Improve stream operator error message.

When a stream operator is applied to an unsupported type.

src/Avalonia.Base/Avalonia.Base.csproj

@@ -44,7 +44,7 @@
     <Compile Include="..\Shared\SharedAssemblyInfo.cs">
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
-    <Compile Include="Data\BindingChainNullException.cs" />
+    <Compile Include="Data\BindingChainException.cs" />
     <Compile Include="Data\BindingNotification.cs" />
     <Compile Include="Data\IndexerBinding.cs" />
     <Compile Include="Diagnostics\INotifyCollectionChangedDebug.cs" />

src/Avalonia.Base/Data/BindingChainNullException.cs → src/Avalonia.Base/Data/BindingChainException.cs

@@ -10,36 +10,39 @@ namespace Avalonia.Data
     /// requested binding expression could not be evaluated because of a null in one of the links
     /// of the binding chain.
     /// </summary>
-    public class BindingChainNullException : Exception
+    public class BindingChainException : Exception
     {
         private string _message;
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
-        public BindingChainNullException()
+        public BindingChainException()
         {
         }
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
-        public BindingChainNullException(string message)
+        /// <param name="message">The error message.</param>
+        public BindingChainException(string message)
         {
             _message = message;
         }
 
         /// <summary>
-        /// Initalizes a new instance of the <see cref="BindingChainNullException"/> class.
+        /// Initalizes a new instance of the <see cref="BindingChainException"/> class.
         /// </summary>
+        /// <param name="message">The error message.</param>
         /// <param name="expression">The expression.</param>
-        /// <param name="expressionNullPoint">
-        /// The point in the expression at which the null was encountered.
+        /// <param name="errorPoint">
+        /// The point in the expression at which the error was encountered.
         /// </param>
-        public BindingChainNullException(string expression, string expressionNullPoint)
+        public BindingChainException(string message, string expression, string errorPoint)
         {
+            _message = message;
             Expression = expression;
-            ExpressionNullPoint = expressionNullPoint;
+            ExpressionErrorPoint = errorPoint;
         }
 
         /// <summary>
@@ -48,37 +51,27 @@ namespace Avalonia.Data
         public string Expression { get; protected set; }
 
         /// <summary>
-        /// Gets the point in the expression at which the null was encountered.
+        /// Gets the point in the expression at which the error occured.
         /// </summary>
-        public string ExpressionNullPoint { get; protected set; }
+        public string ExpressionErrorPoint { get; protected set; }
 
         /// <inheritdoc/>
         public override string Message
         {
             get
             {
-                if (_message == null)
+                if (Expression != null && ExpressionErrorPoint != null)
                 {
-                    _message = BuildMessage();
+                    return $"{_message} in expression '{Expression}' at '{ExpressionErrorPoint}'.";
+                }
+                else if (ExpressionErrorPoint != null)
+                {
+                    return $"{_message} in expression '{ExpressionErrorPoint}'.";
+                }
+                else
+                {
+                    return $"{_message} in expression.";
                 }
-
-                return _message;
-            }
-        }
-
-        private string BuildMessage()
-        {
-            if (Expression != null && ExpressionNullPoint != null)
-            {
-                return $"'{ExpressionNullPoint}' is null in expression '{Expression}'.";
-            }
-            else if (ExpressionNullPoint != null)
-            {
-                return $"'{ExpressionNullPoint}' is null in expression.";
-            }
-            else
-            {
-                return "Null encountered in binding expression.";
             }
         }
     }

src/Avalonia.Controls/TextBox.cs

@@ -547,7 +547,7 @@ namespace Avalonia.Controls
                 var exceptions = aggregate == null ?
                     (IEnumerable<Exception>)new[] { exception } :
                     aggregate.InnerExceptions;
-                var filtered = exceptions.Where(x => !(x is BindingChainNullException)).ToList();
+                var filtered = exceptions.Where(x => !(x is BindingChainException)).ToList();
 
                 if (filtered.Count > 0)
                 {

src/Markup/Avalonia.Markup/Avalonia.Markup.csproj

@@ -43,7 +43,7 @@
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
     <Compile Include="Data\StreamNode.cs" />
-    <Compile Include="Data\MarkupBindingChainNullException.cs" />
+    <Compile Include="Data\MarkupBindingChainException.cs" />
     <Compile Include="Data\CommonPropertyNames.cs" />
     <Compile Include="Data\EmptyExpressionNode.cs" />
     <Compile Include="Data\ExpressionNodeBuilder.cs" />

src/Markup/Avalonia.Markup/Data/ExpressionNode.cs

@@ -88,7 +88,7 @@ namespace Avalonia.Markup.Data
 
         protected virtual void NextValueChanged(object value)
         {
-            var bindingBroken = BindingNotification.ExtractError(value) as MarkupBindingChainNullException;
+            var bindingBroken = BindingNotification.ExtractError(value) as MarkupBindingChainException;
             bindingBroken?.AddNode(Description);
             _observer.OnNext(value);
         }
@@ -152,7 +152,7 @@ namespace Avalonia.Markup.Data
         private BindingNotification TargetNullNotification()
         {
             return new BindingNotification(
-                new MarkupBindingChainNullException(),
+                new MarkupBindingChainException("Null value"),
                 BindingErrorType.Error,
                 AvaloniaProperty.UnsetValue);
         }

