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

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

@@ -61,7 +61,7 @@ namespace Microsoft.AspNetCore.Blazor.Browser.Rendering
         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 @@ namespace Microsoft.AspNetCore.Blazor.Browser.Rendering
         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 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
             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 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
                 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 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
             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 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
         /// <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 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
 
         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 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
         /// </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 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
             }
         }
 
-        internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame)
+        internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame, int parentComponentId)
         {
             if (frame.FrameType != RenderTreeFrameType.Component)
             {
@@ -105,7 +109,7 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
             }
 
             var newComponent = InstantiateComponent(frame.ComponentType);
-            var newComponentId = AssignComponentId(newComponent);
+            var newComponentId = AssignComponentId(newComponent, parentComponentId);
             frame = frame.WithComponentInstance(newComponentId, newComponent);
         }
 
@@ -140,6 +144,15 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
             }
         }
 
+        /// <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 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
             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 @@ namespace Microsoft.AspNetCore.Blazor.Test
         {
             _renderer = new TestRenderer();
             _layoutDisplayComponent = new LayoutDisplay();
-            _layoutDisplayComponentId = _renderer.AssignComponentId(_layoutDisplayComponent);
+            _layoutDisplayComponentId = _renderer.AssignRootComponentId(_layoutDisplayComponent);
         }
 
         [Fact]

test/Microsoft.AspNetCore.Blazor.Test/RendererTest.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;
@@ -28,7 +28,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             });
 
             // Act
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             // Assert
@@ -58,7 +58,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             });
 
             // Act/Assert
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var batch = renderer.Batches.Single();
             var componentFrame = batch.ReferenceFrames
@@ -83,13 +83,46 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 });
         }
 
+        [Fact]
+        public void CanWalkTheAncestorHierarchy()
+        {
+            // TODO: Instead of testing this directly, once there's some other functionaly
+            // that relies on the ancestor hierarchy (e.g., deep params), test that instead
+            // and remove the otherwise unnecessary TemporaryGetParentComponentIdForTest API.
+
+            // Arrange
+            var renderer = new TestRenderer();
+            var rootComponent = new TestComponent(builder =>
+            {
+                builder.AddContent(0, "Hello");
+                builder.OpenComponent<AncestryComponent>(0);
+                builder.AddAttribute(1, nameof(AncestryComponent.NumDescendants), 2);
+                builder.CloseComponent();
+            });
+
+            // Act
+            var componentId = renderer.AssignRootComponentId(rootComponent);
+            rootComponent.TriggerRender();
+
+            // Assert
+            var batch = renderer.Batches.Single();
+            var componentIds = batch.ReferenceFrames
+                .Where(frame => frame.FrameType == RenderTreeFrameType.Component)
+                .Select(f => f.ComponentId);
+            Assert.Equal(new[] { 1, 2, 3 }, componentIds);
+            Assert.Equal(2, renderer.TemporaryGetParentComponentIdForTest(3));
+            Assert.Equal(1, renderer.TemporaryGetParentComponentIdForTest(2));
+            Assert.Equal(0, renderer.TemporaryGetParentComponentIdForTest(1));
+            Assert.Null(renderer.TemporaryGetParentComponentIdForTest(0));
+        }
+
         [Fact]
         public void CanReRenderTopLevelComponents()
         {
             // Arrange
             var renderer = new TestRenderer();
             var component = new MessageComponent { Message = "Initial message" };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
 
