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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xperiandri · 2026-08-17T00:45:35Z

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

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.fs WeakMap.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`.

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`.

xperiandri · 2026-08-21T15:50:07Z

@T-Gro is my approach correct now?

T-Gro

Can you pls revert the whitespace only changes and only pick the essentials of the fix?

T-Gro · 2026-08-24T14:42:34Z

+#if NETSTANDARD2_0
+                    try table.Add(key, value) with | :? ArgumentException -> ()
+#else
+                    table.TryAdd(key, value) |> ignore


🤖🕵️ Please drop the #if NETSTANDARD2_0 / #else here and keep the single try table.Add(key, value) with :? ArgumentException -> () form — it compiles on every TFM. FCS ships netstandard2.0 only, so the #else table.TryAdd branch never ships. Don't move it to a separate PR either; a net-current-only branch would be rejected for the same reason.

@T-Gro is it a correct statement?

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

github-actions · 2026-08-24T15:39:01Z

🔍 Tooling Safety Check — Affects-Bootstrap, Affects-Compiler-Output, Scope-Review-Needed
Affects-Bootstrap: modifies TypedTreePickle.fs (serialization format)
Affects-Compiler-Output: changes TypedTree representation and remapping
Scope-Review-Needed: touches pickle, remapping, transforms, lib.fs, vsintegration beyond stated NullRef fix

Generated by PR Tooling Safety Check · opus46 4.2M · ◷

* 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`.

T-Gro · 2026-08-27T11:24:08Z

-        { entity_typars = Unchecked.defaultof<_>
+    static member NewUnlinked() : Entity =
+        { entity_typars = LazyWithContext.NotLazy []
          entity_flags = Unchecked.defaultof<_>
          entity_stamp = Unchecked.defaultof<_>
-          entity_logical_name = Unchecked.defaultof<_> 
-          entity_range = Unchecked.defaultof<_> 
+          entity_logical_name = "<unknown>"


It's important that this does absolute minimum of allocations, Entity is a hot path object.
The string constant is a risk of incorrect data (as opposed to a NRE).

Should it be handled on the property level as the other nullable fields?
or would you like me just make it nullable and handle on usage?

T-Gro · 2026-08-27T11:25:46Z

            let allFieldsText =
                fields
-                |> Array.map (fun f -> f.LogicalName)
+                |> Seq.map _.LogicalName


There is no need to materialize transformation as the String.concat accepts seq. So why would we allocate an array?

Can we close this?

T-Gro · 2026-08-27T11:28:01Z

          // does not work correctly (they may get incorrectly relinked to a default member)
-          |> List.filter (fun (isExplicitImpl, _) -> not isExplicitImpl)
-          |> List.map (fun (_, vref) -> vref.LogicalName, vref)),
+          |> Seq.filter (fun (isExplicitImpl, _) -> not isExplicitImpl)


Same here there is no need to materialize the list twice. One lazy sequence materialized at the end

Can we close this?

T-Gro · 2026-08-27T11:31:10Z

-    member x.ModuleOrNamespaceType = x.entity_modul_type.Force()
+    member x.ModuleOrNamespaceType =
+        match x.entity_modul_type with
+        | null -> x.entity_modul_type <- MaybeLazy.Strict (Construct.NewEmptyModuleOrNamespaceType ModuleOrType)


What are the consequences and what is the added value of adding a NewEmptyModuleOrNamespaceType here?

The main point was to prevent the NullReferenceException

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

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

github-project-automation Bot 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 entities Aug 17, 2026

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xperiandri commented Aug 17, 2026

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Comment thread src/Compiler/TypedTree/TypedTree.fs

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

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xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026

Fix comment dotnet#20274#discussion_r3815216161

c3d9be5

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

xperiandri commented Aug 20, 2026

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Comment thread FSharpBuild.Directory.Build.props

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Fix comment dotnet#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 4a4d62e Compare 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 comment dotnet#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 ba36e07 Compare August 20, 2026 08:51

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 20, 2026

Fix comment dotnet#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 3f31b99 Compare August 20, 2026 12:57

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xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 21, 2026

Fix comment dotnet#20274#discussion_r3815216161

67bf4ef

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 45d3fb2 to c199150 Compare August 21, 2026 14:36

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xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from c199150 to 9acab97 Compare August 24, 2026 14:55

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Aug 24, 2026

Fix comment dotnet#20274#discussion_r3815216161

a2df00b

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

xperiandri requested a review from T-Gro August 24, 2026 14:57

xperiandri added 4 commits August 26, 2026 21:46

Fix NRE on unlinked placeholder entities and add release notes

b46c727

Fix accessing Entity.entity_modul_type and Entity.entity_tycon_tcaug

2bc2389

* 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 comment dotnet#20274#discussion_r3815216161

bb2d378

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

6be833e

xperiandri force-pushed the fix-NullReferenceException-breaks-IDE-syntax-coloring-on-unlinked-placeholder-entities branch from 9acab97 to 6be833e Compare August 26, 2026 19:54

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xperiandri commented Aug 17, 2026

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