reactiveui · glennawatson · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026
diff --git a/src/ReactiveUI.Binding.SourceGenerators/CodeGeneration/ObservationCodeGenerator.cs b/src/ReactiveUI.Binding.SourceGenerators/CodeGeneration/ObservationCodeGenerator.cs
@@ -720,6 +720,10 @@ internal static void EmitInlineObservation(
     /// for inner segments, so reusing the root's plugin is safe whatever the segment declares.
     /// </param>
     /// <param name="isBeforeChange">Whether before-change notifications are being observed.</param>
+    /// <remarks>
+    /// Missing parents propagate through intermediate stages so downstream subscriptions detach,
+    /// while the leaf suppresses the default value to match the runtime expression-chain fallback.
+    /// </remarks>
     private static void EmitObservationChainInnerSegments(
         StringBuilder sb,
         EquatableArray<PropertyPathSegment> path,
@@ -735,9 +739,25 @@ private static void EmitObservationChainInnerSegments(
             var lambdaParam = $"__parent{s}";
             var segType = seg.PropertyTypeFullName;
 
+            // Only the leaf suppresses. Inner segments keep pushing the null downstream so the
+            // stage below re-parents onto null and drops its subscription on the detached subtree.
+            var nullParentBehavior = s == path.Length - 1
+                ? NullParentObservationBehavior.SuppressEmission
+                : NullParentObservationBehavior.EmitDefault;
+            var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+                ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+                : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
+
             if (rootPlugin is not null)
             {
-                rootPlugin.EmitDeepChainInnerSegment(sb, prevVar, curVar, lambdaParam, seg, isBeforeChange);
+                rootPlugin.EmitDeepChainInnerSegment(
+                    sb,
+                    prevVar,
+                    curVar,
+                    lambdaParam,
+                    seg,
+                    isBeforeChange,
+                    nullParentBehavior);
             }
             else if (IsINPChanging(classInfo) && isBeforeChange)
             {
@@ -750,7 +770,7 @@ private static void EmitObservationChainInnerSegments(
                                                          (global::System.ComponentModel.INotifyPropertyChanging){lambdaParam},
                                                          "{seg.PropertyName}",
                                                          (global::System.ComponentModel.INotifyPropertyChanging __o) => (({seg.DeclaringTypeFullName})__o).{seg.PropertyName})
-                                                     : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                                     : (global::System.IObservable<{segType}>){nullParentObservable}));
                                  """);
             }
             else
@@ -759,8 +779,10 @@ private static void EmitObservationChainInnerSegments(
                     .AppendLine($"""
                                          var {curVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
                                              global::ReactiveUI.Binding.Observables.RxBindingExtensions.Select({prevVar},
-                                                 {lambdaParam} => (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(
-                                                     {lambdaParam} != null ? (({seg.DeclaringTypeFullName}){lambdaParam}).{seg.PropertyName} : default({segType}))));
+                                                 {lambdaParam} => {lambdaParam} != null
+                                                     ? (global::System.IObservable<{segType}>)
+                                                         new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>((({seg.DeclaringTypeFullName}){lambdaParam}).{seg.PropertyName})
+                                                     : (global::System.IObservable<{segType}>){nullParentObservable}));
                                  """);
             }
         }
@@ -779,6 +801,10 @@ private static void EmitObservationChainInnerSegments(
     /// </param>
     /// <param name="isBeforeChange">Whether before-change notifications are being observed.</param>
     /// <param name="varName">The variable-name prefix for the emitted stages.</param>
+    /// <remarks>
+    /// Missing parents propagate through intermediate stages so downstream subscriptions detach,
+    /// while the leaf suppresses the default value to match the runtime expression-chain fallback.
+    /// </remarks>
     private static void EmitDeepChainInnerSegments(
         StringBuilder sb,
         EquatableArray<PropertyPathSegment> path,
@@ -795,9 +821,23 @@ private static void EmitDeepChainInnerSegments(
             var lambdaParam = $"{varName}_p{s}";
             var segType = seg.PropertyTypeFullName;
 