             // Act/Assert: first render
             component.TriggerRender();
@@ -127,7 +160,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.OpenComponent<MessageComponent>(0);
                 builder.CloseComponent();
             });
-            var parentComponentId = renderer.AssignComponentId(parentComponent);
+            var parentComponentId = renderer.AssignRootComponentId(parentComponent);
             parentComponent.TriggerRender();
             var nestedComponentFrame = renderer.Batches.Single()
                 .ReferenceFrames
@@ -173,7 +206,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
                 OnTest = args => { receivedArgs = args; }
             };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var eventHandlerId = renderer.Batches.Single()
@@ -201,7 +234,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
                 OnClick = args => { receivedArgs = args; }
             };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var eventHandlerId = renderer.Batches.Single()
@@ -229,7 +262,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
                 OnClickAction = () => { receivedArgs = new object(); }
             };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var eventHandlerId = renderer.Batches.Single()
@@ -258,7 +291,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.OpenComponent<EventComponent>(0);
                 builder.CloseComponent();
             });
-            var parentComponentId = renderer.AssignComponentId(parentComponent);
+            var parentComponentId = renderer.AssignRootComponentId(parentComponent);
             parentComponent.TriggerRender();
 
             // Arrange: Render nested component
@@ -298,7 +331,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseElement();
             });
 
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var eventHandlerId = renderer.Batches.Single()
@@ -336,9 +369,9 @@ namespace Microsoft.AspNetCore.Blazor.Test
             var renderer1 = new TestRenderer();
             var renderer2 = new TestRenderer();
             var component = new MultiRendererComponent();
-            var renderer1ComponentId = renderer1.AssignComponentId(component);
+            var renderer1ComponentId = renderer1.AssignRootComponentId(component);
-            renderer2.AssignComponentId(new TestComponent(null)); // Just so they don't get the same IDs
+            renderer2.AssignRootComponentId(new TestComponent(null)); // Just so they don't get the same IDs
-            var renderer2ComponentId = renderer2.AssignComponentId(component);
+            var renderer2ComponentId = renderer2.AssignRootComponentId(component);
 
             // Act/Assert
             component.TriggerRender();
@@ -376,7 +409,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseComponent();
             });
 
-            var rootComponentId = renderer.AssignComponentId(component);
+            var rootComponentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var nestedComponentFrame = renderer.Batches.Single()
@@ -417,7 +450,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseComponent();
             });
 
-            var rootComponentId = renderer.AssignComponentId(component);
+            var rootComponentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var originalComponentFrame = renderer.Batches.Single()
@@ -453,7 +486,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseComponent();
             });
 
-            var rootComponentId = renderer.AssignComponentId(component);
+            var rootComponentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
 
             var childComponentId = renderer.Batches.Single()
@@ -495,7 +528,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseComponent();
             });
 
-            var rootComponentId = renderer.AssignComponentId(component);
+            var rootComponentId = renderer.AssignRootComponentId(component);
 
             // Act/Assert 1: First render, capturing child component IDs
             component.TriggerRender();
@@ -534,7 +567,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             var eventCount = 0;
             Action<UIEventArgs> origEventHandler = args => { eventCount++; };
             var component = new EventComponent { OnTest = origEventHandler };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var origEventHandlerId = renderer.Batches.Single()
                 .ReferenceFrames
@@ -571,7 +604,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             var eventCount = 0;
             Action<UIEventArgs> origEventHandler = args => { eventCount++; };
             var component = new EventComponent { OnTest = origEventHandler };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var origEventHandlerId = renderer.Batches.Single()
                 .ReferenceFrames
@@ -611,7 +644,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                     { nameof(EventComponent.OnTest), origEventHandler }
                 }
             };
-            var rootComponentId = renderer.AssignComponentId(component);
+            var rootComponentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var batch = renderer.Batches.Single();
             var rootComponentDiff = batch.DiffsByComponentId[rootComponentId].Single();
@@ -653,7 +686,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             var eventCount = 0;
             Action<UIEventArgs> origEventHandler = args => { eventCount++; };
             var component = new EventComponent { OnTest = origEventHandler };
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var origEventHandlerId = renderer.Batches.Single()
                 .ReferenceFrames
@@ -699,7 +732,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 });
                 builder.CloseComponent();
             });
-            var rootComponentId = renderer.AssignComponentId(rootComponent);
+            var rootComponentId = renderer.AssignRootComponentId(rootComponent);
             rootComponent.TriggerRender();
             var origBatchReferenceFrames = renderer.Batches.Single().ReferenceFrames;
             var childComponentFrame = origBatchReferenceFrames
@@ -781,7 +814,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
                 builder.AddContent(0, $"Render count: {++renderCount}");
             });
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
 
