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. 😅

Yeah, you can't do it with ProjectReference. You need to use PackageReference. There are basically two ways to do what you're trying to do, and you're sort of in-between them:

1. one nix derivation with an assortment of projects linked together with ProjectReference, one deps.json

2. one nix derivation per nuget package with packNupkg = true, using PackageReference