In ComponentState, track parent ComponentState. Be explicit that Renderer only lets you attach root components.

Author: SteveSandersonMS

src/Microsoft.AspNetCore.Blazor.Browser/Rendering/BrowserRenderer.cs

@@ -61,7 +61,7 @@ public void AddComponent<TComponent>(string domElementSelector)
         public void AddComponent(Type componentType, string domElementSelector)
         {
             var component = InstantiateComponent(componentType);
-            var componentId = AssignComponentId(component);
+            var componentId = AssignRootComponentId(component);
 
             // The only reason we're calling this synchronously is so that, if it throws,
             // we get the exception back *before* attempting the first UpdateDisplay

src/Microsoft.AspNetCore.Blazor.Server/Circuits/RemoteRenderer.cs

@@ -64,7 +64,7 @@ public void AddComponent<TComponent>(string domElementSelector)
         public void AddComponent(Type componentType, string domElementSelector)
         {
             var component = InstantiateComponent(componentType);
-            var componentId = AssignComponentId(component);
+            var componentId = AssignRootComponentId(component);
 
             var attachComponentTask = _jsRuntime.InvokeAsync<object>(
                 "Blazor._internal.attachRootComponentToElement",

src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeDiffBuilder.cs

@@ -20,7 +20,7 @@ public static RenderTreeDiff ComputeDiff(
             var editsBuffer = batchBuilder.EditsBuffer;
             var editsBufferStartLength = editsBuffer.Count;
 
-            var diffContext = new DiffContext(renderer, batchBuilder, oldTree.Array, newTree.Array);
+            var diffContext = new DiffContext(renderer, batchBuilder, componentId, oldTree.Array, newTree.Array);
             AppendDiffEntriesForRange(ref diffContext, 0, oldTree.Count, 0, newTree.Count);
 
             var editsSegment = editsBuffer.ToSegment(editsBufferStartLength, editsBuffer.Count);
@@ -638,7 +638,8 @@ private static void InitializeNewComponentFrame(ref DiffContext diffContext, int
                 throw new InvalidOperationException($"Child component already exists during {nameof(InitializeNewComponentFrame)}");
             }
 
-            diffContext.Renderer.InstantiateChildComponentOnFrame(ref frame);
+            var parentComponentId = diffContext.ComponentId;
+            diffContext.Renderer.InstantiateChildComponentOnFrame(ref frame, parentComponentId);
             var childComponentInstance = frame.Component;
 
             // Set initial parameters
@@ -718,16 +719,19 @@ private struct DiffContext
             public readonly ArrayBuilder<RenderTreeEdit> Edits;
             public readonly ArrayBuilder<RenderTreeFrame> ReferenceFrames;
             public readonly Dictionary<string, int> AttributeDiffSet;
+            public readonly int ComponentId;
             public int SiblingIndex;
 
             public DiffContext(
                 Renderer renderer,
                 RenderBatchBuilder batchBuilder,
+                int componentId,
                 RenderTreeFrame[] oldTree,
                 RenderTreeFrame[] newTree)
             {
                 Renderer = renderer;
                 BatchBuilder = batchBuilder;
+                ComponentId = componentId;
                 OldTree = oldTree;
                 NewTree = newTree;
                 Edits = batchBuilder.EditsBuffer;

src/Microsoft.AspNetCore.Blazor/Rendering/ComponentState.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
@@ -15,6 +15,7 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
     internal class ComponentState
     {
         private readonly int _componentId; // TODO: Change the type to 'long' when the Mono runtime has more complete support for passing longs in .NET->JS calls
+        private readonly ComponentState _parentComponentState;
         private readonly IComponent _component;
         private readonly Renderer _renderer;
         private RenderTreeBuilder _renderTreeBuilderCurrent;
@@ -27,9 +28,11 @@ internal class ComponentState
         /// <param name="renderer">The <see cref="Renderer"/> with which the new instance should be associated.</param>
         /// <param name="componentId">The externally visible identifier for the <see cref="IComponent"/>. The identifier must be unique in the context of the <see cref="Renderer"/>.</param>
         /// <param name="component">The <see cref="IComponent"/> whose state is being tracked.</param>
-        public ComponentState(Renderer renderer, int componentId, IComponent component)
+        /// <param name="parentComponentState">The <see cref="ComponentState"/> for the parent component, or null if this is a root component.</param>
+        public ComponentState(Renderer renderer, int componentId, IComponent component, ComponentState parentComponentState)
         {
             _componentId = componentId;
+            _parentComponentState = parentComponentState;
             _component = component ?? throw new ArgumentNullException(nameof(component));
             _renderer = renderer ?? throw new ArgumentNullException(nameof(renderer));
             _renderTreeBuilderCurrent = new RenderTreeBuilder(renderer);
@@ -89,5 +92,10 @@ public void DispatchEvent(EventHandlerInvoker binding, UIEventArgs eventArgs)
 
