MSBuild: How to get custom generated files after regular build process to be treated as content build output - msbuild

Question

I'm using MSBuild SDK style projects with VS 2019. I'm trying to run a custom file generation tool which depends on the output of the build of the current project. The files should be treated as if it was regular content for which CopyToOutputDirectory is set. In dependent projects I expect the files to be part of the output directory as well. The solution I now have works, but not from clean builds, which is obviously not acceptable.
I currently have this in the project file:
<Target Name="Generation" AfterTargets="AfterBuild">
<Exec Command="GeneratedFiles" />
<ItemGroup>
<Content Include="$(TargetDir)\GeneratedFiles.*.xml">
<TargetPath>GeneratedFiles\%(Filename)%(Extension)</TargetPath>
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>
</ItemGroup>
</Target>
This works, but only for non-clean builds.

Answer 1

The reason the Generation target doesn't work is because the logic that performs the copies, based on the presence and value of the CopyToOutputDirectory metadata, runs as part of the 'CoreBuild'. The Generation target has AfterTargets="AfterBuild". It runs after the build (and the copying) has already been completed.
The Generation target doesn't work for the first build and it doesn't work for subsequent incremental builds either.
The question description says that the files are copied for an incremental build. While it may be true that the files are copied, they can't be getting copied because of the Generation target. Without seeing the complete project file, I assume there is another place in the project where the CopyToOutputDirectory metadata is being set for the files.
To have the Generation target run after compilation and before files are copied, the following can be used:
<Target Name="Generation" AfterTargets="AfterCompile" BeforeTargets="GetCopyFilesToOutputDirectoryItems">
The GetCopyFilesToOutputDirectoryItems is publicly documented and it runs before another publicly documented target named CopyFilesToOutputDirectory. The CopyFilesToOutputDirectory is defined with a DependsOnTargets attribute which runs a set of targets that perform the actually copying. This means that the file copies are completed before the CopyFilesToOutputDirectory target is completed.
To ensure the correct order, BeforeTargets="GetCopyFilesToOutputDirectoryItems" is used and not BeforeTargets="CopyFilesToOutputDirectory".
If the scenario were different and <Exec Command="GeneratedFiles" /> didn't depend on the compilation step, i.e. the GeneratedFiles command didn't use the assembly being created by the project, then the Generation target could occur even earlier, e.g.
<Target Name="Generation" BeforeTargets="BeforeBuild">
Update - Execute a Target if ProjectReference has a specific Project
This update is a response to discussion in the comments.
Let's say we have a FooApp project and a BarLib project and FooApp depends on BarLib and needs to copy arbitrary files from the BarLib project directory.
FooApp has a ProjectReference to BarLib, e.g.
<ItemGroup>
<ProjectReference Include="..\BarLib\BarLib.csproj" />
</ItemGroup>
ProjectReference is an ItemGroup and we can check if it includes BarLib.
<Target Name="CopyFilesFromBar" BeforeTargets="BeforeBuild">
<PropertyGroup>
<!-- The name of the project to look for -->
<BarProjectName>BarLib</BarProjectName>
<!-- Use batching to check the ItemGroup -->
<!-- Save the project's directory because we will need it later -->
<BarProjectDirectory Condition="'%(ProjectReference.Filename)' == '$(BarProjectName)'">%(ProjectReference.Directory)</BarProjectDirectory>
<!-- Set up a boolean that indicates if the project was found or not -->
<HasProjectRefToBar>false</HasProjectRefToBar>
<HasProjectRefToBar Condition="$(FooProjectDirectory) != ''">true</HasProjectRefToBar>
</PropertyGroup>
<!-- Copy if the project was found in ProjectReference -->
<Copy SourceFiles="$(TargetDir)$(BarProjectDirectory)\bin\GeneratedFiles\*.*" DestinationFolder="$(OutputPath)GeneratedFiles" Condition="$(HasProjectRefToBar)" />
</Target>
This target could be defined once in a Directory.Build.targets file and shared across a solution.
If the generated files (in BarLib in the example scenario) don't change based on Configuration and Platform, consider using an output path location that doesn't change as in the example - 'bin\GeneratedFiles'. This makes it much easier for consuming projects. Otherwise keep all the projects in sync with regards to using the same Configuration and Platform values and the same $(OutputPath).