.Subscribe(_ => { }))
             {
@@ -30,7 +31,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Set_Value_On_Simple_Property_Chain()
         {
             var data = new Class1 { Foo = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Foo.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar");
 
             using (target.Subscribe(_ => { }))
             {
@@ -44,7 +45,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         public void Should_Not_Try_To_Set_Value_On_Broken_Chain()
         {
             var data = new Class1 { Foo = new Class2 { Bar = "bar" } };
-            var target = new ExpressionObserver(data, "Foo.Bar");
+            var target = ExpressionObserverBuilder.Build(data, "Foo.Bar");
 
             // Ensure the ExpressionObserver's subscriptions are kept active.
             target.OfType<string>().Subscribe(x => { });
@@ -67,7 +68,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         {
             var data = new Class1 { Foo = new Class2 { Bar = "bar" } };
             var rootObservable = new BehaviorSubject<Class1>(data);
-            var target = new ExpressionObserver(rootObservable, "Foo.Bar");
+            var target = ExpressionObserverBuilder.Build(rootObservable, "Foo.Bar");
 
             target.Subscribe(_ => { });
             rootObservable.OnNext(null);

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Task.cs

@@ -7,6 +7,7 @@ using System.Reactive.Linq;
 using System.Threading.Tasks;
 using Avalonia.Data;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -21,7 +22,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var tcs = new TaskCompletionSource<string>();
                 var data = new { Foo = tcs.Task };
-                var target = new ExpressionObserver(data, "Foo");
+                var target = ExpressionObserverBuilder.Build(data, "Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -41,7 +42,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new { Foo = Task.FromResult("foo") };
-                var target = new ExpressionObserver(data, "Foo^");
+                var target = ExpressionObserverBuilder.Build(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -59,7 +60,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var tcs = new TaskCompletionSource<Class2>();
                 var data = new Class1(tcs.Task);
-                var target = new ExpressionObserver(data, "Next^.Foo");
+                var target = ExpressionObserverBuilder.Build(data, "Next^.Foo");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -79,7 +80,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var tcs = new TaskCompletionSource<string>();
                 var data = new { Foo = tcs.Task };
-                var target = new ExpressionObserver(data, "Foo^");
+                var target = ExpressionObserverBuilder.Build(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -105,7 +106,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             using (var sync = UnitTestSynchronizationContext.Begin())
             {
                 var data = new { Foo = TaskFromException(new NotSupportedException()) };
-                var target = new ExpressionObserver(data, "Foo^");
+                var target = ExpressionObserverBuilder.Build(data, "Foo^");
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));
@@ -130,7 +131,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
             {
                 var tcs = new TaskCompletionSource<string>();
                 var data = new { Foo = tcs.Task };
-                var target = new ExpressionObserver(data, "Foo^", true);
+                var target = ExpressionObserverBuilder.Build(data, "Foo^", true);
                 var result = new List<object>();
 
                 var sub = target.Subscribe(x => result.Add(x));

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -515,7 +515,7 @@ namespace Avalonia.Controls.UnitTests
 
             public InstancedBinding ItemsSelector(object item)
             {
-                var obs = new ExpressionObserver(item, nameof(Node.Children));
+                var obs = ExpressionObserver.CreateFromExpression(item, o => (o as Node).Children);
                 return InstancedBinding.OneWay(obs);
             }
 

tests/Avalonia.LeakTests/ExpressionObserverTests.cs

@@ -24,7 +24,7 @@ namespace Avalonia.LeakTests
             Func<ExpressionObserver> run = () =>
             {
                 var source = new { Foo = new AvaloniaList<string> {"foo", "bar"} };
-                var target = new ExpressionObserver(source, "Foo");
+                var target = ExpressionObserver.CreateFromExpression(source, o => o.Foo);
 
                 target.Subscribe(_ => { });
                 return target;
@@ -42,7 +42,7 @@ namespace Avalonia.LeakTests
             Func<ExpressionObserver> run = () =>
             {
                 var source = new { Foo = new AvaloniaList<string> { "foo", "bar" } };
-                var target = new ExpressionObserver(source, "Foo", true);
+                var target = ExpressionObserver.CreateFromExpression(source, o => o.Foo, true);
 
