/tmp 
❯ nix build p#dotnetCorePackages.sdk_9_0_1xx && ls -R ./result/share/dotnet/metadata/workloads/
 9.0.100

./result/share/dotnet/metadata/workloads/9.0.100:
 userlocal

/tmp 
❯ nix build p#dotnetCorePackages.sdk_9_0_1xx-bin && ls -R ./result/share/dotnet/metadata/workloads/
"./result/share/dotnet/metadata/workloads/": No such file or directory (os error 2)
[ble: exit 2]

dotnet-combined =
    (
      with pkgs.dotnetCorePackages;
      combinePackages [
        sdk_10_0-bin
        sdk_9_0-bin
        sdk_8_0-bin
      ]
    ).overrideAttrs
      (old: {
        src = old.src.overrideAttrs (old: {
          postBuild =
            (old.postBuild or "")
            # This is needed to install workload in $HOME
            + ''
              for i in $out/share/dotnet/sdk/*
              do
                i="$(basename $i)"
                i="''${i/-*}" # strip pre-release label
                i="''${i::-2}00" # strip patch

                mkdir -p "$out/share/dotnet/metadata/workloads/$i"
                touch "$out/share/dotnet/metadata/workloads/$i/userlocal"
              done
            '';
        });
      });

this works as a workaround for installing workloads in home

yeah, seems like only the source-built variants have that:

$ nix-locate userlocal               
dotnetCorePackages.dotnet_8.vmr.out                   0 r /nix/store/agr6lyqz3zp4a3llaa0sx1x1i624r55k-dotnet-vmr-8.0.14/dotnet-sdk-8.0.114-linux-x64/metadata/workloads/8.0.100/userlocal
dotnetCorePackages.dotnet_10.vmr.out                  0 r /nix/store/vfd744wlv3338igbai9q711p8647hn75-dotnet-vmr-10.0.0-preview.2/dotnet-sdk-10.0.100-preview.2.25164.1-linux-x64/metadata/workloads/10.0.100/userlocal
(dotnetCorePackages.dotnet_9.sdk.out)                 0 r /nix/store/d3cndkqm57ika3qb2abmvr5snr4fp6pc-dotnet-sdk-9.0.104/share/dotnet/metadata/workloads/9.0.100/userlocal
dotnetCorePackages.dotnet_9.vmr.out                   0 r /nix/store/b679fsfkc571n08cr95zsrh0xgzbn3q4-dotnet-vmr-9.0.3/dotnet-sdk-9.0.104-linux-x64/metadata/workloads/9.0.100/userlocal
(dotnet-sdk_10.out)                                   0 r /nix/store/k1qkscqxcvs801bqihl9rwjm19dpr2xs-dotnet-sdk-10.0.100-preview.2.25164.1/share/dotnet/metadata/workloads/10.0.100/userlocal
(dotnetCorePackages.dotnet_8.sdk.out)                 0 r /nix/store/1nqkg50ljc7rxvhg58f8iws7nqb5mwxa-dotnet-sdk-8.0.114/share/dotnet/metadata/workloads/8.0.100/userlocal

I've got dotnet up and running well with the suggested config for it, even after changing vscode its still not able to open a solution, gives me an error "command 'csdevkit.openSolution' not found". Have the following configuration for that:

systemPackages = with pkgs; [
  (vscode-with-extensions.override {
      vscodeExtensions = with vscode-extensions; [
        ms-dotnettools.csharp
        ms-dotnettools.vscode-dotnet-runtime
        ms-dotnettools.csdevkit
        ms-dotnettools.vscodeintellicode-csharp
      ] ++ vscode-utils.extensionsFromVscodeMarketplace [
        # To get sha256: nix-prefetch-url https://marketplace.visualstudio.com/_apis/public/gallery/publishers/[publisher]/vsextensions/[name]/[version]/vspackage
        {
            name = "vscode-entity-framework";
            publisher = "richardwillis";
            version = "0.0.20";
            sha256 = "0karaxnaalhr08n7dyc89wr5i3y9jxa5nfiyqcxdg4ws0p3zcsbk";
        }
        {
            name = "vscode-avalonia";
            publisher = "avaloniateam";
            version = "0.0.32";
            sha256 = "1vrsnq7v0p508c077g62yy2h9l8dqgad5929nnyqiys3bcx5ksnq";
        }
      ];
  })
];

Any idea what might I be missing here?

well, I think I've figured out why. On the docs for that extension, they have a section with a list of reasons why the command might not be found:

using a readonly OS. C# Dev Kit requires write access to its own extension folder and to the VS Code-provided folder for extensions to write arbitrary state within the operating system, so if you are using an OS that is completely readonly, C# Dev Kit will not work.

Which I think would mean it doesn't work on NixOS, right?

Would this be enough info? Neither C# or C# DevKit have any logs in the output window, only that one. This package is installed though

Extension Host:

2025-04-21 21:22:00.068 [error] Activating extension ms-dotnettools.csharp failed due to an error:
2025-04-21 21:22:00.069 [error] Error: Version 2.2.3 of the .NET Install Tool (ms-dotnettools.vscode-dotnet-runtime) was not found, C# will not activate.
	at /nix/store/igyqc0cisbwz6zj15fb6xl74ljilas2a-vscode-extension-ms-dotnettools-csharp-2.61.28/share/vscode/extensions/ms-dotnettools.csharp/dist/extension.js:2:1205715
	at Generator.next (<anonymous>)
	at s (/nix/store/igyqc0cisbwz6zj15fb6xl74ljilas2a-vscode-extension-ms-dotnettools-csharp-2.61.28/share/vscode/extensions/ms-dotnettools.csharp/dist/extension.js:2:1204371)
2025-04-21 21:22:00.073 [info] Eager extensions activated