SDK-Style project and project.assets.json

Last week I encountered an issue with MSBuild while trying to run it from command line.
The issue did not appear when using VisualStudio right click + build but only appeared when using msbuild.exe CLI directly with a clean project.

Assets file 'C:\[...]\obj\project.assets.json' not found. Run a NuGet package restore to generate this file.

When I first saw the error, few questions came to my mind which I will share today in 3 points:

1. Overview of project.assets.json
2. Slim SDK-Style project
3. Mixing SDK-Style project and old projects

Special shoutout to @enricosada who provided me with all the answers regarding the SDK-Style project.

1. Overview of project.assets.json

project.assets.json lists all the dependencies of the project. It is created in the /obj folder when using dotnet restore or dotnet build as it implicitly calls restore before build, or msbuid.exe /t:restore with msbuild CLI.

To simulate dotnet build (restore + build) for .NET Framework project with msbuild, we can do msbuild /t:restore;build which use the target parameter /t.

Building requires the project.assets.json file, which lists the dependencies of your application. The file is created when dotnet restore is executed. Without the assets file in place, the tooling cannot resolve reference assemblies, which results in errors. With .NET Core 1.x SDK, you needed to explicitly run the dotnet restore before running dotnet build. Starting with .NET Core 2.0 SDK, dotnet restore runs implicitly when you run dotnet build. If you want to disable implicit restore when running the build command, you can pass the --no-restore option.

Source:
https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-build?tabs=netcore2x

2. Slim SDK-Style project

In the past, all dependencies were listed directly in the old project file:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>453c2b22-326a-4d20-a87a-e43285785f0f</ProjectGuid>
    <OutputType>Library</OutputType>
    <RootNamespace>Library1</RootNamespace>
    <AssemblyName>Library1</AssemblyName>
    <UseStandardResourceNames>true</UseStandardResourceNames>
    <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
    <TargetFSharpCoreVersion>4.4.3.0</TargetFSharpCoreVersion>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
    <Name>Library1</Name>
  </PropertyGroup>
  <PropertyGroup>
    <MinimumVisualStudioVersion Condition="'$(MinimumVisualStudioVersion)' == ''">11</MinimumVisualStudioVersion>
  </PropertyGroup>
  
  ... lots of other configs ...

  <ItemGroup>
    <Reference Include="mscorlib" />
    <Reference Include="FSharp.Core">
      <Name>FSharp.Core</Name>
      <AssemblyName>FSharp.Core.dll</AssemblyName>
      <HintPath>$(MSBuildProgramFiles32)\Reference Assemblies\Microsoft\FSharp\.NETFramework\v4.0\$(TargetFSharpCoreVersion)\FSharp.Core.dll</HintPath>
    </Reference>
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="System.Numerics" />
    <Reference Include="System.ValueTuple">
      <HintPath>..\packages\System.ValueTuple.4.4.0\lib\net461\System.ValueTuple.dll</HintPath>
      <Private>True</Private>
    </Reference>
  </ItemGroup>
</Project>

But since MSBuild 15.0, a new kind of project called SDK-Style project is available. It is a slimmer version of the project file and works directly with dotnet CLI.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net472</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Library.fs" />
  </ItemGroup>

</Project>

This has the big advantage of having the project file now being developer friendly. In the past it would be filled with XML tags added by the VS tooling which would prevent developers from changing the project file for fear of breaking the project.
With the SDK-Style or slim project, dependencies are specified in project.assets.json automatically generated during restore in /obj, it is now transparent to developers and is now a build step rather than a development step.

{
  "version": 3,
  "targets": {
    ".NETFramework,Version=v4.7.2": {...}
  },
  "libraries": {
    "FSharp.Core/4.3.4": {
      "sha512": "...",
      "type": "package",
      "path": "fsharp.core/4.3.4",
      "files": [...]
    },
    "System.ValueTuple/4.5.0": {
      "sha512": "...",
      "type": "package",
      "path": "system.valuetuple/4.5.0",
      "files": [...]
    }
  },
  "projectFileDependencyGroups": {
    ".NETFramework,Version=v4.7.2": [
      "FSharp.Core >= 4.3.4",
      "System.ValueTuple >= 4.5.0"
    ]
  },
  "packageFolders": {...},
  "project": {
    "version": "1.0.0",
    "restore": {...},
    "frameworks": {
      "net472": {
        "dependencies": {
          "FSharp.Core": {
            "target": "Package",
            "version": "[4.3.4, )"
          }
        }
      }
    }
  }
}

We can see here all the dependencies, project references, nuget packages, target frameworks, etc… All the information which were previously in the project file are now generated at build time. Once generated, it is then updated dynamically when removing/adding reference in Visual Studio without the need to build.

There are many advantages for using the SDK-Style projects and those are the ones that affects my daily work:

1. Usage of dotnet CLI
2. Slimmer project file more understandable
3. Allow direct editing of the file from VS without the need to unload

Note:
It isn’t mandatory to be creating a .NET Core application to use the SDK-Style project file. The project file is linked to the version of MSBuild and whether it supports the compilation of the project. Therefore only MSBuild 15.0 will be able to understand it and comes installed with VS2017.
In fact when creating an ASP.NET Core application project on .NET Framework using the VS template, it will create a SDK-Style project targeting .NET Framework.

3. Mixing SDK-Style project and old projects

It is not always possible to use the dotnet CLI with a SDK-Style project. If the project has dependencies on other projects that aren’t SDK-Style projects, the only way to build will still remain using msbuild.exe /t:restore;build as the dependent libraries can’t be compiled with dotnet CLI.

Conclusion

Today we saw what project.assets.json file used for and how we can fix the Assets file 'C:\[...]\obj\project.assets.json' not found. Run a NuGet package restore to generate this file. issue. We then saw what were the differences between a project prior MSBuild 15.0 and a SDK-Style project and what are the advantages of SDK-Style projects. Hope you liked this post, see you next time!