dotnet-watch: F# hot reload integration (inspection draft) by NatElkins · Pull Request #1 · NatElkins/sdk

NatElkins · 2026-06-12T12:55:27Z

dotnet-watch: F# hot reload integration (inspection draft)

The dotnet-watch half of F# hot reload, paired with the compiler work in
NatElkins/fsharp#3 (full implementation).
Not for upstream until the FCS API ships publicly — this draft exists for review and for
anyone who wants to run the end-to-end experience.

🚀 Try it

hot-reload-quickstart.md
— two clones, two builds, copy-paste steps, ending with dotnet watch run hot-patching a
running F# console app (including adding lambdas to live methods). This branch builds the
complete .NET CLI (artifacts/bin/redist/Debug/dotnet) that the guide uses.

What this branch contains

FSharpHotReloadService + FSharpProjectInfo (src/Dotnet.Watch/Watch/HotReload/FSharp/):
F# project discovery from the MSBuild graph, source/dependency change mapping, forced-build
orchestration, and a reflection bridge into FSharp.Compiler.Service — no compile-time FCS
dependency, graceful restart-fallback on stock toolsets
Session-object mode: one FSharpHotReloadSession per watch run (the FCS-side
DebuggingSession analogue), every F# project added at prestart, per-project deltas with
solution-wide commit. Multi-project interleaved edits apply in place — e2e
ChangeFilesInFSharpAppAndLib_InterleavedEditsApplyInPlace edits a referenced library,
then the app, then the library again, all without restart. Legacy single-project path
preserved as the fallback for older FCS builds
(DOTNET_WATCH_FSHARP_USE_SESSION_OBJECT=0 kill switch)
CompilationHandler integration: merged Roslyn + F# update flow; session-owned F# files
kept out of the Roslyn workspace (prevents spurious ENC1009 restarts); runtime EnC
capability forwarding with in-place refresh once agents connect (the session is prestarted
before the app launches)
SupportsHotReload project capability in Microsoft.NET.Sdk.FSharp.targets (C#/VB parity)
MVID pinning at AddProject time: forced design-time rebuilds move the on-disk MVID;
deltas must keep targeting the module the runtime actually loaded
Trace diagnostics throughout via DOTNET_WATCH_TRACE_FSHARP_HOTRELOAD=1

Evidence

19/19 F# watch test classes (FSharpHotReloadTests, FSharpHotReloadServiceTests,
FSharpReflectionHostTests) including the multi-project e2e, no-op edit handling (no
restart on whitespace saves), and dependency-edit handling — run against the locally built
FCS from hot-reload-v2.

Design docs

The architecture (session entity, snapshot contract, capability negotiation, determinism)
is documented on the compiler branch — start at
hot-reload-architecture.md.

NatElkins · 2026-06-12T20:09:12Z

Compiler-side counterpart is now under upstream review at dotnet/fsharp#19941. This branch gained: a one-variable master kill switch (DOTNET_WATCH_FSHARP_HOTRELOAD=0), the SupportsHotReload capability now advertised only under dotnet-watch builds (no VS blast radius), and session-object-only FCS support (the bridge no longer requires the retired legacy checker surface — e2e suite 8/0 against current FCS).

ChangeFileInFSharpProjectWithLoop_FirstEditAppliesInPlace asserted only the applied/no-restart log messages, so a delta that "applies" without changing behavior would still pass. Assert the running loop now prints the edited string in place (mirrors ChangeFileInFSharpProjectWithLoop_AppliesOrRestarts). Addresses review feedback on #1.

…t-watch The F# bridge collapsed the rude-edit reason to a record/DU ToString() dump and logged it at Debug only, so an edit that forces a restart gave the user no reason. Now that FSharpHotReloadError.UnsupportedEdit carries a structured rude-edit list (Id + Severity + Message), read it via reflection (TryFormatRudeEdits) into a clean "{Id}: {Message}" reason and report it as a warning instead of at Debug. The extraction is defensive: any shape mismatch falls back to ToString(), so the bridge stays correct against older FCS builds (where UnsupportedEdit still carries a string). Pairs with the FCS-side structured diagnostics channel on dotnet/fsharp#19941. Addresses review feedback on #1.

