NET 9-windows project can't project-reference a net8-windows project

[dotMorten]

Describe the bug

If I create a .NET 8 WinUI Class library, and then reference that in a .NET 9 MAUI project, I get the following build error:

C:\Program Files\dotnet\sdk\9.0.101\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets(266,5): error NETSDK1005: Assets file 'E:\source\WinUIClassLibraryNet8\WinUIClassLibraryNet8\obj\project.assets.json' doesn't have a target for 'net9.0-windows10.0.19041.0'. Ensure that restore has run and that you have included 'net9.0-windows10.0.19041.0' in the TargetFrameworks for your project.

To Reproduce

• Create a WinUI .NET 8 Class Library
• Create a .NET MAUI .NET 9 Class Library
• Add a project reference to the NET8 WinUI project from the NET9 MAUI project
  <ProjectReference Include="..\WinUIClassLibraryNet8\WinUIClassLibraryNet8.csproj" Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">
• Build the project and observe the build error mentioned above.

Repro project: WinUIClassLibraryNet8.zip

Exceptions (if any)

project.assets.json' doesn't have a target for 'net9.0-windows10.0.19041.0'. Ensure that restore has run and that you have included 'net9.0-windows10.0.19041.0' in the TargetFrameworks for your project.

Further technical details

dotnet --info

.NET SDK:
Version: 9.0.101
Commit: eedb237
Workload version: 9.0.100-manifests.4a280210
MSBuild version: 17.12.12+1cce77968

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.101\

.NET workloads installed:
[ios]
Installation Source: VS 17.12.35527.113
Manifest Version: 18.1.9163/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.ios\18.1.9163\WorkloadManifest.json
Install Type: Msi

[maui-windows]
Installation Source: VS 17.12.35527.113
Manifest Version: 9.0.0/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maui\9.0.0\WorkloadManifest.json
Install Type: Msi

[android]
Installation Source: VS 17.12.35527.113
Manifest Version: 35.0.7/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.android\35.0.7\WorkloadManifest.json
Install Type: Msi

[maccatalyst]
Installation Source: VS 17.12.35527.113
Manifest Version: 18.1.9163/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maccatalyst\18.1.9163\WorkloadManifest.json
Install Type: Msi

Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.0
Architecture: x64
Commit: 9d5a6a9aa4

.NET SDKs installed:
8.0.404 [C:\Program Files\dotnet\sdk]
9.0.101 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: VS2022 17.12.3

Workarounds:

Add the following to the project reference:
<AdditionalProperties>TargetFramework=net8.0-windows10.0.19041.0</AdditionalProperties>
or change the WinUI class library to use TargetFrameworks instead of TargetFramework which seem silly when there's just one TFM.

[baronfel]

The proximate cause is that the GetMrtPackagingOutputs Target from the Microsoft.WindowsAppSDK package ends up calling the ResolvePackageAssets Target on the .NET 8-targeted project, but using the .NET 9 TFM from the MAUI app. It feels like there should be a negotiation step that should happen somewhere. Here's the binlog showing this. @dsplaisted / @rainersigwald do we have any guidance that we could give to the WindowsAppSDK team to help negotiate better TFMs?

msbuild.binlog.zip

[dsplaisted]

The zip with the binlog seem like it's corrupted, so I couldn't look at it.

Here's the documentation for how we handle TargetFramework negotiation for project references: https://github.com/dotnet/msbuild/blob/main/documentation/ProjectReference-Protocol.md

I'm not sure how GetMrtPackagingOutputs should work, but probably it should hook into the existing ProjectReference handling or follow a similar pattern.

[baronfel]

Oh the zip isn't really a zip - it's just a renamed binlog.