C# Extension Crashes When Saving New File Created via ".NET New File" in VS Code

[florestankorp]

Type: Bug

Issue Description

Whenever I create a new file in Visual Studio Code using the ".NET New File" command and attempt to save it, the C# extension crashes. The output window shows repeated errors such as "Failed to get language for textDocument/diagnostic" and "Document pull failed for text document untitled:Untitled-1". This issue occurs consistently and prevents saving new files created through the ".NET New File" workflow.

Steps to Reproduce

1. Create a new file in VS Code using the ".NET New File" command
2. Try to save the file
3. Observe that C# crashes with the error above

Expected Behavior

The file should be saved without crashing the C# extension.

Actual Behavior

C# crashes with the error message above when saving a new file created via ".NET New File".

Extension version: 2.90.60
VS Code version: Code 1.104.2 (e3a5acfb517a443235981655413d566533107e92, 2025-09-24T11:21:37.073Z)
OS version: Darwin arm64 24.6.0
Modes:

Logs

C# log

2025-10-02 11:37:36.318 [info] Locating .NET runtime version 9.0.1
2025-10-02 11:37:36.377 [info] Dotnet path: /usr/local/share/dotnet/dotnet
2025-10-02 11:37:36.377 [info] Activating C# + C# Dev Kit...
2025-10-02 11:37:36.454 [info] [stdout] info: Program[0]
      Server started with process ID 3563

2025-10-02 11:37:36.708 [info] [stdout] {"pipeName":"/var/folders/lc/nf0p2stx0rld6r95vd22s28h0000gn/T/7403409d.sock"}

2025-10-02 11:37:36.708 [info] received named pipe information from server
2025-10-02 11:37:36.709 [info] client has connected to server
2025-10-02 11:37:36.762 [info] [Program] Language server initialized
2025-10-02 11:37:39.764 [info] [textDocument/diagnostic] [Microsoft.CodeAnalysis.LanguageServer.DotnetCliHelper] Using dotnet executable configured on the PATH
2025-10-02 11:37:39.769 [info] [textDocument/diagnostic] [Microsoft.CodeAnalysis.LanguageServer.FileBasedPrograms.FileBasedProgramsProjectSystem] Failed to obtain virtual project for '/Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Api/Contracts/Products/test.cs' using dotnet run-api. Falling back to directly creating the virtual project.
2025-10-02 11:37:39.788 [warning] [textDocument/diagnostic] [LanguageServerProjectLoader] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Api/Contracts/Products/test.csproj has unresolved dependencies
2025-10-02 11:37:39.793 [info] [textDocument/diagnostic] [LanguageServerProjectLoader] Successfully completed load of /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Api/Contracts/Products/test.cs
2025-10-02 11:37:40.134 [error] [workspace/_roslyn_restore] [Microsoft.CodeAnalysis.LanguageServer.Handler.RestoreHandler] Restore completed with errors.
2025-10-02 11:37:40.146 [info] [textDocument/diagnostic] [LanguageServerProjectLoader] Completed (re)load of all projects in 00:00:01.1999756
2025-10-02 11:37:41.971 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Infrastructure/WebShop.Infrastructure.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.978 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/tests/WebShop.Api.Tests/WebShop.Api.Tests.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.982 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/tests/WebShop.Integration.Tests/WebShop.Integration.Tests.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.982 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Api/WebShop.Api.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.984 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Domain/WebShop.Domain.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.984 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/src/WebShop.Application/WebShop.Application.csproj loaded by C# Dev Kit
2025-10-02 11:37:41.985 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project /Users/florestankorp/Documents/Coding/WEBDEV/dotnet/Webshop/tests/WebShop.Unit.Tests/WebShop.Unit.Tests.csproj loaded by C# Dev Kit
2025-10-02 11:38:50.885 [error] [textDocument/diagnostic] [LSP] Failed to get language for untitled:Untitled-1 with language 
2025-10-02 11:38:50.887 [error] [textDocument/diagnostic] [LSP] System.Exception: Failed to get language for textDocument/diagnostic
2025-10-02 11:38:50.891 [error] Request textDocument/diagnostic failed.
  Message: Failed to get language for textDocument/diagnostic
  Code: -32000 
[object Object]
2025-10-02 11:38:50.891 [error] Document pull failed for text document untitled:Untitled-1
  Message: Failed to get language for textDocument/diagnostic
  Code: -32000 
[object Object]