Co-authored-by: Viktor Hofer <7412651+ViktorHofer@users.noreply.github.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Co-authored-by: Jan Provaznik <janprovaznik@microsoft.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…#54719) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

TestAsset.WithSource applied ~14 property/package-version substitutions by calling UpdateProjProperty/ReplacePackageVersionVariable in a loop, each of which reloaded and rewrote every project file in the asset. That meant ~14 XDocument load+save round-trips per project file during the setup of every test that calls WithSource (the standard setup path for hundreds of SDK integration tests). Extract the per-document mutation bodies into private static helpers and apply all substitutions in one WithProjectChanges pass, so each project file is loaded and saved once. Behavior is unchanged: verified byte-identical output across all 352 test-asset csproj files. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: a22eead4-88ca-4142-a6d8-a7199db73289

WithSource copied every source file to the destination and then, in the substitution pass, reloaded and re-saved each project/.xml file with the target-framework, runtime-identifier, and package-version substitutions applied. That meant every project file was written twice: once by File.Copy and again by the substitution save that immediately overwrote it. Skip the File.Copy for files that the substitution pass rewrites; instead load those directly from the source and write the substituted result straight to the destination once. Non-project files are still copied verbatim. Because File.Copy is an exact byte copy, loading from the source is equivalent to loading from the copied destination, so output is unchanged: verified byte-identical across all 257 test assets (1439 files, 0 differences) while eliminating 404 redundant writes (~22% of WithSource's file writes). Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: a22eead4-88ca-4142-a6d8-a7199db73289

Address review feedback: use OfType<XAttribute>() to filter out null attributes returned by the LINQ chain instead of a redundant inner null-check. Behavior is unchanged. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: a22eead4-88ca-4142-a6d8-a7199db73289

…55259) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

…5218) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…s on x64 (dotnet#55205) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…ovider.EnsureInstallPrerequisites (dotnet#55105) Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

CODEOWNERS_PAT_0 through _2 would need to exist as secrets containing a token with exclusively organization Members: read and repository Issues: read for this to function. This is scope creep so I will be likely removing this and deferring it to another PR but including here so we have it for future reference to build on top of.

Copilot-Session: 1094e91b-1c9c-4704-998f-0a5959e900aa

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 366a002c-9ad2-4938-a79a-af9d2f425d24

dotnet#55388) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 76a86ef3-d73c-451c-8f6e-cef56bf3509c

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

…otnet#55425)

Resolve the TestFX dependency conflicts using main's newer coherent MSTest and Microsoft.Testing.Platform build. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8bf6402b-2114-4555-97d4-9708f64b60da

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 99b4cd14-14dc-459c-bac7-f984c9d2632b

Co-authored-by: Chet Husk <chusk3@gmail.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

Copilot-Session: 2331cf72-dd7c-4507-ba0f-792de68bd434

Update MSTest and Microsoft.Testing.Platform to TestFX build 324182, which includes microsoft/testfx#10169, and re-enable live capture in CI. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8bf6402b-2114-4555-97d4-9708f64b60da

…otnet#55433) Copilot-Session: ffc47f67-df45-4968-a755-7f30f3d3da22

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ed opt-out property (dotnet#55447) Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: javiercn <6995051+javiercn@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

…#55117) Co-authored-by: Jan Kratochvíl <kratochvil.jan@outlook.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 44aacce5-4a69-493c-989a-6bacbe193e08

…rp-hotreload-watch-v2

This was referenced Jun 12, 2026

F# hot reload: full implementation (inspection overview) NatElkins/fsharp#3

Closed

Hot Reload dotnet/fsharp#11636

Open

F# hot reload: Edit-and-Continue delta emission behind --test:HotReloadDeltas dotnet/fsharp#19941

Open

T-Gro reviewed Jun 23, 2026

View reviewed changes

Comment thread test/dotnet-watch.Tests/HotReload/FSharpHotReloadTests.cs

nagilson and others added 23 commits July 13, 2026 16:12

Allow assignment broadly to prevent maintaining codeowners sync script

a934eb3

Update issue triage assignment guidance.

a24bc21

needs more info should ping issue author + sometimes ask for a binlog

6dbd42b

reissue issue triage lock

0fa3b9c

Clean up issue triage language

342c847

[automated] Merge branch 'release/10.0.4xx' => 'main' (dotnet#55146)

c76f7a4

manual review of language in the skill file

fd8051a

[main] Source code updates from dotnet/dotnet (dotnet#55245)

83265f8

Co-authored-by: Viktor Hofer <7412651+ViktorHofer@users.noreply.github.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Co-authored-by: Jan Provaznik <janprovaznik@microsoft.com>

Stop shipping dnx from the SDK component on macOS (dotnet#55241)

7ee70d1

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Run NativeAOT CLI tests in CI (win-x64, linux-x64, osx-arm64) (dotnet…

1c43067

…#54719) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Merge origin/main and resolve MTP run properties conflict

8c32e76

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Default BuildProjectReferences to false when NoBuild is true (dotnet#…

bd3e0e5

…55259) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Enable cross-architecture Native AOT tool packaging (dotnet#55250)

d6efb39

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Enable MSBuild server by default in MSBuildForwardingApp (dotnet#55231)

16e4424

Consolidate generated-file guardrails in agent instructions (dotnet#5…

9d1b3a6

…5218) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Do not bundle the ILCompiler (NativeAOT) on ppc64le (dotnet#55051)

