Fix `NullReferenceException` in `Entity` on unlinked placeholder entities by xperiandri · Pull Request #20274 · dotnet/fsharp

xperiandri · 2026-08-17T00:32:55Z

Summary

Fixes#20269 by making typed-tree remapping and classification resilient to placeholder/unlinked entities created while the file is in a broken mid-edit state. Instead of allowing a single null placeholder to crash the entire IDE classification pipeline, the compiler now treats absent module-type augmentation data as null and skips the remap/guard path safely.

Changes

Added null guards in the typed-tree remapping path for placeholder TyconAugmentation and entity_modul_type values.
Updated typed-tree signatures to reflect the nullable TyconAugmentation shape.
Kept the WeakMap cache update compatible across target frameworks while preserving existing behavior.
Added a compiler release-note entry for issue NullReferenceException breaks IDE syntax coloring on unlinked placeholder entities (e.g. broken code mid-edit) #20269.

github-actions · 2026-08-17T00:33:08Z

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`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md	No release notes found or release notes format is not correct

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xperiandri · 2026-08-17T00:45:35Z

@T-Gro this is another fix (I can move to another PR if you want)

xperiandri · 2026-08-17T00:49:23Z

The Trailing Whitespace extension removed all trailing whitespaces.
I hope it is not an issue.

So if you hide whitespaces you will see the real several lines changed

T-Gro

Thanks for digging into this — resilient partial classification during mid-edit is a real, worthwhile goal, and the guarded remapTyconAug / entity_modul_type remap in TypedTreeOps.Remapping.fs are the right shape. A few blocking items before this can go in:

It doesn't build. Marking entity_modul_type and entity_tycon_tcaug as | null in TypedTree.fsi propagates nullability to consumers that weren't updated, and nullness warnings are errors here (outside Proto). A local -c Release build fails with FS3261 at:

TypedTreeOps.Transforms.fs(991) — d.entity_modul_type.Value is the same unguarded deref you fixed in Remapping.fs, missed here.
TypedTreePickle.fs(2818) — p_maybe_lazy p_modul_typ x.entity_modul_type passes a nullable to a non-nullable target.
TypedTreePickle.fs(2837–2852) — eight TyconAugmentation | null incompatibilities in the tcaug pickle/unpickle path.

That's why every Build_And_Test_* leg is red. If you make these fields nullable you have to sweep all consumers, including the pickler.

~1360 lines of the TypedTree.fs diff are trailing-whitespace churn — only ~20 lines are real change. This trips CheckCodeFormatting, buries the actual fix, and will conflict with everything. Please revert the whitespace-only edits (and the lone one in lib.fs) so the diff is just the guards.

Scope of the type change. These fields were already effectively null for NewUnlinked() placeholders (Unchecked.defaultof<_>), so | null documents an existing latent invariant — but flipping two core Entity fields to nullable forces null-checks everywhere and is a big hammer for an IDE-resilience fix. Worth considering whether the dangling unlinked entity reaching the classification path is itself the bug to fix, rather than making the field type nullable tree-wide.

NewUnlinked() value changes are unnecessary and risky. Switching the placeholder to entity_logical_name = "<unknown>", entity_tycon_repr = TNoRepr, entity_typars = NotLazy [] changes observable pre-Link state on the metadata-unpickling hot path. IsLinked keys off entity_attribs (still defaultof), so linkage detection survives, but none of this is needed for the NRE fix — suggest reverting to Unchecked.defaultof<_>.

No regression test. The linked issue has a clean repro (break a type decl mid-edit); please encode it so this can't silently come back.

lib.fsWeakMap.TryAdd behind #if NETSTANDARD2_0 is correct and a nice cleanup, but it's unrelated to the NRE — fine to keep, just calling it out.

One more: the issue body references files/functions not in this diff (vsintegration/src/FSharp.Editor/*, NewModified*, FreeVars, a NullRef-TypedTree-Classification-Fix.md). Reconciling the description with the actual change will help reviewers follow along.

T-Gro

One note on the null-fallback design (complements the root-cause point).

T-Gro · 2026-08-19T17:21:33Z

- member x.TypeContents = x.entity_tycon_tcaug
+ member x.TypeContents =
+ match x.entity_tycon_tcaug with
+ | null -> TyconAugmentation.Create()


🤖🕵️ On the null branch TypeContents returns a fresh TyconAugmentation.Create() each access, but callers mutate the result in place (tcaug_super <-, tcaug_abstract <-, tcaug_closed <-). If an unlinked placeholder ever reaches such a path, those writes land on a throwaway object and are silently lost — turning a fail-fast NRE into silent typed-tree corruption.
|null-> TyconAugmentation.Create()
ModuleOrNamespaceType (line 884) has the same shape. A shared sentinel, or keeping unlinked entities out of these paths (the root-cause fix), is safer.

So do you think we must save TyconAugmentation.Create() on empty Entity creation?

