Get the names of directories under a specified directory in MSBuild

build.targets

<PropertyGroup>
  <ParentDir>$(MSBuildProjectDirectory)\some\folder</ParentDir><!-- This folder has subfolders folders a, b, c-->
</PropertyGroup>

<Target>
  <ItemGroup>
    <SubfolderFullPaths Include="$([System.IO.Directory]::GetDirectories('$(ParentDir)'))" />
    <SubfolderNames Include="@(SubfolderFullPaths->'$([System.IO.Path]::GetFileName('%(SubfolderFullPaths.Identity)'))')" />
  </ItemGroup>
  
  <!-- SubfolderNames now has 3 items, with their Identity metadata "a", "b", "c" -->
  
  <!-- If you want to use the well-known item metadata to define further metadata,
       it seems you have to us the above as an intermediate step: -->
  <SubfolderNames2 Include="$(SubfolderNames)" >
    <!-- Identity refers to that of the Included items, so can't do this in SubfolderNames directly -->
    <OutputDir>%(Identity)</OutputDir>
  </SubfolderNames2>
</Target>