Add Custom Metadata to Already-defined ItemGroup from Another ItemGroup - msbuild

Question

I have the following:
<ItemGroup>
<Files Include="C:\Versioning\**\file.version" />
<ItemGroup>
<ReadLinesFromFile File="%(Files.Identity)">
<Output TaskParameter="Lines" ItemName="_Version"/>
</ReadLinesFromFile>
where each file.version file contains simply one line which is - you guessed it - a version of the form Major.Minor.Build.Revision.
I want to be able to associate each item in the Files ItemGroup with its _Version by adding the latter as metadata, so that I can do something like:
<Message Text="%(Files.Identity): %(Files.Version)" />
and have MSBuild print out a nice list of file-version associations.
Is this possible?

Answer 1

This can be achieved by using target batching to add your Version member to the metadata. This involves moving your ReadLinesFromFile operation to its own target, using the #(Files) ItemGroup as an input.
This causes the target to be executed for each item in your ItemGroup, allowing you to read the contents (i.e. version number) from each individual file and subsequently update that item to add the Version metadata:
<Project DefaultTargets="OutputFilesAndVersions" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<Files Include="C:\Versioning\**\file.version" />
</ItemGroup>
<Target Name="OutputFilesAndVersions" DependsOnTargets="RetrieveVersions">
<Message Text="#(Files->'%(Identity): %(Version)')" />
</Target>
<Target Name="RetrieveVersions" Inputs="#(Files)" Outputs="%(Files.Identity)">
<ReadLinesFromFile File="%(Files.Identity)">
<Output TaskParameter="Lines" PropertyName="_Version"/>
</ReadLinesFromFile>
<PropertyGroup>
<MyFileName>%(Files.Identity)</MyFileName>
</PropertyGroup>
<ItemGroup>
<Files Condition="'%(Files.Identity)'=='$(MyFileName)'">
<Version>$(_Version)</Version>
</Files>
</ItemGroup>
</Target>
</Project>