2e11375

Enable same-OS cross-arch NativeAOT builds; cross-build arm64 AoT leg…

5ef6b4b

…s on x64 (dotnet#55205) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Fix case-sensitive path comparison in GlobalSettingsTemplatePackagePr…

8354130

…ovider.EnsureInstallPrerequisites (dotnet#55105) Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

make issue target deterministic

c825e13

JanProvaznik and others added 30 commits July 23, 2026 10:36

Bump minimum MSBuild version to 18.7 (dotnet#55230)

72524e5

Merge branch 'main' into dev/amauryleve/traversal-deep-nested-e2e-tests

ed821a5

Add deep-nested traversal e2e tests for dotnet test (MTP) (dotnet#55407)

a81cf7c

Forward MSBuild build/tasks/details telemetry event (dotnet#54092)

b792a18

Copilot-Session: 1094e91b-1c9c-4704-998f-0a5959e900aa

Normalize SDK build leg naming (dotnet#55400)

0135f03

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 366a002c-9ad2-4938-a79a-af9d2f425d24

Update Strings.resx

f03c38c

Merge branch 'main' into Fixed-many-typos

ad92021

Add test authoring guidance for Helix compatibility and TFM properties (

cbb5d5a

dotnet#55388) Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 76a86ef3-d73c-451c-8f6e-cef56bf3509c

Add MTP artifact post-processing to dotnet test (dotnet#55419)

5e25224

Honor Configuration environment variable in CLI options

f03b697

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

Bump MSTest.Sdk from 4.4.0-preview.26367.7 to 4.4.0-preview.26372.14 (d…

008a7da

…otnet#55425)

Merge main into MSTest live output update

53bb3be

Resolve the TestFX dependency conflicts using main's newer coherent MSTest and Microsoft.Testing.Platform build. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 8bf6402b-2114-4555-97d4-9708f64b60da

Document multi-repo ownership boundaries (dotnet#55432)

e83a7f9

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 99b4cd14-14dc-459c-bac7-f984c9d2632b

Fix multi-arch container label parsing (dotnet#55414)

360efc5

[main] Fix Native AOT SDK version lookup (dotnet#55413)

bb37a53

Co-authored-by: Chet Husk <chusk3@gmail.com>

Address configuration option review feedback

b45f18c

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

Add PartitionByClass support to Helix test scheduling (dotnet#55434)

3f3c7c3

Copilot-Session: 2331cf72-dd7c-4507-ba0f-792de68bd434

Merge branch 'main' into dev/amauryleve/fix-sdk-issue-52358

546c10e

Fixed a lot of typos in the codebase and documentation (dotnet#55327)

24dcbf7

Fix ValidatePatternBaseDir for wildcard-in-directory-name patterns (d…

d9d7ec4

…otnet#55433) Copilot-Session: ffc47f67-df45-4968-a755-7f30f3d3da22

Add copilot-app LLM environment telemetry detection (dotnet#55440)

36c0dfd

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add telemetry for --help, --version, and --info options (dotnet#55442)

83e4e51

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

[Blazor] Fix Blazor WebAssembly diagnostics defaults to use the defin…

b3edf14

…ed opt-out property (dotnet#55447) Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: javiercn <6995051+javiercn@users.noreply.github.com>

Fix CLI default value test regressions

b931bfe

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 72bf4a4e-15d3-4177-9246-c49ad3071b13

Enable live output for MSTest tests (dotnet#55418)

b4c5c63

Migrate DefineStaticWebAssets to multithreaded task execution (dotnet…

cc56ad4

…#55117) Co-authored-by: Jan Kratochvíl <kratochvil.jan@outlook.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Honor Configuration environment variable in CLI options (dotnet#55431)

01968e2

Preserve whitespace paths in static web asset tasks (dotnet#55266)

aa2ca0b

Copilot-Session: 44aacce5-4a69-493c-989a-6bacbe193e08

Merge remote-tracking branch 'origin/main' into refresh/20260717/fsha…

227e1df

…rp-hotreload-watch-v2

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

dotnet-watch: F# hot reload integration (inspection draft)#1

dotnet-watch: F# hot reload integration (inspection draft)#1
NatElkins wants to merge 705 commits into
mainfrom
fsharp-hotreload-watch-v2

NatElkins commented Jun 12, 2026 •

edited

Loading

Uh oh!

NatElkins commented Jun 12, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

20 participants

Conversation

NatElkins commented Jun 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

dotnet-watch: F# hot reload integration (inspection draft)

🚀 Try it

What this branch contains

Evidence

Design docs

Uh oh!

NatElkins commented Jun 12, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

20 participants

NatElkins commented Jun 12, 2026 •

edited

Loading