Nuget restore issues

This is a repro for an issue in solutions that contain a mix of packages.config (old) and PackageReference (new) based projects when using the 16.8 tooling (msbuild, visual studio)

Scenario

Dependency chain::

TestProject (netcoreapp3.1) 
╚ MvcWebApplication (net461, packages.config) 
  ╚ DependencyA (netstandard2) 
    ╚ TransientProjectB (netstandard2)

The TestProject use classes from TransientProjectB in a test.

• ❗️ Fails: building the master branch using msbuild.exe
• ✅ Works: builds the master branch using the IDE, VS2019 16.8.X.
• ✅ Works: modify the TestProject adding a direct reference to TransientProjectB, then build using msbuild.exe. As in #1

Microsoft recommendations:

• It has earlier been recommended from MS to run nuget.exe restore for the legacy projects (packages.config). Source.

• ℹ️ Update: MS recommends to use msbuild -restore /p:RestorePackagesConfig=true for 16.8 + 5.8 to avoid version conflicts between nuget.exe and msbuild versions. Source.

• It's recommended to match nuget.exe version with the msbuild.exe/ Visual Studio / dotnet.exe:

It is advised that your toolchain includes VS 16.8+, MSBuild 16.8+ and .NET 5.0+ when using NuGet 5.8 or later. If you restored using an earlier version of NuGet than 5.8 or 16.8, then please delete your obj/ folders and restore & compile with the tools listed above or higher.

Sources: [1], [2]

Known bugs:

.NET 5 SDK, v.5.0.100

• "This will fix issues where CI builds fail because they download an older version of NuGet.exe and use this for restore." dotnet/sdk#14517 (❗️ merged, but not released)

Errors

'project.assets.json' doesn't have a target for {TFM}'

If mismatching nuget.exe version with msbuild version (i.e using nuget 5.7 with 16.8, instead of nuget 5.8):

C:\Program%20Files\dotnet\sdk\5.0.100\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets(241,5): error NETSDK1005: Assets file 'D:\a\mixed-repro\mixed-repro\TransientProjectB\obj\project.assets.json' doesn't have a target for 'netstandard2.0'. Ensure that restore has run and that you have included 'netstandard2.0' in the TargetFrameworks for your project. [D:\a\mixed-repro\mixed-repro\TransientProjectB\TransientProjectB.csproj]

Solution: Downgrade to 16.7 tooling (uninstall .NET 5 SDK, and downgrade msbuild 16.8 to 16.7)

Unable to resolve transient references

Hypothesis: Resolving the transient dependency worked in 16.7, but no longer works in 16.8

D:\a\mixed-repro\mixed-repro\TestProject\UnitTest1.cs(3,7): error CS0246: The type or namespace name 'TransientProjectB' could not be found (are you missing a using directive or an assembly reference?) [D:\a\mixed-repro\mixed-repro\TestProject\TestProject.csproj]

Solution: Make the transient reference a direct reference in the test project.

Gotchas

Cake

Building via Cake and their boostrapper build.ps1 will use the installed nuget.exe version, if it already exists. If not, it will download and install the latest. You might end up using a nuget.exe version not compatible with the msbuild / sdk tooling you are using.