dotnet watch run completely broken for F#

[isaacabraham]

Describe the bug

Using dotnet watch run does not work for F# applications. It won't start the application at all.

This is the output I get:

PS D:\code\TodoService\src\app> dotnet watch run
dotnet watch 🔥 Hot reload enabled. For a list of supported edits, see https://aka.ms/dotnet/hot-reload.
  💡 Press "Ctrl + R" to restart.
Using launch settings from D:\code\TodoService\src\app\Properties\launchSettings.json...
dotnet watch ⏳ Waiting for a file to change before restarting dotnet...

At this point, the application should be running, but isn't. Notice how it is trying to use hot reload which doesn't support F#.

When I make a change to the application code (which should trigger a standard watch run restart), sure enough I get this:

dotnet watch ❌ System.InvalidOperationException: Cannot open project 'D:\code\TodoService\src\app\app.fsproj' because the file extension '.fsproj' is not associated with a language.
   at Microsoft.CodeAnalysis.MSBuild.DiagnosticReporter.Report(DiagnosticReportingMode mode, String message, Func`2 createException)
   at Microsoft.CodeAnalysis.MSBuild.ProjectFileExtensionRegistry.TryGetLanguageNameFromProjectPath(String projectFilePath, DiagnosticReportingMode mode, String& languageName)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.Worker.LoadProjectFileInfosAsync(String projectPath, DiagnosticReportingOptions reportingOptions, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.Worker.LoadProjectInfosFromPathAsync(String projectPath, DiagnosticReportingOptions reportingOptions, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.Worker.LoadAsync(CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.LoadProjectInfoAsync(String projectFilePath, ProjectMap projectMap, IProgress`1 progress, ILogger msbuildLogger, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.LoadProjectInfoAsync(String projectFilePath, ProjectMap projectMap, IProgress`1 progress, ILogger msbuildLogger, CancellationToken cancellationToken)
   at Microsoft.DotNet.Watcher.Tools.IncrementalMSBuildWorkspace.UpdateProjectConeAsync(String rootProjectPath, CancellationToken cancellationToken)
   at Microsoft.DotNet.Watcher.HotReloadDotNetWatcher.WatchAsync(CancellationToken shutdownCancellationToken)
   at Microsoft.DotNet.Watcher.HotReloadDotNetWatcher.WatchAsync(CancellationToken shutdownCancellationToken)
   at Microsoft.DotNet.Watcher.HotReloadDotNetWatcher.WatchAsync(CancellationToken shutdownCancellationToken)
   at Microsoft.DotNet.Watcher.HotReloadDotNetWatcher.WatchAsync(CancellationToken shutdownCancellationToken)
   at Microsoft.DotNet.Watcher.HotReloadDotNetWatcher.WatchAsync(CancellationToken shutdownCancellationToken)
   at Microsoft.DotNet.Watcher.Program.RunAsync()
dotnet watch ❌ An unexpected error occurred

Fsproj is indeed associated with a language - F#. It's just that hot reload doesn't know about it. It's completely broken.

Using dotnet run works, thankfully:

PS D:\code\TodoService\src\app> dotnet run
Using launch settings from D:\code\TodoService\src\app\Properties\launchSettings.json...
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5000
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: D:\code\TodoService\src\app

To Reproduce

I have an F#9 project which uses the Web SDK. It doesn't use anything else really - standard nuget etc.

Further technical details

• Include the output of dotnet --info
• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version

.NET SDK:
 Version:           9.0.100
 Commit:            59db016f11
 Workload version:  9.0.100-manifests.c6f19616
 MSBuild version:   17.12.7+5b8665660

Running on Win 11.

[baronfel]

cc @tmat

[forki]

Noticed that as well and I assumed I messed up my setup

[isaacabraham]

This would be extremely undesirable to leave in for any period of time - F# web applications rely heavily on this (the entire SAFE Stack is built upon a watch mode for both client and server for example).

[forki]

Also dotnet aspire is broken for F# users.
/cc @davidfowl

[baronfel]

Using the Roslyn MSBuild APIs directly likely won't work for any project type that is non-Roslyn. I'd expect watch for esproj for example to be broken similarly based on this stack.

[forki]

@vzarytovskii

[vzarytovskii]

@vzarytovskii

Nothing actionable for us here, unfortunately, Roslyn/watch team needs to fix it.

[vzarytovskii]

Also, I assume --no-hot-reload still works? It should be automatically enabled for F# projects

[tmat]

Yes, a workaround is to pass --no-hot-reload.

[forki]

How is that a workaround?

[tmat]

@forki Passing --no-hot-reload makes dotnet-watch work as expected until a fix is available.

[vzarytovskii]

How is that a workaround?

Proper hot reload was never supported in F#. @tmat do you know what changed to default behaviour, so it's failing in F# now, did no-hot-reload become default in net8, and now is off-by-default in net9?

[forki]

Well it worked in fable. But yes for dotnet watch run, changes in fsproj or fs files did a recompile and restart. Which would be OK if this works properly again

[isaacabraham]

How is that a workaround?

Proper hot reload was never supported in F#. @tmat do you know what changed to default behaviour, so it's failing in F# now, did no-hot-reload become default in net8, and now is off-by-default in net9?

Even if that were the case, it should only be on-by-default for C# projects - the rest of everything will break, and there's no need to enforce an extra cmdline parameter on everyone because of this.

[tmat]

@isaacabraham What about F# projects that have C# dependencies and vice versa? Would you expect Hot Reload work for C# code and trigger rebuild for changes in F# code?

[JordanMarr]

For anyone else affected by this issue, my workaround is to add the following file:

global.json

{
  "sdk": {
    "version": "8.0.100",
    "rollForward": "latestMinor",
    "allowPrerelease": false
  }
}