Dotnet

Local Development Workflow

For local development, it's recommended to use nix-shell to create a dotnet environment:

# shell.nix
with import <nixpkgs> {};

mkShell {
  name = "dotnet-env";
  packages = [
    dotnet-sdk_3
  ];
}

Using many sdks in a workflow

It's very likely that more than one sdk will be needed on a given project. Dotnet provides several different frameworks (E.g dotnetcore, aspnetcore, etc.) as well as many versions for a given framework. Normally, dotnet is able to fetch a framework and install it relative to the executable. However, this would mean writing to the nix store in nixpkgs, which is read-only. To support the many-sdk use case, one can compose an environment using dotnetCorePackages.combinePackages:

with import <nixpkgs> {};

mkShell {
  name = "dotnet-env";
  packages = [
    (with dotnetCorePackages; combinePackages [
      sdk_3_1
      sdk_5_0
    ])
  ];
}

This will produce a dotnet installation that has the dotnet 3.1, 3.0, and 2.1 sdk. The first sdk listed will have it's cli utility present in the resulting environment. Example info output:

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   3.1.101
 Commit:    b377529961

...

.NET Core SDKs installed:
  2.1.803 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/sdk]
  3.0.102 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/sdk]
  3.1.101 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.15 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.15 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.2 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.1 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.15 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.2 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.1 [/nix/store/iiv98i2jdi226dgh4jzkkj2ww7f8jgpd-dotnet-core-combined/shared/Microsoft.NETCore.App]

dotnet-sdk vs dotnetCorePackages.sdk

The dotnetCorePackages.sdk_X_Y is preferred over the old dotnet-sdk as both major and minor version are very important for a dotnet environment. If a given minor version isn't present (or was changed), then this will likely break your ability to build a project.

dotnetCorePackages.sdk vs dotnetCorePackages.runtime vs dotnetCorePackages.aspnetcore

The dotnetCorePackages.sdk contains both a runtime and the full sdk of a given version. The runtime and aspnetcore packages are meant to serve as minimal runtimes to deploy alongside already built applications.

Packaging a Dotnet Application

To package Dotnet applications, you can use buildDotnetModule. This has similar arguments to stdenv.mkDerivation, with the following additions:

projectFile is used for specifying the dotnet project file, relative to the source root. These usually have .sln or .csproj file extensions. This can be a list of multiple projects as well. Most of the time dotnet can figure this location out by itself, so this should only be set if necessary.
nugetDeps takes either a path to a deps.nix file, or a derivation. The deps.nix file can be generated using the script attached to passthru.fetch-deps. This file can also be generated manually using nuget-to-nix tool, which is available in nixpkgs. If the argument is a derivation, it will be used directly and assume it has the same output as mkNugetDeps.
packNupkg is used to pack project as a nupkg, and installs it to $out/share. If set to true, the derivation can be used as a dependency for another dotnet project by adding it to projectReferences.
projectReferences can be used to resolve ProjectReference project items. Referenced projects can be packed with buildDotnetModule by setting the packNupkg = true attribute and passing a list of derivations to projectReferences. Since we are sharing referenced projects as NuGets they must be added to csproj/fsproj files as PackageReference as well. For example, your project has a local dependency:

    <ProjectReference Include="../foo/bar.fsproj" />

To enable discovery through projectReferences you would need to add:

    <ProjectReference Include="../foo/bar.fsproj" />
    <PackageReference Include="bar" Version="*" Condition=" '$(ContinuousIntegrationBuild)'=='true' "/>

executables is used to specify which executables get wrapped to $out/bin, relative to $out/lib/$pname. If this is unset, all executables generated will get installed. If you do not want to install any, set this to []. This gets done in the preFixup phase.
runtimeDeps is used to wrap libraries into LD_LIBRARY_PATH. This is how dotnet usually handles runtime dependencies.
buildType is used to change the type of build. Possible values are Release, Debug, etc. By default, this is set to Release.
selfContainedBuild allows to enable the self-contained build flag. By default, it is set to false and generated applications have a dependency on the selected dotnet runtime. If enabled, the dotnet runtime is bundled into the executable and the built app has no dependency on Dotnet.
dotnet-sdk is useful in cases where you need to change what dotnet SDK is being used.
dotnet-runtime is useful in cases where you need to change what dotnet runtime is being used. This can be either a regular dotnet runtime, or an aspnetcore.
dotnet-test-sdk is useful in cases where unit tests expect a different dotnet SDK. By default, this is set to the dotnet-sdk attribute.
testProjectFile is useful in cases where the regular project file does not contain the unit tests. It gets restored and build, but not installed. You may need to regenerate your nuget lockfile after setting this.
disabledTests is used to disable running specific unit tests. This gets passed as: dotnet test --filter "FullyQualifiedName!={}", to ensure compatibility with all unit test frameworks.
dotnetRestoreFlags can be used to pass flags to dotnet restore.
dotnetBuildFlags can be used to pass flags to dotnet build.
dotnetTestFlags can be used to pass flags to dotnet test. Used only if doCheck is set to true.
dotnetInstallFlags can be used to pass flags to dotnet install.
dotnetPackFlags can be used to pass flags to dotnet pack. Used only if packNupkg is set to true.
dotnetFlags can be used to pass flags to all of the above phases.

When packaging a new application, you need to fetch its dependencies. You can run nix-build -A package.fetch-deps to generate a script that will build a lockfile for you. After running the script you should have the location of the generated lockfile printed to the console, which can be copied to a stable directory. Then set nugetDeps = ./deps.nix and you're ready to build the derivation.

Here is an example default.nix, using some of the previously discussed arguments:

{ lib, buildDotnetModule, dotnetCorePackages, ffmpeg }:

let
  referencedProject = import ../../bar { ... };
in buildDotnetModule rec {
  pname = "someDotnetApplication";
  version = "0.1";

  src = ./.;

  projectFile = "src/project.sln";
  nugetDeps = ./deps.nix; # File generated with `nix-build -A package.passthru.fetch-deps`.

  projectReferences = [ referencedProject ]; # `referencedProject` must contain `nupkg` in the folder structure.

  dotnet-sdk = dotnetCorePackages.sdk_3_1;
  dotnet-runtime = dotnetCorePackages.net_5_0;

  executables = [ "foo" ]; # This wraps "$out/lib/$pname/foo" to `$out/bin/foo`.
  executables = []; # Don't install any executables.

  packNupkg = true; # This packs the project as "foo-0.1.nupkg" at `$out/share`.

  runtimeDeps = [ ffmpeg ]; # This will wrap ffmpeg's library path into `LD_LIBRARY_PATH`.
}

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