fix: Check for loops and invalide children (fixes #1935)

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to **bUnit** will be documented in this file. The project ad
 ### Fixed
 
 - `DisposeComponentsAsync()` could dispose nothing when the renderer's dispatcher was busy, e.g. with a timer calling `InvokeAsync`. Reported by [@xfunc-tim-schneider](https://github.com/xfunc-tim-schneider) in #1939. Fixed by [@linkdotnet](https://github.com/linkdotnet).
+- Markup generation and `FindComponents` could recurse until the test host crashed with a stack overflow when a render threw while instantiating a child component. Reported in #1935. Fixed by [@linkdotnet](https://github.com/linkdotnet).
 
 ## [2.11.3] - 2026-09-13
 

src/bunit/Rendering/BunitRenderer.cs

@@ -661,7 +661,7 @@ void FindComponentsInRenderTree(int componentId)
 			for (var i = 0; i < frames.Count; i++)
 			{
 				ref var frame = ref frames.Array[i];
-				if (frame.FrameType == RenderTreeFrameType.Component)
+				if (frame.FrameType == RenderTreeFrameType.Component && frame.Component is not null)
 				{
 					if (frame.Component is TComponent)
 					{

src/bunit/Rendering/Internal/Htmlizer.cs

@@ -131,6 +131,14 @@ int position
 	)
 	{
 		var frame = frames.Array[position];
+
+		// A component frame is left uninstantiated (ComponentId 0) when the render that added it threw mid-diff.
+		// Following that id would recurse into the first root component, which can be an ancestor.
+		if (frame.Component is null)
+		{
+			return position + frame.ComponentSubtreeLength;
+		}
+
 		var childFrames = context.GetRenderTreeFrames(frame.ComponentId);
 		RenderFrames(context, childFrames, 0, childFrames.Count);
 		return position + frame.ComponentSubtreeLength;

tests/bunit.tests/Rendering/BunitRendererTest.cs

@@ -450,6 +450,27 @@ public void Test211()
 		cut.Find("h1").TextContent.ShouldBe($"Hello world {RenderMode.InteractiveWebAssembly}");
 	}
 
+	[Fact(DisplayName = "Markup generation skips child components whose instantiation threw during a render")]
+	public async Task Test212()
+	{
+		var cut = Render<ShowsThrowingChild>();
+		var trigger = cut.FindComponent<RenderTrigger>();
+		await cut.Instance.ShowThrowingChild();
+
+		await trigger.Instance.TriggerWithValue("after");
+
+		cut.Markup.ShouldBe("after");
+	}
+
+	[Fact(DisplayName = "FindComponents skips child components whose instantiation threw during a render")]
+	public async Task Test213()
+	{
+		var cut = Render<ShowsThrowingChild>();
+		await cut.Instance.ShowThrowingChild();
+
+		cut.FindComponents<RenderTrigger>().Count.ShouldBe(1);
+	}
+
 	internal sealed class LifeCycleMethodInvokeCounter : ComponentBase
 	{
 		public int InitilizedCount { get; private set; }
@@ -532,6 +553,33 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
 		}
 	}
 
+	internal sealed class ThrowsOnConstruction : ComponentBase
+	{
+		public ThrowsOnConstruction() => throw new InvalidOperationException("THROWS ON PURPOSE");
+	}
+
+	internal sealed class ShowsThrowingChild : ComponentBase
+	{
+		private bool showThrowing;
+
+		public Task ShowThrowingChild()
+		{
+			showThrowing = true;
+			return InvokeAsync(StateHasChanged);
+		}
+
+		protected override void BuildRenderTree(RenderTreeBuilder builder)
+		{
+			builder.OpenComponent<RenderTrigger>(0);
+			builder.CloseComponent();
+			if (showThrowing)
+			{
+				builder.OpenComponent<ThrowsOnConstruction>(1);
+				builder.CloseComponent();
+			}
+		}
+	}
+
 	internal sealed class ToggleChild : ComponentBase
 	{
 		private bool showing = true;