src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs

@@ -235,7 +235,7 @@ namespace Avalonia.Markup.Data
             }
             else
             {
-                var broken = BindingNotification.ExtractError(o) as MarkupBindingChainNullException;
+                var broken = BindingNotification.ExtractError(o) as MarkupBindingChainException;
 
                 if (broken != null)
                 {

src/Markup/Avalonia.Markup/Data/MarkupBindingChainException.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Avalonia.Data;
+
+namespace Avalonia.Markup.Data
+{
+    internal class MarkupBindingChainException : BindingChainException
+    {
+        private IList<string> _nodes = new List<string>();
+
+        public MarkupBindingChainException(string message)
+            : base(message)
+        {
+        }
+
+        public MarkupBindingChainException(string message, string node)
+            : base(message)
+        {
+            AddNode(node);
+        }
+
+        public MarkupBindingChainException(string message, string expression, string expressionNullPoint)
+            : base(message, expression, expressionNullPoint)
+        {
+            _nodes = null;
+        }
+
+        public bool HasNodes => _nodes.Count > 0;
+        public void AddNode(string node) => _nodes.Add(node);
+
+        public void Commit(string expression)
+        {
+            Expression = expression;
+            ExpressionErrorPoint = string.Join(".", _nodes.Reverse())
+                .Replace(".!", "!")
+                .Replace(".[", "[")
+                .Replace(".^", "^");
+            _nodes = null;
+        }
+    }
+}

src/Markup/Avalonia.Markup/Data/MarkupBindingChainNullException.cs

@@ -1,33 +0,0 @@
-using System.Collections.Generic;
-using System.Linq;
-using Avalonia.Data;
-
-namespace Avalonia.Markup.Data
-{
-    internal class MarkupBindingChainNullException : BindingChainNullException
-    {
-        private IList<string> _nodes = new List<string>();
-
-        public MarkupBindingChainNullException()
-        {
-        }
-
-        public MarkupBindingChainNullException(string expression, string expressionNullPoint)
-            : base(expression, expressionNullPoint)
-        {
-            _nodes = null;
-        }
-
-        public bool HasNodes => _nodes.Count > 0;
-        public void AddNode(string node) => _nodes.Add(node);
-
-        public void Commit(string expression)
-        {
-            Expression = expression;
-            ExpressionNullPoint = string.Join(".", _nodes.Reverse())
-                .Replace(".!", "!")
-                .Replace(".[", "[");
-            _nodes = null;
-        }
-    }
-}

src/Markup/Avalonia.Markup/Data/StreamNode.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Markup.Data
 
             // TODO: Improve error.
             return Observable.Return(new BindingNotification(
-                new InvalidCastException("Value could not be streamed."),
+                new MarkupBindingChainException("Stream operator applied to unsupported type", Description),
                 BindingErrorType.Error));
         }
     }

tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs

@@ -143,7 +143,7 @@ namespace Avalonia.Markup.UnitTests.Data
             Assert.Equal(new[]
             {
                 new BindingNotification(
-                    new MarkupBindingChainNullException("Inner.MustBePositive", "Inner"),
+                    new MarkupBindingChainException("Null value", "Inner.MustBePositive", "Inner"),
                     BindingErrorType.Error,
                     AvaloniaProperty.UnsetValue),
             }, result);

tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Observable.cs

@@ -110,6 +110,31 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
+        [Fact]
+        public void Should_Return_BindingNotification_If_Stream_Operator_Applied_To_Not_Supported_Type()
+        {
+            using (var sync = UnitTestSynchronizationContext.Begin())
+            {
+                var data = new Class2("foo");
+                var target = new ExpressionObserver(data, "Foo^", true);
+                var result = new List<object>();
+
+                var sub = target.Subscribe(x => result.Add(x));
+                sync.ExecutePostedCallbacks();
+
+                Assert.Equal(
+                    new[]
+                    {
+                        new BindingNotification(
+                            new MarkupBindingChainException("Stream operator applied to unsupported type", "Foo^", "Foo^"),
+                            BindingErrorType.Error)
+                    },
+                    result);
+
+                sub.Dispose();
+            }
+        }
+
         private class Class1 : NotifyingBase
         {
             public Subject<Class2> Next { get; } = new Subject<Class2>();

tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Property.cs

@@ -146,7 +146,7 @@ namespace Avalonia.Markup.UnitTests.Data
                 new[]
                 {
                     new BindingNotification(
-                    new MarkupBindingChainNullException("Foo.Bar.Baz", "Foo"),
+                        new MarkupBindingChainException("Null value", "Foo.Bar.Baz", "Foo"),
                         BindingErrorType.Error,
                         AvaloniaProperty.UnsetValue),
                 },
@@ -274,7 +274,7 @@ namespace Avalonia.Markup.UnitTests.Data
                 {
                     "bar",
                     new BindingNotification(
-                        new MarkupBindingChainNullException("Next.Next.Bar", "Next.Next"),
+                        new MarkupBindingChainException("Null value", "Next.Next.Bar", "Next.Next"),
                         BindingErrorType.Error,
                         AvaloniaProperty.UnsetValue),
                     "bar"