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homebrews cellar version of mono breaks omnisharp

20 Dec 2019

Note, August 2026: this post is from December 2019. The screenshots were originally hotlinked from Twitter’s image CDN. They are now served from this site so the post does not break if those links stop working. The text and the images are otherwise unchanged.

So today, I raised a GitHub issue because after I had opened the result of dotnet new mvc in VSCode the problems window would have approximately 120 issues and the code editor window would be full of red squiggles.

VSCode showing HomeController.cs covered in red squiggles, with the problems panel reporting 119 errors such as "The type or namespace name 'System' could not be found"

I was running the very latest version of dotnetcore

Terminal output from dotnet --info showing .NET Core SDK 3.1.100 on Mac OS X 10.14

And the very latest version of VSCode

The Visual Studio Code about dialog showing version 1.41.0

I had not changed anything in the .csproj file. It was fresh from running dotnet new mvc from the terminal.

MvcApp.csproj open in VSCode, unchanged and targeting netcoreapp3.1, while the problems panel still reports 120 errors

So, I raised an issue over on GitHub.

The GitHub issue raised against the omnisharp-vscode repository

Big thanks to the rapid response and answer from @filipw, who discovered that it was the homebrew cellar version of mono that was the issue and that intstalling the stable version of mono was the fix.

Terminal output from mono --version showing the Mono JIT compiler at 6.4.0.198

The same project open in VSCode after installing the stable Mono release, with the problems panel down to two minor style hints

Success 🎉

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