Actually looks like dotnet 9+ no longer try to guess, and just give:

MSBUILD : error MSB1011: Specify which project or solution file to use because this folder contains more than one project or solution file.

dotnet 8 will prefer .sln, but not .slnf and doesn't understand .slnx

_dotnetIsSolution() {
  local -r projectFile="${1-}"
  [[ $projectFile == *.sln || \
       ! -f $projectFile && \
         -n $(find "${projectFile:-.}" -maxdepth 1 -name \*.sln -print -quit) ]]
}

This is all it does in the PR. It just wants to know if you're attempting to build a solution

let
  Application = import ../Application { pkgs = pkgs; };
  DataMatrixGenerator = import ../../utils/DataMatrixGenerator { pkgs = pkgs; };
  Infrastrcture = import ../Infrastructure { pkgs = pkgs; };
  WebUI = import ../WebUI { pkgs = pkgs; };
in
pkgs.buildDotnetModule {
  pname = "API.csproj";
  version = "0.1";

  src = ../..;

  projectFile = "src/API/API.csproj";

  buildInputs = \[
    Application
    DataMatrixGenerator
    Infrastrcture
    WebUI
  \];

  dotnetPackFlags = \[ "-p:RuntimeIdentifier=linux-x64" \];
  nugetDeps = ./deps.json;
  packNupkg = false;
}

let
  Application = import ../Application { pkgs = pkgs; };
  DataMatrixGenerator = import ../../utils/DataMatrixGenerator { pkgs = pkgs; };
  Infrastrcture = import ../Infrastructure { pkgs = pkgs; };
  WebUI = import ../WebUI { pkgs = pkgs; };
in
pkgs.buildDotnetModule {
  pname = "API.csproj";
  version = "0.1";

  src = ../..;

  projectFile = "src/API/API.csproj";

  buildInputs = [
    Application
    DataMatrixGenerator
    Infrastrcture
    WebUI
  ];

  dotnetPackFlags = [ "-p:RuntimeIdentifier=linux-x64" ];
  nugetDeps = ./deps.json;
  packNupkg = false;
}

So because my computer needs replaced: I had to do an unplanned power down, my WSL filesystem seems to have corrupted, and it looks like e2fsck basically destroyed the filesystem as WSL now refuses to boot it. 🙃

I may not be able to do the build test in the near future. 😅