[Feature Request]: Add JoinItems task

[tmat]

Summary

Add JoinItems task that's currently defined in .NET SDK to msbuild as an officially supported task.

Background and Motivation

Joining items is generally useful operation that should be provided by msbuild.

Proposed Feature

The current implementation as provided in .NET SDK, maybe add extra parameters to customize the join (inner join vs left/right outer join).

Alternative Designs

No response

[AR-May]

Team triage: we want to gather feedback on this proposal.

[jrdodds]

For any task that will be a 'public' task, the following questions should probably have answers.

• Is the task providing something that is not otherwise available?
• Is the task general-purpose?

A task is not needed to perform many types of joins with Item collections.

For me, this raises the question - why does this task exist? What was the purpose and motivation for creating the JoinItems task originally? I hope that it is not the case that a task was written instead of understanding how to join in MSBuild.

If the functionality of JoinItems is redundant, that doesn't necessarily break the case for adding the task. Performance and/or simplicity have value.

The FormatUrl task is not a general purpose task. I'm guessing FormatUrl was created for a specific internal need. In its current form, it should not have been exposed as a 'public' task. That can't be undone at this point. (The lack of a true general-purpose task for URIs is part of why I proposed #8613.) But the situation of FormatUrl should not be repeated.

From my own quick review, JoinItems seems general-purpose and would not create an issue like FormatUrl.

Independently I have noodled with creating a similar task wrapping IEnumerable<ITaskItem>.Join() and IEnumerable<ITaskItem>Group.Join(). I can share details and code if there is interest.