csproj include (recursive) folders based on a list (batching) - msbuild

Question

I want to add some files recursively to a single project that are located in folders inside my solution root.
Right now, I found a working solution to add those files by specifying each folder manually:
<ItemGroup>
<Content Include="..\Folder1\**\*">
<Link>Folder1\%(RecursiveDir)\%(Filename)%(Extension)</Link>
</Content>
<Content Include="..\Folder2\**\*">
<Link>Folder2\%(RecursiveDir)\%(Filename)%(Extension)</Link>
</Content>
<Content Include="..\Folder3\**\*">
<Link>Folder3\%(RecursiveDir)\%(Filename)%(Extension)</Link>
</Content>
</ItemGroup>
As those can be a handfull folders and they can be different depending on my solution, I want to have an easy list to define the folder names.
Something like this:
<PropertyGroup>
<DefinedResourceFolders>Folder1;Folder2;Folder3</DefinedResourceFolders>
</PropertyGroup>
This would also allow me to input this as a property directly when calling msbuild to extend the files that are going to the build output, maybe.
I tried to look into the MSBuild Batching documentation and several other online sources, but I could not figure it out. Most batching examples I found were working with Targets, not Includes into the solution items.
Is this possible? How would I define the <ItemGroup> for the content then?
P.S.: I don't care about wildcard issues with .csproj files when new files are added, etc. This will either be a single "Resources" project only containing those displayed files, or I am using my external .props file that I am importing into each .csproj file anyway.

Answer 1

Suppose you want to include all files in folders root/Folder1, root/Folder2 and root/Folder3 recursively.
This is how the builds (both VS and CLI) do what you want. However, VS will not show the files as part of the project.
<Project Sdk="Microsoft.NET.Sdk" InitialTargets="__AddBatchedContent;$(InitialTargets)">
<!-- the usual properties -->
<!-- You could of course define the folder array directly in an Item,
but this is what you wanted :-) -->
<PropertyGroup>
<Lookups>Folder1;Folder2;Folder3</Lookups>
</PropertyGroup>
<ItemGroup>
<LookupDir Include="$(Lookups)" />
</ItemGroup>
<Target Name="__AddBatchedContent">
<ItemGroup>
<!-- save the original source folder of the globbed file name in custom
metadata "Folder" so we can use it later as its output base folder -->
<__BatchedFiles Include="..\%(LookupDir.Identity)\**\*" Folder="%(LookupDir.Identity)" />
<Content Include="#(__BatchedFiles)" Link="%(Folder)\%(RecursiveDir)\%(Filename)%(Extension)" CopyToOutputDirectory="Always" />
</ItemGroup>
</Target>
</Project>
Note that if you want to put this in a .targets file to use this technique in multiple projects, you should replace the ..\ with $(MSBuildThisFileDirectory)..\ so that the path is relative to the (known) location of the .targets file instead of the (unknown) location of the importing project.
Thanks to #Ilya Kozhevnikov for the basis of this answer.
You are not the only one who would like this to be simpler :-): https://github.com/dotnet/msbuild/issues/3274