fix: prevent null WrappedElement during concurrent component re-renders (#1946)

Author: Nacl-HCl

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to **bUnit** will be documented in this file. The project ad
 
 ### Fixed
 
+- `WrapperBase.WrappedElement` could return `null` and throw `NullReferenceException` when an element was accessed concurrently with component re-renders. Reported by [@psu-prog](https://github.com/psu-prog) in #1937. Fixed by [@Nacl-HCl](https://github.com/Nacl-HCl).
 - `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).
 - `Find()` could keep returning DOM nodes from the previous render when a render, e.g. from a timer, happened while the nodes were parsed, causing `UnknownEventHandlerIdException`. Reported by [@xfunc-tim-schneider](https://github.com/xfunc-tim-schneider) in #1938. Fixed by [@linkdotnet](https://github.com/linkdotnet).

src/bunit.generators.internal/Web.AngleSharp/WrapperBase.cs

@@ -24,13 +24,7 @@ public abstract class WrapperBase<TElement> : IElementWrapper<TElement>
 	public TElement WrappedElement
 	{
 		[MethodImpl(MethodImplOptions.AggressiveInlining)]
-		get
-		{
-			if (element is null)
-				element = elementFactory.GetElement<TElement>();
-
-			return element;
-		}
+		get => element ??= elementFactory.GetElement<TElement>();
 	}
 
 	/// <summary>

tests/bunit.generators.tests/Web.AngleSharp/WrapperElementsGeneratorTest.Generator#WrapperBase.g.verified.cs

@@ -1,4 +1,4 @@
-//HintName: WrapperBase.g.cs
+//HintName: WrapperBase.g.cs
 #nullable enable
 using System.CodeDom.Compiler;
 using System.Diagnostics;
@@ -25,13 +25,7 @@ public abstract class WrapperBase<TElement> : IElementWrapper<TElement>
 	public TElement WrappedElement
 	{
 		[MethodImpl(MethodImplOptions.AggressiveInlining)]
-		get
-		{
-			if (element is null)
-				element = elementFactory.GetElement<TElement>();
-
-			return element;
-		}
+		get => element ??= elementFactory.GetElement<TElement>();
 	}
 
 	/// <summary>

tests/bunit.tests/Extensions/RefreshingWrappedElementTest.cs

@@ -76,6 +76,53 @@ public void Test031()
 		Should.Throw<ElementRemovedFromDomException>(() => btn.TextContent);
 	}
 
+	[Fact(DisplayName = "Found element is never null when accessed concurrently with component re-renders")]
+	public async Task Test040()
+	{
+		var cut = Render<Counter>();
+		var elm = cut.Find("span");
+		using var cts = CancellationTokenSource.CreateLinkedTokenSource(Xunit.TestContext.Current.CancellationToken);
+		cts.CancelAfter(TimeSpan.FromSeconds(1));
+		var renders = Task.Run(async () =>
+		{
+			while (!cts.IsCancellationRequested)
+			{
+				await cut.InvokeAsync(cut.Instance.Increment);
+			}
+		}, Xunit.TestContext.Current.CancellationToken);
+
+		try
+		{
+			while (!cts.IsCancellationRequested)
+			{
+				elm.TextContent.ShouldNotBeNull();
+			}
+		}
+		finally
+		{
+			await cts.CancelAsync();
+			await renders;
+		}
+	}
+
+	private sealed class Counter : ComponentBase
+	{
+		private int count;
+
+		public void Increment()
+		{
+			count++;
+			StateHasChanged();
+		}
+
+		protected override void BuildRenderTree(RenderTreeBuilder builder)
+		{
+			builder.OpenElement(0, "span");
+			builder.AddContent(1, count);
+			builder.CloseElement();
+		}
+	}
+
 	private sealed class Markup : ComponentBase
 	{
 		[Parameter] public string Base { get; set; } = string.Empty;