C# LSP Trace Logs log

Environment Information

VSCode version: 1.104.2
C# Extension: 2.90.60
Using OmniSharp: false

Dotnet Information

.NET SDK: Version: 9.0.301 Commit: a596cd22e2 Workload version: 9.0.300-manifests.9bcbba73 MSBuild version: 17.14.5+edd3bbf37

Runtime Environment:
OS Name: Mac OS X
OS Version: 15.6
OS Platform: Darwin
RID: osx-arm64
Base Path: /usr/local/share/dotnet/sdk/9.0.301/

.NET workloads installed:
There are no installed workloads to display.
Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.6
Architecture: arm64
Commit: 3875b54e7b

.NET SDKs installed:
9.0.301 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 9.0.6 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 9.0.6 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

Visual Studio Code Extensions

Extension	Author	Version	Folder Name
angular-console	nrwl	18.70.0	nrwl.angular-console-18.70.0
angular-essentials	johnpapa	18.0.0	johnpapa.angular-essentials-18.0.0
Angular2	johnpapa	18.0.2	johnpapa.angular2-18.0.2
astro-vscode	astro-build	2.15.4	astro-build.astro-vscode-2.15.4-darwin-arm64
auto-close-tag	formulahendry	0.5.15	formulahendry.auto-close-tag-0.5.15
auto-complete-tag	formulahendry	0.1.0	formulahendry.auto-complete-tag-0.1.0
code-spell-checker	streetsidesoftware	4.2.6	streetsidesoftware.code-spell-checker-4.2.6
code-spell-checker-dutch	streetsidesoftware	0.5.4	streetsidesoftware.code-spell-checker-dutch-0.5.4
copilot	GitHub	1.372.0	github.copilot-1.372.0
copilot-chat	GitHub	0.31.3	github.copilot-chat-0.31.3
cpptools	ms-vscode	1.27.7	ms-vscode.cpptools-1.27.7-darwin-arm64
csdevkit	ms-dotnettools	1.50.51	ms-dotnettools.csdevkit-1.50.51-darwin-arm64
csharp	ms-dotnettools	2.90.60	ms-dotnettools.csharp-2.90.60-darwin-arm64
data-workspace-vscode	ms-mssql	0.6.3	ms-mssql.data-workspace-vscode-0.6.3
docker	docker	0.17.0	docker.docker-0.17.0-darwin-arm64
dupchecker	jianbingfang	0.1.7	jianbingfang.dupchecker-0.1.7
EditorConfig	EditorConfig	0.17.4	editorconfig.editorconfig-0.17.4
errorlens	usernamehw	3.26.0	usernamehw.errorlens-3.26.0
explorer	vitest	1.30.0	vitest.explorer-1.30.0
gc-excelviewer	GrapeCity	4.2.64	grapecity.gc-excelviewer-4.2.64
gitlens	eamodio	17.5.1	eamodio.gitlens-17.5.1
go	golang	0.50.0	golang.go-0.50.0
grafana-alloy	Grafana	0.2.0	grafana.grafana-alloy-0.2.0
hexeditor	ms-vscode	1.11.1	ms-vscode.hexeditor-1.11.1
html-css-class-completion	Zignd	1.20.0	zignd.html-css-class-completion-1.20.0
LiveServer	ritwickdey	5.7.9	ritwickdey.liveserver-5.7.9
material-icon-theme	PKief	5.27.0	pkief.material-icon-theme-5.27.0
ng-template	Angular	20.2.2	angular.ng-template-20.2.2
output-colorizer	IBM	0.1.2	ibm.output-colorizer-0.1.2
playwright	ms-playwright	1.1.15	ms-playwright.playwright-1.1.15
prettier-vscode	esbenp	11.0.0	esbenp.prettier-vscode-11.0.0
pretty-ts-errors	YoavBls	0.6.1	yoavbls.pretty-ts-errors-0.6.1
properties-validator	vivek-kasture	0.0.6	vivek-kasture.properties-validator-0.0.6
rainbow-csv	mechatroner	3.22.0	mechatroner.rainbow-csv-3.22.0
remote-containers	ms-vscode-remote	0.429.0	ms-vscode-remote.remote-containers-0.429.0
remote-explorer	ms-vscode	0.5.0	ms-vscode.remote-explorer-0.5.0
