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.
I was running the very latest version of dotnetcore
And the very latest version of VSCode
I had not changed anything in the .csproj file. It was fresh from running
dotnet new mvc from the terminal.
So, I raised an issue over on GitHub.
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.