LSP diagnostics fail when you have a net9.0 target framework project and a net9.0-windows target framework project in a solution

[badly-drawn-wizards]

The LSP selects net9.0 as the overall framework when net9.0-windows will subsume it.
The LSP fails to pick up on referenced assemblies.

[razzmatazz]

Hey @badly-drawn-wizards! Could you provide a more detailed example (excerpts from .csproj's)–as I may probably fail to produce a corresponding test case correctly for this issue?

[themayarose]

I have this exact same problem, and I'm trying to generate a test case -- except I can't. The setup I'm using on my project is a root slnx, a WinUI app project on Root.App, a SignalR server project on Root.Server and a common shared library project on Root.Core. I'm trying to reproduce the bug creating a new, empty project with that same structure, but it works correctly. I don't know what exactly on my original project structure is triggering this bug, but I'll report it if I find out.

For reference, this only happens on csharp-ls 0.19 - not on 0.18.

Edit:

These are the errors that show up on LspLog:

[ERROR][2025-11-06 11:15:56] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: Initialization[0]\r\n      initializing, csharp-ls version 0.19.0.0\r\n"
[ERROR][2025-11-06 11:15:56] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: Initialization[0]\r\n      csharp-ls is released under MIT license and is not affiliated with Microsoft Corp.; see https://github.com/razzmatazz/csharp-language-server\r\n"
[ERROR][2025-11-06 11:15:56] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	'info: Initialization[0]\r\n      MSBuildLocator: will register ".NET Core SDK", Version=9.0.306 as default instance\r\n'
[ERROR][2025-11-06 11:15:58] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: serverEventLoop[0]\r\n      Will use these MSBuild props: map [(TargetFramework, net9.0)]\r\n"
[ERROR][2025-11-06 11:16:00] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: serverEventLoop[0]\r\n      msbuildWorkspace.Diagnostics: [Failure] Msbuild failed when processing the file 'C:\\Users\\maya\\Development\\[Project]\\[Project]\\[Project].csproj' with message: The Windows App SDK requires a specific target OS version. Please set the <TargetFramework> property to an appropriate value. For more information, visit https://aka.ms/winappsdk.TFM.\r\n"```

[themayarose]

I was finally able to generate a test case that reliably reproduces the bug.

I'm using nvim with coq_nvim to get completions. I use a custom function to get the root_dir for the solution in the vim.lsp.config call:

root_dir = vim.fs.root(
            0,
            function (name, path)
                return name:match('%.slnx?$') ~= nil
            end
),

Firstly, I generated a new project with the following commands (using WinUI templates from https://github.com/egvijayanand/winui-templates to make it easier to reproduce):

dotnet new install VijayAnand.WinUITemplates
dotnet new winui -n TestCase.App -o TestCase/TestCase.App -mvvm -nsf
dotnet new classlib -n TestCase.Core -o TestCase/TestCase.Core
cd TestCase
dotnet new sln -f slnx
dotnet sln add .\TestCase.App\TestCase.App.csproj
dotnet sln add .\TestCase.Core\TestCase.Core.csproj
dotnet reference add .\TestCase.Core\TestCase.Core.csproj --project .\TestCase.App\TestCase.App.csproj
dotnet add .\TestCase.Core\TestCase.Core.csproj package CommunityToolkit.Mvvm::8.*

What this does is generate a MVVM-using WinUI app, and a Core class library. I then add the MVVM reference to the Core library project (that is useful, for instance, to make app models inherently observable).

There is now one way to get csharp-ls 0.19 working and three ways to break everything:

root_dir match	Working directory for nvim	Result
'%.slnx?$'	TestCase/	Failure
'%.slnx?$'	TestCase.App/	Failure
'%.csproj$'	TestCase/	Failure
'%.csproj$'	TestCase.App/	Success

Then, when it breaks, LspLog shows this information:

[ERROR][2025-11-06 16:04:58] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: Initialization[0]\r\n      initializing, csharp-ls version 0.19.0.0\r\n"
[ERROR][2025-11-06 16:04:58] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: Initialization[0]\r\n      csharp-ls is released under MIT license and is not affiliated with Microsoft Corp.; see https://github.com/razzmatazz/csharp-language-server\r\n"
[ERROR][2025-11-06 16:04:58] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	'info: Initialization[0]\r\n      MSBuildLocator: will register ".NET Core SDK", Version=9.0.306 as default instance\r\n'
[ERROR][2025-11-06 16:04:58] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: serverEventLoop[0]\r\n      Will use these MSBuild props: map [(TargetFramework, net9.0)]\r\n"
[ERROR][2025-11-06 16:05:01] ...p/_transport.lua:36	"rpc"	"C:\\Users\\maya\\.dotnet\\tools\\csharp-ls.EXE"	"stderr"	"info: serverEventLoop[0]\r\n      msbuildWorkspace.Diagnostics: [Failure] Msbuild failed when processing the file 'C:\\Users\\maya\\Development\\TestCase\\TestCase.App\\TestCase.App.csproj' with message: The Windows App SDK requires a specific target OS version. Please set the <TargetFramework> property to an appropriate value. For more information, visit https://aka.ms/winappsdk.TFM.\r\n"

Then, if I downgrade csharp-ls with dotnet tool install --global csharp-ls --version 0.18.0 --allow-downgrade, all cases work correctly.

[themayarose]

Just to let you know, I have just tested it with the 0.20.0 version and it still picks up the wrong TFM...

Actually, scratch that, it's working now! 🎉

[razzmatazz]

Actually, scratch that, it's working now! 🎉

Awesome! I will be closing the issue then