+            var nullParentBehavior = s == path.Length - 1
+                ? NullParentObservationBehavior.SuppressEmission
+                : NullParentObservationBehavior.EmitDefault;
+            var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+                ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+                : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
+
             if (rootPlugin is not null)
             {
-                rootPlugin.EmitDeepChainInnerSegment(sb, prevObsVar, curObsVar, lambdaParam, seg, isBeforeChange);
+                rootPlugin.EmitDeepChainInnerSegment(
+                    sb,
+                    prevObsVar,
+                    curObsVar,
+                    lambdaParam,
+                    seg,
+                    isBeforeChange,
+                    nullParentBehavior);
             }
             else if (IsINPChanging(classInfo) && isBeforeChange)
             {
@@ -810,7 +850,7 @@ private static void EmitDeepChainInnerSegments(
                                                          (global::System.ComponentModel.INotifyPropertyChanging){lambdaParam},
                                                          "{seg.PropertyName}",
                                                          (global::System.ComponentModel.INotifyPropertyChanging __o) => (({seg.DeclaringTypeFullName})__o).{seg.PropertyName})
-                                                     : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                                     : (global::System.IObservable<{segType}>){nullParentObservable}));
                                  """);
             }
             else
@@ -819,8 +859,10 @@ private static void EmitDeepChainInnerSegments(
                     .AppendLine($"""
                   var {curObsVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
                       global::ReactiveUI.Binding.Observables.RxBindingExtensions.Select({prevObsVar},
-                          {lambdaParam} => (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(
-                              {lambdaParam} != null ? (({seg.DeclaringTypeFullName}){lambdaParam}).{seg.PropertyName} : default({segType}))));
+                          {lambdaParam} => {lambdaParam} != null
+                              ? (global::System.IObservable<{segType}>)
+                                  new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>((({seg.DeclaringTypeFullName}){lambdaParam}).{seg.PropertyName})
+                              : (global::System.IObservable<{segType}>){nullParentObservable}));
           """);
             }
         }
@@ -836,6 +878,7 @@ private static void EmitDeepChainInnerSegments(
     /// <param name="classInfo">The root type's binding info, when known.</param>
     /// <param name="plugin">The observation plugin for the root type, when one matched.</param>
     /// <param name="variableName">The name of the variable the chain result is assigned to.</param>
+    /// <remarks>Missing parents emit defaults here so binding consumers can clear their targets.</remarks>
     private static void EmitInlineDeepChain(
         StringBuilder sb,
         string rootVar,
@@ -866,7 +909,14 @@ private static void EmitInlineDeepChain(
 
             if (plugin is not null)
             {
-                plugin.EmitDeepChainInnerSegment(sb, prevVar, curVar, lambdaParam, seg, false);
+                plugin.EmitDeepChainInnerSegment(
+                    sb,
+                    prevVar,
+                    curVar,
+                    lambdaParam,
+                    seg,
+                    isBeforeChange: false,
+                    nullParentBehavior: NullParentObservationBehavior.EmitDefault);
                 continue;
             }
 
@@ -876,8 +926,10 @@ private static void EmitInlineDeepChain(
                 .AppendLine($"""
                                  var {curVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
                                      global::ReactiveUI.Binding.Observables.RxBindingExtensions.Select({prevVar},
-                                         {lambdaParam} => (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(
-                                             {lambdaParam} != null ? (({declType}){lambdaParam}).{seg.PropertyName} : default({segType}))));
+                                         {lambdaParam} => {lambdaParam} != null
+                                             ? (global::System.IObservable<{segType}>)
+                                                 new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>((({declType}){lambdaParam}).{seg.PropertyName})
+                                             : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
                              """);
         }
 

diff --git a/src/ReactiveUI.Binding.SourceGenerators/Plugins/IObservationPlugin.cs b/src/ReactiveUI.Binding.SourceGenerators/Plugins/IObservationPlugin.cs
@@ -102,13 +102,15 @@ void EmitDeepChainRootSegment(
     /// <param name="lambdaParam">The lambda parameter name for the parent value.</param>
     /// <param name="segment">The current property path segment.</param>
     /// <param name="isBeforeChange">True for WhenChanging (before-change).</param>
+    /// <param name="nullParentBehavior">The behavior to use while the parent segment is null.</param>
     void EmitDeepChainInnerSegment(
         StringBuilder sb,
         string prevVar,
         string curVar,
         string lambdaParam,
         PropertyPathSegment segment,
-        bool isBeforeChange);
+        bool isBeforeChange,
+        NullParentObservationBehavior nullParentBehavior);
 
     /// <summary>Emits an inline observation variable for binding generators. Used by BindOneWay/BindTwoWay for direct observation code.</summary>
     /// <param name="sb">The string builder to append to.</param>

diff --git a/src/ReactiveUI.Binding.SourceGenerators/Plugins/NullParentObservationBehavior.cs b/src/ReactiveUI.Binding.SourceGenerators/Plugins/NullParentObservationBehavior.cs
@@ -0,0 +1,15 @@
+// Copyright (c) 2019-2026 ReactiveUI Association Incorporated. All rights reserved.
+// ReactiveUI Association Incorporated licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+namespace ReactiveUI.Binding.SourceGenerators.Plugins;
+
+/// <summary>Specifies what a generated deep-chain observer emits while its parent segment is null.</summary>
+internal enum NullParentObservationBehavior
+{
+    /// <summary>Suppresses values until the parent becomes non-null.</summary>
+    SuppressEmission,
+
+    /// <summary>Emits the leaf type's default value so binding consumers can clear their target.</summary>
+    EmitDefault,
+}
diff --git a/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/AndroidObservationPlugin.cs b/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/AndroidObservationPlugin.cs
@@ -102,17 +102,23 @@ public void EmitDeepChainInnerSegment(
         string curVar,
         string lambdaParam,
         PropertyPathSegment segment,
-        bool isBeforeChange)
+        bool isBeforeChange,
+        NullParentObservationBehavior nullParentBehavior)
     {
         var segType = segment.PropertyTypeFullName;
         var declType = segment.DeclaringTypeFullName;
+        var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+            ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+            : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
 
         _ = sb.AppendLine()
             .AppendLine($"""
                                  var {curVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
                                      global::ReactiveUI.Binding.Observables.RxBindingExtensions.Select({prevVar},
-                                         {lambdaParam} => (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(
-                                             {lambdaParam} != null ? (({declType}){lambdaParam}).{segment.PropertyName} : default({segType}))));
+                                         {lambdaParam} => {lambdaParam} != null
+                                             ? (global::System.IObservable<{segType}>)
+                                                 new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>((({declType}){lambdaParam}).{segment.PropertyName})
+                                             : (global::System.IObservable<{segType}>){nullParentObservable}));
                          """);
     }
 

diff --git a/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/INPCObservationPlugin.cs b/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/INPCObservationPlugin.cs
@@ -119,9 +119,13 @@ public void EmitDeepChainInnerSegment(
         string curVar,
         string lambdaParam,
         PropertyPathSegment segment,
-        bool isBeforeChange)
+        bool isBeforeChange,
+        NullParentObservationBehavior nullParentBehavior)
     {
         var segType = segment.PropertyTypeFullName;
+        var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+            ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+            : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
 