remote-server	ms-vscode	1.5.3	ms-vscode.remote-server-1.5.3
remote-ssh	ms-vscode-remote	0.120.0	ms-vscode-remote.remote-ssh-0.120.0
remote-ssh-edit	ms-vscode-remote	0.87.0	ms-vscode-remote.remote-ssh-edit-0.87.0
remote-wsl	ms-vscode-remote	0.104.3	ms-vscode-remote.remote-wsl-0.104.3
sonarlint-vscode	SonarSource	4.31.0	sonarsource.sonarlint-vscode-4.31.0-darwin-arm64
svelte-vscode	svelte	109.11.1	svelte.svelte-vscode-109.11.1
terraform	hashicorp	2.37.2	hashicorp.terraform-2.37.2-darwin-arm64
vscode-azureappservice	ms-azuretools	0.26.4	ms-azuretools.vscode-azureappservice-0.26.4
vscode-azureresourcegroups	ms-azuretools	0.11.4	ms-azuretools.vscode-azureresourcegroups-0.11.4
vscode-containers	ms-azuretools	2.2.0	ms-azuretools.vscode-containers-2.2.0
vscode-counter	uctakeoff	3.7.2	uctakeoff.vscode-counter-3.7.2
vscode-css-formatter	aeschli	1.0.2	aeschli.vscode-css-formatter-1.0.2
vscode-diff	fabiospampinato	2.1.2	fabiospampinato.vscode-diff-2.1.2
vscode-docker	ms-azuretools	2.0.0	ms-azuretools.vscode-docker-2.0.0
vscode-dotnet-runtime	ms-dotnettools	2.3.7	ms-dotnettools.vscode-dotnet-runtime-2.3.7
vscode-eslint	dbaeumer	3.0.16	dbaeumer.vscode-eslint-3.0.16
vscode-github-actions	github	0.27.2	github.vscode-github-actions-0.27.2
vscode-icons	vscode-icons-team	12.14.0	vscode-icons-team.vscode-icons-12.14.0
vscode-jest	Orta	6.4.4	orta.vscode-jest-6.4.4
vscode-jest-runner	firsttris	0.4.84	firsttris.vscode-jest-runner-0.4.84
vscode-pylance	ms-python	2025.8.3	ms-python.vscode-pylance-2025.8.3
vscode-remote-extensionpack	ms-vscode-remote	0.26.0	ms-vscode-remote.vscode-remote-extensionpack-0.26.0
vscode-speech	ms-vscode	0.16.0	ms-vscode.vscode-speech-0.16.0-darwin-arm64
vscode-sqlite	alexcvzz	0.14.1	alexcvzz.vscode-sqlite-0.14.1
vscode-stylelint	stylelint	1.5.3	stylelint.vscode-stylelint-1.5.3
vscode-svgviewer	cssho	2.0.0	cssho.vscode-svgviewer-2.0.0
vscode-tailwindcss	bradlc	0.14.26	bradlc.vscode-tailwindcss-0.14.26
vscode-xml	redhat	0.29.0	redhat.vscode-xml-0.29.0-darwin-arm64
vscode-yaml	redhat	1.19.0	redhat.vscode-yaml-1.19.0

C# Settings

Setting	Value
preferCSharpExtension	false
compilerDiagnosticScope	openFiles
analyzerDiagnosticScope	openFiles
enableXamlTools	true
useServerGC	true

System Info

Item	Value
CPUs	Apple M2 Pro (10 x 2400)
GPU Status	2d_canvas: enabled direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: enabled_on trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	3, 6, 6
Memory (System)	16.00GB (0.31GB free)
Process Argv
Screen Reader	no
VM	0%

[JoeRobich]

From the C# log: 2025-10-02 11:38:50.885 [error] [textDocument/diagnostic] [LSP] Failed to get language for untitled:Untitled-1 with language

Since the new file is unsaved and has no extension, the CDK tells VS Code in advance that the file should be treated as a C# file. However, this could potentially be overridden by Auto-Detect. I was able to reproduce this issue by manually switching from C# to Plain Text.

@florestankorp What language is shown in the status bar for the new file?

[florestankorp]

@JoeRobich Once I open the output and the IDE "settles down", I get C#: