[P0 blocker][XAML SourceGen Hot Reload] Subsequent updates fail with ENC1002 after generated method identity is lost

[rmarinho]

Description

.NET 11 Preview 7's opt-in Incremental XAML Hot Reload / XAML SourceGen path can fail on a subsequent Hot Reload update with:

ENC1002: Cannot apply changes -- unexpected error

System.NullReferenceException / InvalidOperationException
at Microsoft.CodeAnalysis.Emit.DefinitionMap.GetPreviousMethodHandle(...)
at Microsoft.CodeAnalysis.Emit.DeltaMetadataWriter.CreateDeletedMemberDefs(...)
at Microsoft.CodeAnalysis.Emit.EmitHelpers.EmitDifference(...)

The complete minimal analysis and EnC logs are in:

• EnC/Hot Reload: GetPreviousMethodHandle fails for a delta‑added source‑generated method on a later update (MethodsAdded tracking lost across generations) roslyn#84569
• Related crash variant: Hot Reload (EnC): NullReferenceException in DefinitionMap.GetPreviousMethodHandle deleting a delta-added source-generated method (MAUI XAML SourceGen UpdateComponent) roslyn#84560

Roslyn closed both issues as Resolution-External. The maintainer diagnosis on #84569 was:

The root cause of the problem is that the source generator is non-deterministic.

This therefore needs a canonical MAUI-side blocker rather than being considered fixed because the Roslyn issue is closed.

Why this is a P0 blocker

This blocks trustworthy validation of the .NET 11 Preview 7 opt-in XAML SourceGen/Incremental Hot Reload feature:

• A first XAML update can succeed, while a later update crashes Roslyn's EnC delta emitter.
• The failure affects the core repeated-edit workflow that Hot Reload must support.
• It can affect both Visual Studio and VS Code because both consume the generated code through Roslyn EnC.
• A closed Roslyn issue currently gives a false impression that the defect was fixed.

Scope is intentionally narrow: this blocks the opt-in .NET 11 XAML SourceGen/Incremental path. It is not evidence that Legacy/default XAML Hot Reload is broken.

Root cause

The Incremental XAML Hot Reload generator can emit UpdateComponent() only in an EnC delta rather than in generation 0. On a later generation, Roslyn can no longer map that delta-added method:

1. UpdateComponent() is introduced in generated *.xaml.uc.xsg.g.cs output after the baseline.
2. A subsequent generator output does not deterministically preserve/re-emit the method.
3. Roslyn's definition mapping loses the corresponding EmitBaseline.MethodsAdded entry.
4. A later update/delete reaches DefinitionMap.GetPreviousMethodHandle, which cannot resolve the previous method and throws.

The MAUI generator must produce stable output across every generation, including C#-only edits, invalid/intermediate XAML, structural resets, and no-op patches.

Steps to reproduce

Use the minimal reproduction and diagnostic logs from:

dotnet/roslyn#84569

At a high level:

1. Use a .NET 11 Preview 7 MAUI app with XAML SourceGen/Incremental Hot Reload enabled:

   <MauiXamlInflator>SourceGen</MauiXamlInflator>
   <MauiXamlHotReload>SourceGen</MauiXamlHotReload>
   <EnableIncrementalHotReload>true</EnableIncrementalHotReload>

2. Start a managed Hot Reload session.

3. Apply the first XAML update that introduces the generated patch method.

4. Apply subsequent XAML and/or interleaved C# updates.

5. Observe ENC1002 and the GetPreviousMethodHandle exception on a later update.

Expected behavior

Every subsequent XAML or interleaved C# update applies successfully. Generated patch methods retain deterministic identity across all EnC generations.

Actual behavior

A later update fails in Roslyn's EnC delta emitter because a previously delta-added generated method can no longer be resolved.

Current fix status

No fix is merged or shipped as of 2026-07-27:

• [XAML] Incremental XAML Hot Reload (source-generated patch chains) #34338 merged the Incremental XAML Hot Reload feature into net11.0.
• [XAML HR] Keep UpdateComponent() stable across generations (avoid EnC "deleted member" crash) #36674 was the first MAUI-side stabilization attempt and closed unmerged when its base branch was retired.
• [XAML HR] Keep UpdateComponent() stable across generations to avoid E… #36680 re-targeted that approach to net11.0, but closed unmerged.
• XAML Hot Reload: Improve incremental generation and coverage #36730 is an open adjacent generator-hardening PR, but does not currently claim or demonstrate that it fixes this issue.
• Live net11.0 generator state does not contain the UpdateComponent() lifetime/stability tracking proposed by [XAML HR] Keep UpdateComponent() stable across generations to avoid E… #36680.

Related issues that should remain separate

• [XAML Hot Reload] SourceGen HR: successive DataTemplate edits under dotnet watch crash the app / poison HR / kill the watcher (PR #34338) #36482: successive DataTemplate edits and unstable synthesized closure/lambda identity.
• [Apple mobile] dotnet watch: XAML edits do not update the UI, C# edits crash with a Roslyn EnC NullReferenceException #35928: Apple dotnet watch Hot Reload failures with a similar Roslyn stack, but a different surface and ownership path.
• [HR] Exception while emitting update: System.NullReferenceException #27612: an older EnC exception with a different root cause, fixed in Roslyn by EnC: Track stale projects roslyn#78166.

Suggested fix direction

Revive the intent of #36680 while addressing its review concerns:

• deterministically preserve/re-emit UpdateComponent() across all generator branches;
• avoid unbounded static state or retain state only for the bounded lifetime of the active compilation/session;
• avoid broad exception swallowing;
• add regression tests for:
  • repeated XAML edits;
  • XAML edit followed by C#-only edit;
  • invalid/intermediate XAML followed by recovery;
  • structural reset/full-page transitions;
  • multiple pages/projects without state leaking between compilations.

Exit criteria

• A deterministic MAUI generator fix is merged into net11.0.
• The fix is present in the .NET 11 Preview/MAUI build used by VS and VS Code.
• Repeated-edit regression tests cover the exact #84569 failure.
• Device/desktop E2E verifies multiple consecutive XAML updates and interleaved XAML/C# updates.
• No ENC1002, GetPreviousMethodHandle exception, stale generator state, or memory retention across repeated sessions.

Version and platforms

• Version: .NET 11 Preview 7 / MAUI net11.0 CI builds with Incremental XAML Hot Reload enabled.
• Platforms: generator/EnC-layer issue; potentially cross-platform. The Roslyn repro used Windows, while related SourceGen repeated-edit failures have also been observed on Android.
• Regression: new opt-in feature introduced by [XAML] Incremental XAML Hot Reload (source-generated patch chains) #34338; no prior stable version of this feature.