         _ = sb.AppendLine()
             .AppendLine(isBeforeChange
@@ -133,7 +137,7 @@ public void EmitDeepChainInnerSegment(
                                            (global::System.ComponentModel.INotifyPropertyChanging){lambdaParam},
                                            "{segment.PropertyName}",
                                            (global::System.ComponentModel.INotifyPropertyChanging __o) => (({segment.DeclaringTypeFullName})__o).{segment.PropertyName})
-                                       : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                       : (global::System.IObservable<{segType}>){nullParentObservable}));
                    """
                 : $"""
                            var {curVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
@@ -144,7 +148,7 @@ public void EmitDeepChainInnerSegment(
                                            "{segment.PropertyName}",
                                            (global::System.ComponentModel.INotifyPropertyChanged __o) => (({segment.DeclaringTypeFullName})__o).{segment.PropertyName},
                                            false)
-                                       : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                       : (global::System.IObservable<{segType}>){nullParentObservable}));
                    """);
     }
 

diff --git a/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/KVOObservationPlugin.cs b/src/ReactiveUI.Binding.SourceGenerators/Plugins/Observation/KVOObservationPlugin.cs
@@ -125,11 +125,15 @@ public void EmitDeepChainInnerSegment(
         string curVar,
         string lambdaParam,
         PropertyPathSegment segment,
-        bool isBeforeChange)
+        bool isBeforeChange,
+        NullParentObservationBehavior nullParentBehavior)
     {
         var segType = segment.PropertyTypeFullName;
         var declType = segment.DeclaringTypeFullName;
         var keyPath = ToKvoKeyPath(segment.PropertyName, segment.PropertyTypeFullName);
+        var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+            ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+            : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
 
         _ = sb.AppendLine()
             .AppendLine($"""
@@ -142,7 +146,7 @@ public void EmitDeepChainInnerSegment(
                                                  (global::Foundation.NSObject __o) => (({declType})__o).{segment.PropertyName},
                                                  false,
                                                  {BoolLiteral(isBeforeChange)})
-                                             : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                             : (global::System.IObservable<{segType}>){nullParentObservable}));
                          """);
     }
 

diff --git a/...eactiveUI.Binding.SourceGenerators/Plugins/Observation/ReactiveObjectObservationPlugin.cs b/...eactiveUI.Binding.SourceGenerators/Plugins/Observation/ReactiveObjectObservationPlugin.cs
@@ -120,9 +120,13 @@ public void EmitDeepChainInnerSegment(
         string curVar,
         string lambdaParam,
         PropertyPathSegment segment,
-        bool isBeforeChange)
+        bool isBeforeChange,
+        NullParentObservationBehavior nullParentBehavior)
     {
         var segType = segment.PropertyTypeFullName;
+        var nullParentObservable = nullParentBehavior == NullParentObservationBehavior.EmitDefault
+            ? $"new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))"
+            : $"global::ReactiveUI.Binding.Observables.EmptyObservable<{segType}>.Instance";
 
         _ = sb.AppendLine()
             .AppendLine(isBeforeChange
@@ -134,7 +138,7 @@ public void EmitDeepChainInnerSegment(
                                            (global::System.ComponentModel.INotifyPropertyChanging){lambdaParam},
                                            "{segment.PropertyName}",
                                            (global::System.ComponentModel.INotifyPropertyChanging __o) => (({segment.DeclaringTypeFullName})__o).{segment.PropertyName})
-                                       : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                       : (global::System.IObservable<{segType}>){nullParentObservable}));
                    """
                 : $"""
                            var {curVar} = global::ReactiveUI.Binding.Observables.RxBindingExtensions.Switch(
@@ -145,7 +149,7 @@ public void EmitDeepChainInnerSegment(
                                            "{segment.PropertyName}",
                                            (global::System.ComponentModel.INotifyPropertyChanged __o) => (({segment.DeclaringTypeFullName})__o).{segment.PropertyName},
                                            false)
-                                       : (global::System.IObservable<{segType}>)new global::ReactiveUI.Binding.Observables.ReturnObservable<{segType}>(default({segType}))));
+                                       : (global::System.IObservable<{segType}>){nullParentObservable}));
                    """);
     }