reactivemarbles
diff --git a/‎src/DynamicData.Tests/Binding/WhenPropertyChangedBehaviorFixture.cs‎
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+// Copyright (c) 2011-2025 Roland Pheasant. All rights reserved.
+// Roland Pheasant licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+
+using DynamicData.Binding;
+
+using FluentAssertions;
+
+using Xunit;
+
+namespace DynamicData.Tests.Binding;
+
+/// <summary>
+/// Single-threaded contract tests for <see cref="NotifyPropertyChangedEx.WhenPropertyChanged{TObject, TProperty}"/>:
+/// handler attachment ordering, no-dedup semantics, deep-chain re-walks on swaps.
+/// </summary>
+public sealed class WhenPropertyChangedBehaviorFixture
+{
+    [Fact]
+    public void Shallow_NotifyInitialFalse_SubscribesHandlerBeforeReturning()
+    {
+        // notifyOnInitialValue=false: Subscribe must return only after the PropertyChanged handler
+        // is attached. A setter that fires immediately after Subscribe returns must reach the
+        // observer.
+        var model = new TestModel { Value = 10 };
+        var emissions = new List<int>();
+
+        using var sub = model.WhenPropertyChanged(m => m.Value, notifyOnInitialValue: false)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        model.Value = 20;
+
+        emissions.Should().Equal(new[] { 20 });
+    }
+
+    [Fact]
+    public void Shallow_NotifyInitialTrue_DoesNotDedupSameValuedEvents()
+    {
+        var model = new TestModel { Value = 10 };
+        var emissions = new List<int>();
+
+        using var sub = model.WhenPropertyChanged(m => m.Value, notifyOnInitialValue: true)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        model.Value = 10;
+        model.Value = 10;
+        model.Value = 10;
+
+        emissions.Should().Equal(new[] { 10, 10, 10, 10 });
+    }
+
+    [Fact]
+    public void Shallow_NotifyInitialFalse_DoesNotDedupSameValuedEvents()
+    {
+        var model = new TestModel { Value = 10 };
+        var emissions = new List<int>();
+
+        using var sub = model.WhenPropertyChanged(m => m.Value, notifyOnInitialValue: false)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        model.Value = 42;
+        model.Value = 42;
+
+        emissions.Should().Equal(new[] { 42, 42 });
+    }
+
+    [Fact]
+    public void DeepChain_NotifyInitialTrue_DoesNotDedupSameValuedEvents()
+    {
+        var parent = new ParentModel { Child = new ChildModel { Age = 1 } };
+        var emissions = new List<int>();
+
+        using var sub = parent.WhenPropertyChanged(p => p.Child!.Age, notifyOnInitialValue: true)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        parent.Child!.Age = 1;
+        parent.Child!.Age = 1;
+        parent.Child!.Age = 1;
+
+        emissions.Should().Equal(new[] { 1, 1, 1, 1 });
+    }
+
+    [Fact]
+    public void DeepChain_NotifyInitialFalse_DoesNotDedupSameValuedEvents()
+    {
+        var parent = new ParentModel { Child = new ChildModel { Age = 1 } };
+        var emissions = new List<int>();
+
+        using var sub = parent.WhenPropertyChanged(p => p.Child!.Age, notifyOnInitialValue: false)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        parent.Child!.Age = 7;
+        parent.Child!.Age = 7;
+
+        emissions.Should().Equal(new[] { 7, 7 });
+    }
+
+    [Fact]
+    public void DeepChain_PostSwap_LeafEventOnNewChild_Captured()
+    {
+        // After parent.Child is reassigned, the leaf-level subscription must be re-attached
+        // against the new child. A subsequent leaf mutation on the new child must be captured.
+        var parent = new ParentModel { Child = new ChildModel { Age = 10 } };
+        var emissions = new List<int>();
+
+        using var sub = parent.WhenPropertyChanged(p => p.Child!.Age, notifyOnInitialValue: true)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        var newChild = new ChildModel { Age = 20 };
+        parent.Child = newChild;
+        newChild.Age = 30;
+
+        emissions.Should().Equal(new[] { 10, 20, 30 });
+    }
+
+    [Fact]
+    public void DeepChain_MidChainSwap_DeeperLevelsRetargetCorrectly()
+    {
+        // Mid-chain swap on a 4-level chain. When level 3 is reassigned, the leaf subscription
+        // must re-attach against the new subtree; events on the old subtree must be ignored
+        // (its notifier subscription was disposed).
+        var l1 = new Level1
+        {
+            Child = new Level2
+            {
+                Child = new Level3
+                {
+                    Child = new Level4 { Leaf = 10 },
+                },
+            },
+        };
+
+        var emissions = new List<int>();
+        using var sub = l1.WhenPropertyChanged(x => x.Child!.Child!.Child!.Leaf, notifyOnInitialValue: true)
+            .Subscribe(pv => emissions.Add(pv.Value));
+
+        emissions.Should().Equal(new[] { 10 }, "initial emission");
+
+        var originalLeaf = l1.Child!.Child!.Child!;
+
+        var newL4 = new Level4 { Leaf = 20 };
+        l1.Child!.Child!.Child = newL4;
+
+        emissions.Should().Equal(new[] { 10, 20 }, "mid-chain swap emits the new leaf value");
+
+        newL4.Leaf = 30;
+        emissions.Should().Equal(new[] { 10, 20, 30 }, "leaf event on new subtree is captured");
+
+        originalLeaf.Leaf = 999;
+        emissions.Should().Equal(new[] { 10, 20, 30 }, "leaf event on detached subtree is ignored");
+    }
+
+    private sealed class TestModel : INotifyPropertyChanged
+    {
+        private int _value;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public int Value
+        {
+            get => _value;
+            set
+            {
+                _value = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Value)));
+            }
+        }
+    }
+
+    private sealed class ParentModel : INotifyPropertyChanged
+    {
+        private ChildModel? _child;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public ChildModel? Child
+        {
+            get => _child;
+            set
+            {
+                _child = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Child)));
+            }
+        }
+    }
+
+    private sealed class ChildModel : INotifyPropertyChanged
+    {
+        private int _age;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public int Age
+        {
+            get => _age;
+            set
+            {
+                _age = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Age)));
+            }
+        }
+    }
+
+    private sealed class Level1 : INotifyPropertyChanged
+    {
+        private Level2? _child;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public Level2? Child
+        {
+            get => _child;
+            set
+            {
+                _child = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Child)));
+            }
+        }
+    }
+
+    private sealed class Level2 : INotifyPropertyChanged
+    {
+        private Level3? _child;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public Level3? Child
+        {
+            get => _child;
+            set
+            {
+                _child = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Child)));
+            }
+        }
+    }
+
+    private sealed class Level3 : INotifyPropertyChanged
+    {
+        private Level4? _child;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public Level4? Child
+        {
+            get => _child;
+            set
+            {
+                _child = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Child)));
+            }
+        }
+    }
+
+    private sealed class Level4 : INotifyPropertyChanged
+    {
+        private int _leaf;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public int Leaf
+        {
+            get => _leaf;
+            set
+            {
+                _leaf = value;
+                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Leaf)));
+            }
+        }
+    }
+}