                 target.Subscribe(_ => { });
                 return target;
@@ -60,7 +60,7 @@ namespace Avalonia.LeakTests
             Func<ExpressionObserver> run = () =>
             {
                 var source = new { Foo = new NonIntegerIndexer() };
-                var target = new ExpressionObserver(source, "Foo");
+                var target = ExpressionObserver.CreateFromExpression(source, o => o.Foo);
 
                 target.Subscribe(_ => { });
                 return target;
@@ -78,7 +78,7 @@ namespace Avalonia.LeakTests
             Func<ExpressionObserver> run = () =>
             {
                 var source = new { Foo = new MethodBound() };
-                var target = new ExpressionObserver(source, "Foo.A");
+                var target = ExpressionObserver.CreateFromExpression(source, o => (Action)o.Foo.A);
                 target.Subscribe(_ => { });
                 return target;
             };

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionNodeBuilderTests.cs → tests/Avalonia.Markup.UnitTests/Parsers/ExpressionNodeBuilderTests.cs

@@ -4,16 +4,18 @@
 using System.Collections.Generic;
 using System.Linq;
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
+using Avalonia.Markup.Parsers.Nodes;
 using Xunit;
 
-namespace Avalonia.Base.UnitTests.Data.Core
+namespace Avalonia.Markup.UnitTests.Parsers
 {
-    public class ExpressionNodeBuilderTests
+    public class ExpressionObserverBuilderTests
     {
         [Fact]
         public void Should_Build_Single_Property()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo"));
 
             AssertIsProperty(result[0], "Foo");
         }
@@ -21,7 +23,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Underscored_Property()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("_Foo"));
+            var result = ToList(ExpressionObserverBuilder.Parse("_Foo"));
 
             AssertIsProperty(result[0], "_Foo");
         }
@@ -29,7 +31,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Property_With_Digits()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("F0o"));
+            var result = ToList(ExpressionObserverBuilder.Parse("F0o"));
 
             AssertIsProperty(result[0], "F0o");
         }
@@ -37,7 +39,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Property_Chain()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo.Bar.Baz"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo.Bar.Baz"));
 
             Assert.Equal(3, result.Count);
             AssertIsProperty(result[0], "Foo");
@@ -48,7 +50,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Negated_Property_Chain()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("!Foo.Bar.Baz"));
+            var result = ToList(ExpressionObserverBuilder.Parse("!Foo.Bar.Baz"));
 
             Assert.Equal(4, result.Count);
             Assert.IsType<LogicalNotNode>(result[0]);
@@ -60,7 +62,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Double_Negated_Property_Chain()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("!!Foo.Bar.Baz"));
+            var result = ToList(ExpressionObserverBuilder.Parse("!!Foo.Bar.Baz"));
 
             Assert.Equal(5, result.Count);
             Assert.IsType<LogicalNotNode>(result[0]);
@@ -73,29 +75,29 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Indexed_Property()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo[15]"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo[15]"));
 
             Assert.Equal(2, result.Count);
             AssertIsProperty(result[0], "Foo");
             AssertIsIndexer(result[1], "15");
-            Assert.IsType<IndexerNode>(result[1]);
+            Assert.IsType<StringIndexerNode>(result[1]);
         }
 
         [Fact]
         public void Should_Build_Indexed_Property_StringIndex()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo[Key]"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo[Key]"));
 
             Assert.Equal(2, result.Count);
             AssertIsProperty(result[0], "Foo");
             AssertIsIndexer(result[1], "Key");
-            Assert.IsType<IndexerNode>(result[1]);
+            Assert.IsType<StringIndexerNode>(result[1]);
         }
 
         [Fact]
         public void Should_Build_Multiple_Indexed_Property()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo[15,6]"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo[15,6]"));
 