         public void NotifyRenderCompleted()
             => (_component as IHandleAfterRender)?.OnAfterRender();
+
+        // TODO: Remove this once we can remove TemporaryGetParentComponentIdForTest
+        // from Renderer.cs and corresponding unit test.
+        public int? TemporaryParentComponentIdForTests
+            => _parentComponentState?._componentId;
     }
 }

src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs

@@ -48,10 +48,14 @@ protected IComponent InstantiateComponent(Type componentType)
         /// </summary>
         /// <param name="component">The component.</param>
         /// <returns>The component's assigned identifier.</returns>
-        protected int AssignComponentId(IComponent component)
+        protected int AssignRootComponentId(IComponent component)
+            => AssignComponentId(component, -1);
+
+        private int AssignComponentId(IComponent component, int parentComponentId)
         {
             var componentId = _nextComponentId++;
-            var componentState = new ComponentState(this, componentId, component);
+            var parentComponentState = GetOptionalComponentState(parentComponentId);
+            var componentState = new ComponentState(this, componentId, component, parentComponentState);
             _componentStateById.Add(componentId, componentState);
             component.Init(new RenderHandle(this, componentId));
             return componentId;
@@ -92,7 +96,7 @@ public void DispatchEvent(int componentId, int eventHandlerId, UIEventArgs event
             }
         }
 
-        internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame)
+        internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame, int parentComponentId)
         {
             if (frame.FrameType != RenderTreeFrameType.Component)
             {
@@ -105,7 +109,7 @@ internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame)
             }
 
             var newComponent = InstantiateComponent(frame.ComponentType);
-            var newComponentId = AssignComponentId(newComponent);
+            var newComponentId = AssignComponentId(newComponent, parentComponentId);
             frame = frame.WithComponentInstance(newComponentId, newComponent);
         }
 
@@ -140,6 +144,15 @@ internal void AddToRenderQueue(int componentId, RenderFragment renderFragment)
             }
         }
 
+        /// <summary>
+        /// This only needs to exist until there's some other unit-testable functionality
+        /// that makes use of walking the ancestor hierarchy.
+        /// </summary>
+        /// <param name="componentId">The component ID.</param>
+        /// <returns>The parent component's ID, or null if the component was at the root.</returns>
+        internal int? TemporaryGetParentComponentIdForTest(int componentId)
+            => GetRequiredComponentState(componentId).TemporaryParentComponentIdForTests;
+
         private ComponentState GetRequiredComponentState(int componentId)
             => _componentStateById.TryGetValue(componentId, out var componentState)
                 ? componentState

test/Microsoft.AspNetCore.Blazor.Build.Test/RazorIntegrationTestBase.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
@@ -368,7 +368,7 @@ public TestRenderer() : base(new TestServiceProvider())
             public RenderTreeFrame[] LatestBatchReferenceFrames { get; private set; }
 
             public void AttachComponent(IComponent component)
-                => AssignComponentId(component);
+                => AssignRootComponentId(component);
 
             protected override void UpdateDisplay(in RenderBatch renderBatch)
             {

test/Microsoft.AspNetCore.Blazor.Test/LayoutTest.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using Microsoft.AspNetCore.Blazor.Components;
@@ -21,7 +21,7 @@ public LayoutTest()
         {
             _renderer = new TestRenderer();
             _layoutDisplayComponent = new LayoutDisplay();
-            _layoutDisplayComponentId = _renderer.AssignComponentId(_layoutDisplayComponent);
+            _layoutDisplayComponentId = _renderer.AssignRootComponentId(_layoutDisplayComponent);
         }
 
         [Fact]