             // Act/Assert: Can trigger initial render
             Assert.Equal(0, renderCount);
@@ -817,7 +850,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseComponent();
                 builder.AddContent(2, $"Parent render count: {++parentRenderCount}");
             });
-            var parentComponentId = renderer.AssignComponentId(parent);
+            var parentComponentId = renderer.AssignRootComponentId(parent);
 
             // Act
             parent.TriggerRender();
@@ -891,7 +924,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 }
             });
 
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var childComponentId = renderer.Batches.Single()
                 .ReferenceFrames
@@ -928,7 +961,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             // Arrange: Rendered with textbox enabled
             var renderer = new TestRenderer();
             var component = new BindPlusConditionalAttributeComponent();
-            var componentId = renderer.AssignComponentId(component);
+            var componentId = renderer.AssignRootComponentId(component);
             component.TriggerRender();
             var checkboxChangeEventHandlerId = renderer.Batches.Single()
                 .ReferenceFrames
@@ -971,7 +1004,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 builder.CloseElement();
             });
             var renderer = new TestRenderer();
-            renderer.AssignComponentId(component);
+            renderer.AssignRootComponentId(component);
 
             // Act: Update the attribute value on the parent
             component.TriggerRender();
@@ -1001,7 +1034,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
                 OnUpdateDisplay = _ => onAfterRenderCallCountLog.Add(component.OnAfterRenderCallCount)
             };
-            renderer.AssignComponentId(component);
+            renderer.AssignRootComponentId(component);
 
             // Act
             component.TriggerRender();
@@ -1042,7 +1075,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 }
             });
             var renderer = new TestRenderer();
-            var parentComponentId = renderer.AssignComponentId(parentComponent);
+            var parentComponentId = renderer.AssignRootComponentId(parentComponent);
 
             // Act: First render
             parentComponent.TriggerRender();
@@ -1077,8 +1110,8 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
             }
 
-            public new int AssignComponentId(IComponent component)
+            public new int AssignRootComponentId(IComponent component)
-                => base.AssignComponentId(component);
+                => base.AssignRootComponentId(component);
 
             protected override void UpdateDisplay(in RenderBatch renderBatch)
             {
@@ -1330,5 +1363,25 @@ namespace Microsoft.AspNetCore.Blazor.Test
             {
             }
         }
+
+        private class AncestryComponent : AutoRenderComponent
+        {
+            [Parameter] public int NumDescendants { get; private set; }
+
+            protected override void BuildRenderTree(RenderTreeBuilder builder)
+            {
+                // Recursively renders more of the same until NumDescendants == 0
+                if (NumDescendants > 0)
+                {
+                    builder.OpenComponent<AncestryComponent>(0);
+                    builder.AddAttribute(1, nameof(NumDescendants), NumDescendants - 1);
+                    builder.CloseComponent();
+                }
+                else
+                {
+                    builder.AddContent(1, "I'm the final descendant");
+                }
+            }
+        }
     }
 }

test/shared/TestRenderer.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;
@@ -24,8 +24,8 @@ namespace Microsoft.AspNetCore.Blazor.Test.Helpers
         public List<CapturedBatch> Batches { get; }
             = new List<CapturedBatch>();
 
-        public new int AssignComponentId(IComponent component)
+        public new int AssignRootComponentId(IComponent component)
-            => base.AssignComponentId(component);
+            => base.AssignRootComponentId(component);
 
         public new void DispatchEvent(int componentId, int eventHandlerId, UIEventArgs args)
             => base.DispatchEvent(componentId, eventHandlerId, args);