             Assert.Equal(2, result.Count);
             AssertIsProperty(result[0], "Foo");
@@ -105,7 +107,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Multiple_Indexed_Property_With_Space()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo[5, 16]"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo[5, 16]"));
 
             Assert.Equal(2, result.Count);
             AssertIsProperty(result[0], "Foo");
@@ -115,7 +117,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Consecutive_Indexers()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo[15][16]"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo[15][16]"));
 
             Assert.Equal(3, result.Count);
             AssertIsProperty(result[0], "Foo");
@@ -126,7 +128,7 @@ namespace Avalonia.Base.UnitTests.Data.Core
         [Fact]
         public void Should_Build_Indexed_Property_In_Chain()
         {
-            var result = ToList(ExpressionNodeBuilder.Build("Foo.Bar[5, 6].Baz"));
+            var result = ToList(ExpressionObserverBuilder.Parse("Foo.Bar[5, 6].Baz"));
 
             Assert.Equal(4, result.Count);
             AssertIsProperty(result[0], "Foo");
@@ -145,9 +147,9 @@ namespace Avalonia.Base.UnitTests.Data.Core
 
         private void AssertIsIndexer(ExpressionNode node, params string[] args)
         {
-            Assert.IsType<IndexerNode>(node);
+            Assert.IsType<StringIndexerNode>(node);
 
-            var e = (IndexerNode)node;
+            var e = (StringIndexerNode)node;
             Assert.Equal(e.Arguments.ToArray(), args);
         }
 

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionNodeBuilderTests_Errors.cs → tests/Avalonia.Markup.UnitTests/Parsers/ExpressionNodeBuilderTests_Errors.cs

@@ -2,73 +2,74 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using Avalonia.Data.Core;
+using Avalonia.Markup.Parsers;
 using Xunit;
 
-namespace Avalonia.Base.UnitTests.Data.Core
+namespace Avalonia.Markup.UnitTests.Parsers
 {
-    public class ExpressionNodeBuilderTests_Errors
+    public class ExpressionObserverBuilderTests_Errors
     {
         [Fact]
         public void Identifier_Cannot_Start_With_Digit()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("1Foo"));
+                () => ExpressionObserverBuilder.Parse("1Foo"));
         }
 
         [Fact]
         public void Identifier_Cannot_Start_With_Symbol()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.%Bar"));
+                () => ExpressionObserverBuilder.Parse("Foo.%Bar"));
         }
 
         [Fact]
         public void Expression_Cannot_End_With_Period()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar."));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar."));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Empty_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[]"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[]"));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Extra_Comma_At_Start_Of_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[,3,4]"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[,3,4]"));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Extra_Comma_In_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[3,,4]"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[3,,4]"));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Extra_Comma_At_End_Of_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[3,4,]"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[3,4,]"));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Digit_After_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[3,4]5"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[3,4]5"));
         }
 
         [Fact]
         public void Expression_Cannot_Have_Letter_After_Indexer()
         {
             Assert.Throws<ExpressionParseException>(
-                () => ExpressionNodeBuilder.Build("Foo.Bar[3,4]A"));
+                () => ExpressionObserverBuilder.Parse("Foo.Bar[3,4]A"));
         }
     }
 }

tests/Avalonia.Markup.UnitTests/Parsers/SelectorGrammarTests.cs

@@ -6,7 +6,7 @@ using Avalonia.Markup.Parsers;
 using Sprache;
 using Xunit;
 
-namespace Avalonia.Markup.UnitTest.Parsers
+namespace Avalonia.Markup.UnitTests.Parsers
 {
     public class SelectorGrammarTests
     {

tests/Avalonia.Markup.UnitTests/Parsers/SelectorParserTests.cs

@@ -3,7 +3,7 @@ using Avalonia.Controls;
 using Avalonia.Markup.Parsers;
 using Xunit;
 
-namespace Avalonia.Markup.Xaml.UnitTests.Parsers
+namespace Avalonia.Markup.UnitTests.Parsers
 {
     public class SelectorParserTests
     {