Hi. This is probably because you have the C# Dev Kit installed and hence OmniSharp is not used here.

Step 1. Make sure that the C# Dev Kit extension is uninstalled
Step 2. Make sure that "Use Omnisharp" is ticked in the settings.

Microsoft basically killed OmniSharp (and dotnet-script language services) when they decided to write their own backend.
It did not land well in the OSS community. Most of the discussion can be found here
dotnet/vscode-csharp#5276

@seesharper Thanks for your response, I've gotten it to work.

I had already stumbled across that discussion while trying to find ways around Visual Studio to develop .NET applications.

As I understand it, Omnisharp is being phased out for a new mechanism called the LSP. It sounds like this new mechanism will not be open source, but will expose enough functionality to still be able to do the things we have been able to do so far in Omnisharp, and perhaps more. I wonder if my understanding is correct?

I guess people are ticked off because it won't be open source, but if it exposes enough functionality doesn't that make it good enough for creating a solid workflow outside of Visual Studio?

I have not fully understood this myself. Although Microsoft is developing their own closed source LSP-based host to provide language services, there is still people from Microsoft working with OmniSharp-Roslyn which we now need to opt in to use.
There is also an effort to bring LSP to Omnisharp through this project https://github.com/OmniSharp/csharp-language-server-protocol. This is also mostly driven by Joey Robichaud which is employed at Microsoft.
So to me it seems that maybe they are keeping Omnisharp alive just in case this new thing goes south. I don't know 😃

It's supposedly on their table, let's see if they can get the integration in. I was interested as well and found the issue on their open-source repo for that: dotnet/vscode-csharp#5722