Fixed in the latest commit

Also, should we then initialize these objects on empty entity creation instead of doing that lazily?

Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri · 2026-08-20T00:38:38Z

@T-Gro without this I still cannot build anything in VS 2026.
I will remove this from this PR. It is here for testing purposes only.
cc @kerams

Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

The previous run was SIGKILL'd by the OOM-killer mid-suite (0 real test failures; 229 tests never ran). Empty commit to re-run the pipeline. Same exit-137 flake also hit unrelated PRs #20274 and #20235 at the same time. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

github-actions · 2026-08-20T18:38:16Z

🔍 Tooling Safety Check — Affects-Bootstrap, Affects-Build-Infra, Affects-Compiler-Output, Scope-Review-Needed
Affects-Bootstrap: FSharpBuild.Directory.Build.props changes bootstrap fsc path resolution
Affects-Build-Infra: eng/Build.ps1, build.sh, test-determinism.ps1, Directory.Build.props modified
Affects-Compiler-Output: TypedTree, TypedTreePickle, TypedTreeOps changes affect compiled metadata
Scope-Review-Needed: Build infra + compiler internals changes beyond stated NRE fix

Generated by PR Tooling Safety Check · opus46 4.1M · ◷

* Used safe `ModuleOrNamespaceType` property instead of `entity_modul_type`. * Used safe `TypeContents` property instead of `entity_tycon_tcaug`. * Optimized `TextViewEventsHandler` memory usage with `voption`

Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri · 2026-08-21T15:50:07Z

@T-Gro is my approach correct now?

xperiandri requested a review from a team as a code owner August 17, 2026 00:32

github-project-automationBot added this to F# Compiler and Tooling Aug 17, 2026

github-project-automationBot moved this to New in F# Compiler and Tooling Aug 17, 2026

xperiandri changed the title ~~Fix NRE on unlinked placeholder entities and add release notes~~Fix NullReferenceException in Entity on unlinked placeholder entitiesAug 17, 2026

github-actionsBot added the AI-Tooling-Check-Scanned-Clean Tooling check: diff analyzed, no interesting infrastructure files label Aug 17, 2026

xperiandri commented Aug 17, 2026
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Comment threadsrc/Compiler/Utilities/lib.fs
xperiandriAug 17, 2026
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@T-Gro this is another fix (I can move to another PR if you want)

xperiandri commented Aug 17, 2026
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T-Gro requested changes Aug 17, 2026
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github-project-automationBot moved this from New to In Progress in F# Compiler and Tooling Aug 17, 2026

T-Gro added the AI-reviewed PR reviewed by AI review council label Aug 17, 2026

T-Gro self-requested a review August 17, 2026 08:41

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from d9c55d0 to 026148dCompare August 17, 2026 11:26

T-Gro reviewed Aug 19, 2026
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xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch 2 times, most recently from a9fc80b to 24f2fc9Compare August 19, 2026 22:13

github-actionsBot added ⚠️ Affects-Bootstrap Tooling check: PR touches compiler bootstrap chain ⚠️ Affects-Build-Infra Tooling check: PR touches build infrastructure ⚠️ Scope-Review-Needed Tooling check: PR scope exceeds title/description labels Aug 19, 2026

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xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026
Fix commentdotnet#20274#discussion_r3815216161
c3d9be5
Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri commented Aug 20, 2026
View reviewed changes

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026
Fix commentdotnet#20274#discussion_r3815216161
4a4d62e
Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from c3d9be5 to 4a4d62eCompare August 20, 2026 00:56

xperiandri requested a review from T-Gro August 20, 2026 08:46

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026
Fix commentdotnet#20274#discussion_r3815216161
ba36e07
Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from 4a4d62e to ba36e07Compare August 20, 2026 08:51

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026
Fix commentdotnet#20274#discussion_r3815216161
3f31b99
Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from ba36e07 to 3f31b99Compare August 20, 2026 12:57

github-actionsBot added the ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen label Aug 20, 2026

xperiandri added 5 commits August 21, 2026 16:35

Fix missing .NET 11 framework build issue
289e8a8

Fix NRE on unlinked placeholder entities and add release notes
3e13fa1

Fix accessing Entity.entity_modul_type and Entity.entity_tycon_tcaug
16fe0f4
* Used safe `ModuleOrNamespaceType` property instead of `entity_modul_type`. * Used safe `TypeContents` property instead of `entity_tycon_tcaug`. * Optimized `TextViewEventsHandler` memory usage with `voption`

Fix commentdotnet#20274#discussion_r3815216161
67bf4ef
Always return initialized value for `Entity.entity_modul_type` and `Entity.entity_tycon_tcaug`.

fixup! Fix NRE on unlinked placeholder entities and add release notes
c199150

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from 45d3fb2 to c199150Compare August 21, 2026 14:36

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xperiandri commented Aug 17, 2026

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