Fix setting expression value on broken chain.

Closes #254.

src/Markup/Perspex.Markup/Binding/PropertyAccessorNode.cs

@@ -81,6 +81,8 @@ namespace Perspex.Markup.Binding
             {
                 inpc.PropertyChanged -= PropertyChanged;
             }
+
+            _propertyInfo = null;
         }
 
         private static PropertyInfo FindProperty(object target, string propertyName)

tests/Perspex.Markup.UnitTests/Binding/ExpressionObserverTests_SetValue.cs

@@ -0,0 +1,80 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Collections.Generic;
+using System.Reactive.Linq;
+using Perspex.Markup.Binding;
+using Xunit;
+
+namespace Perspex.Markup.UnitTests.Binding
+{
+    public class ExpressionObserverTests_SetValue
+    {
+        [Fact]
+        public void Should_Set_Simple_Property_Value()
+        {
+            var data = new { Foo = "foo" };
+            var target = new ExpressionObserver(data, "Foo");
+
+            target.SetValue("bar");
+
+            Assert.Equal("foo", data.Foo);
+        }
+
+        [Fact]
+        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");
+
+            target.SetValue("foo");
+
+            Assert.Equal("foo", data.Foo.Bar);
+        }
+
+        [Fact]
+        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");
+
+            // Ensure the ExpressionObserver's subscriptions are kept active.
+            target.OfType<string>().Subscribe(x => { });
+
+            data.Foo = null;
+
+            Assert.False(target.SetValue("foo"));
+        }
+
+        private class Class1 : NotifyingBase
+        {
+            private Class2 _foo;
+
+            public Class2 Foo
+            {
+                get { return _foo; }
+                set
+                {
+                    _foo = value;
+                    RaisePropertyChanged(nameof(Foo));
+                }
+            }
+        }
+
+        private class Class2 : NotifyingBase
+        {
+            private string _bar;
+
+            public string Bar
+            {
+                get { return _bar; }
+                set
+                {
+                    _bar = value;
+                    RaisePropertyChanged(nameof(Bar));
+                }
+            }
+        }
+    }
+}

tests/Perspex.Markup.UnitTests/Perspex.Markup.UnitTests.csproj

@@ -74,6 +74,7 @@
   <ItemGroup>
     <Compile Include="Binding\ExpressionNodeBuilderTests_Errors.cs" />
     <Compile Include="Binding\ExpressionObserverTests_Observable.cs" />
+    <Compile Include="Binding\ExpressionObserverTests_SetValue.cs" />
     <Compile Include="Binding\ExpressionObserverTests_Task.cs" />
     <Compile Include="Binding\ExpressionObserverTests_Indexer.cs" />
     <Compile Include="Binding\ExpressionObserverTests_Negation.cs" />