Fix Components: prevent collection modification during nested component navigation with SupplyParameterFromQuery

src/Components/Components/src/Routing/SupplyParameterFromQueryValueProvider.cs

@@ -17,6 +17,7 @@ internal sealed class SupplyParameterFromQueryValueProvider(NavigationManager na
     private string? _lastUri;
     private bool _isSubscribedToLocationChanges;
     private bool _queryChanged;
+    private bool _isProcessingLocationChange;
 
     public bool IsFixed => false;
 
@@ -34,7 +35,7 @@ public bool CanSupplyValue(in CascadingParameterInfo parameterInfo)
 
     public void Subscribe(ComponentState subscriber, in CascadingParameterInfo parameterInfo)
     {
-        if (_pendingSubscribers?.Count > 0 || (TryUpdateUri() && _isSubscribedToLocationChanges))
+        if (_pendingSubscribers?.Count > 0 || (TryUpdateUri() && _isSubscribedToLocationChanges) || _isProcessingLocationChange)
         {
             // Renderer.RenderInExistingBatch triggers Unsubscribe via ProcessDisposalQueueInExistingBatch after subscribing with any new components,
             // so this branch should be taken iff there's a pending OnLocationChanged event for the current Uri that we're already subscribed to.
@@ -125,26 +126,34 @@ private void OnLocationChanged(object? sender, LocationChangedEventArgs args)
     {
         Debug.Assert(_subscribers is not null);
 
-        TryUpdateUri();
-
-        if (_queryChanged)
+        _isProcessingLocationChange = true;
+        try
         {
-            foreach (var subscriber in _subscribers)
+            TryUpdateUri();
+
+            if (_queryChanged)
             {
-                subscriber.NotifyCascadingValueChanged(ParameterViewLifetime.Unbound);
-            }
+                foreach (var subscriber in _subscribers)
+                {
+                    subscriber.NotifyCascadingValueChanged(ParameterViewLifetime.Unbound);
+                }
 
-            _queryChanged = false;
-        }
+                _queryChanged = false;
+            }
 
-        if (_pendingSubscribers is not null)
-        {
-            foreach (var subscriber in _pendingSubscribers)
+            if (_pendingSubscribers is not null)
             {
-                _subscribers.Add(subscriber);
-            }
+                foreach (var subscriber in _pendingSubscribers)
+                {
+                    _subscribers.Add(subscriber);
+                }
 
-            _pendingSubscribers.Clear();
+                _pendingSubscribers.Clear();
+            }
+        }
+        finally
+        {
+            _isProcessingLocationChange = false;
         }
     }
 

src/Components/Components/test/Routing/SupplyParameterFromQueryValueProviderTest.cs

@@ -0,0 +1,104 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+using Microsoft.AspNetCore.Components.Rendering;
+using Microsoft.AspNetCore.Components.Test.Helpers;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Components.Routing;
+
+public class SupplyParameterFromQueryValueProviderTest
+{
+    [Fact]
+    public void NavigationWithNestedComponentsDoesNotThrowCollectionModifiedException()
+    {
+        // This test reproduces the bug where navigating causes a child component with
+        // [SupplyParameterFromQuery] to be rendered, which modifies the subscribers collection
+        // while it's being enumerated during OnLocationChanged
+
+        // Arrange
+        var serviceCollection = new ServiceCollection();
+        var navigationManager = new FakeNavigationManager();
+        serviceCollection.AddSingleton<NavigationManager>(navigationManager);
+        serviceCollection.AddSupplyValueFromQueryProvider();
+        var services = serviceCollection.BuildServiceProvider();
+
+        var renderer = new TestRenderer(services);
+
+        // Create a parent component that conditionally renders a child based on query parameter
+        var parentComponent = new ConditionalParentComponent();
+        var parentComponentId = renderer.AssignRootComponentId(parentComponent);
+
+        // Initial render - parent without child
+        navigationManager.NotifyLocationChanged("http://localhost/test", false);
+        parentComponent.TriggerRender();
+
+        // Assert initial state
+        Assert.Single(renderer.Batches);
+
+        // Act - Navigate to URL that causes child to be rendered
+        // This should trigger OnLocationChanged which enumerates subscribers,
+        // and the child component subscribes during that enumeration
+        var exception = Record.Exception(() =>
+        {
+            navigationManager.NotifyLocationChanged("http://localhost/test?parentParam=showChild", false);
+        });
+
+        // Assert - No exception should be thrown
+        Assert.Null(exception);
+
+        // Verify the components rendered correctly
+        Assert.True(renderer.Batches.Count >= 2, "Should have rendered at least twice");
+    }
+
+    private class FakeNavigationManager : NavigationManager
+    {
+        public FakeNavigationManager()
+        {
+            Initialize("http://localhost/", "http://localhost/test");
+        }
+
+        public void NotifyLocationChanged(string uri, bool isInterceptedLink)
+        {
+            Uri = uri;
+            NotifyLocationChanged(isInterceptedLink);
+        }
+
+        protected override void NavigateToCore(string uri, NavigationOptions options)
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    private class ConditionalParentComponent : AutoRenderComponent
+    {
+        [SupplyParameterFromQuery]
+        public string ParentParam { get; set; }
+
+        protected override void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            builder.OpenElement(0, "div");
+            builder.AddContent(1, $"Parent: {ParentParam}");
+
+            if (ParentParam == "showChild")
+            {
+                builder.OpenComponent<ChildWithQueryParamComponent>(2);
+                builder.CloseComponent();
+            }
+
+            builder.CloseElement();
+        }
+    }
+
+    private class ChildWithQueryParamComponent : AutoRenderComponent
+    {
+        [SupplyParameterFromQuery]
+        public string ChildParam { get; set; }
+
+        protected override void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            builder.OpenElement(0, "div");
+            builder.AddContent(1, $"Child: {ChildParam}");
+            builder.CloseElement();
+        }
+    }
+}