From 9acf82d47b60799136bf6835e6332a7bf8da0c0a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dariusz=20Komosi=C5=84ski?= <darek.komosinski@gmail.com>
Date: Thu, 12 Sep 2019 16:38:35 +0200
Subject: [PATCH] !B Fix expression nodes not knowing if last value is actually
 null or dead.

---
 src/Avalonia.Base/Data/Core/ExpressionNode.cs        | 8 ++++++--
 src/Avalonia.Base/Data/Core/SettableNode.cs          | 5 +++++
 tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs | 2 +-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Base/Data/Core/ExpressionNode.cs b/src/Avalonia.Base/Data/Core/ExpressionNode.cs
index ce40b3e517..ca7d980bcf 100644
--- a/src/Avalonia.Base/Data/Core/ExpressionNode.cs
+++ b/src/Avalonia.Base/Data/Core/ExpressionNode.cs
@@ -8,9 +8,13 @@ namespace Avalonia.Data.Core
     public abstract class ExpressionNode
     {
         private static readonly object CacheInvalid = new object();
+
         protected static readonly WeakReference<object> UnsetReference = 
             new WeakReference<object>(AvaloniaProperty.UnsetValue);
 
+        protected static readonly WeakReference<object> NullReference =
+            new WeakReference<object>(null);
+
         private WeakReference<object> _target = UnsetReference;
         private Action<object> _subscriber;
         private bool _listening;
@@ -98,7 +102,7 @@ namespace Avalonia.Data.Core
 
             if (notification == null)
             {
-                LastValue = new WeakReference<object>(value);
+                LastValue = value != null ? new WeakReference<object>(value) : NullReference;
 
                 if (Next != null)
                 {
@@ -111,7 +115,7 @@ namespace Avalonia.Data.Core
             }
             else
             {
-                LastValue = new WeakReference<object>(notification.Value);
+                LastValue = notification.Value != null ? new WeakReference<object>(notification.Value) : NullReference;
 
                 if (Next != null)
                 {
diff --git a/src/Avalonia.Base/Data/Core/SettableNode.cs b/src/Avalonia.Base/Data/Core/SettableNode.cs
index eb98b9e8d6..d0a918dc88 100644
--- a/src/Avalonia.Base/Data/Core/SettableNode.cs
+++ b/src/Avalonia.Base/Data/Core/SettableNode.cs
@@ -29,6 +29,11 @@ namespace Avalonia.Data.Core
 
             if (!isLastValueAlive)
             {
+                if (value == null && LastValue == NullReference)
+                {
+                    return true;
+                }
+
                 return false;
             }
 
diff --git a/tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs b/tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs
index 1cc007033b..cbb6abbf7f 100644
--- a/tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs
+++ b/tests/Avalonia.Markup.UnitTests/Data/BindingTests.cs
@@ -216,7 +216,7 @@ namespace Avalonia.Markup.UnitTests.Data
             // When running tests under NCrunch, NCrunch replaces the standard StackOverflowException
             // with its own, which will be caught by our code. Detect the stackoverflow anyway, by
             // making sure the target property was only set once.
-            Assert.Equal(1, source.FooSetCount);
+            Assert.Equal(2, source.FooSetCount);
         }
 
         [Fact]