Element: NRE by accessing RealParent

[MartyIX]

Description

Running LayoutBenchmarker reports a NullReferenceException.

Steps to Reproduce

cd src/Core/tests/Benchmarks
dotnet build -c Release --framework net8.0 && dotnet run --framework net8.0 -c Release -- --runtimes net8.0 --filter *LayoutBenchmarker*

and I can observe in the output:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Maui.Controls.Element.SetParent(Element value) in Z:\maui\src\Controls\src\Core\Element\Element.cs:line 399
   at Microsoft.Maui.Controls.Element.OnChildAdded(Element child) in Z:\maui\src\Controls\src\Core\Element\Element.cs:line 578
   at Microsoft.Maui.Benchmarks.LayoutBenchmarker.GetLayoutHandlerIndex() in Z:\maui\src\Core\tests\Benchmarks\Benchmarks\LayoutBenchmarker.cs:line 26
   at BenchmarkDotNet.Autogenerated.Runnable_2.WorkloadActionNoUnroll(Int64 invokeCount) in Z:\maui\srtifacts\bin\Core.Benchmarks\Release\net8.0\44823321-481f-4fe8-b2f3-29b8a75b64f6\44823321-481f-4fe8-b2f3-29b8a75b64f6.notcs:line 695
   at BenchmarkDotNet.Engines.Engine.RunIteration(IterationData data)
   at BenchmarkDotNet.Engines.EngineStage.RunIteration(IterationMode mode, IterationStage stage, Int32 index, Int64 invokeCount, Int32 unrollFactor)
   at BenchmarkDotNet.Engines.EnginePilotStage.RunAuto()
   at BenchmarkDotNet.Engines.EnginePilotStage.Run()
   at BenchmarkDotNet.Engines.Engine.Run()
   at BenchmarkDotNet.Autogenerated.Runnable_2.Run(IHost host, String benchmarkName) in Z:\maui\srtifacts\bin\Core.Benchmarks\Release\net8.0\44823321-481f-4fe8-b2f3-29b8a75b64f6\44823321-481f-4fe8-b2f3-29b8a75b64f6.notcs:line 562
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)
   --- End of inner exception stack trace ---
   at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)
   at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at BenchmarkDotNet.Autogenerated.UniqueProgramName.AfterAssemblyLoadingAttached(String[] args) in Z:\maui\srtifacts\bin\Core.Benchmarks\Release\net8.0\44823321-481f-4fe8-b2f3-29b8a75b64f6\44823321-481f-4fe8-b2f3-29b8a75b64f6.notcs:line 57

The problem is here:

maui/src/Controls/src/Core/Element/Element.cs

Lines 397 to 403 in 45a8686

	if (RealParent != null)
	{
	((IElementDefinition)RealParent).RemoveResourcesChangedListener(OnParentResourcesChanged);
	if (value != null && (RealParent is Layout || RealParent is IControlTemplated))
	Application.Current?.FindMauiContext()?.CreateLogger<Element>()?.LogWarning($"{this} is already a child of {RealParent}. Remove {this} from {RealParent} before adding to {value}.");
	}

The issue seems to be that RealParent is de facto WeakReference<Element> and as such checking for null here is not sufficient to avoid an NRE.

Link to public reproduction project repository

No response

Version with bug

Nightly / CI build (Please specify exact version)

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

Unknown/Other

Affected platforms

Windows, I was not able test on other platforms

Affected platform versions

No response

Did you find any workaround?

I added var realParent = RealParent; and the benchmark finished OK.

Relevant log output

No response

[MartyIX]

@samhouts I believe that this affects all platforms. And if I read the code properly it can randomly crash for people under some scenarios.

[PureWeen]

AFAICT this isn't going to be something that happens in the wild much

The reason this happen in the benchmark is that the child views are all static

And the layout is purely scoped to the method body.
This means on subsequent runs these children are going to keep getting reparented and the previous parent is going to get GC'd

My gut tells me that recreating that set of conditions in the wild isn't going to happen very frequently.

We should still pattern match that if statement just to be safe.