What does the MSBuild -> (arrow/dash greater/->) operator do?

Question

What does the -> (or ->) operator do inside MSBuild?

Some example code from another question: File Tracker Log file format

<!-- Appended tlog to track custom build events -->
<WriteLinesToFile
  File="$(IntDir)$(ProjectName).write.1.tlog"
  Lines="@(CustomBuildStep-&gt;'^%(Identity)');@(CustomBuildStep-&gt;MetaData('Outputs')-&gt;FullPath()-&gt;Distinct())"/>

Bonus question: What does @(CustomBuildStep->'^%(Identity)') do in the above code?

Answer 1

The -> operator in MSBuild is the transformation operator. It transforms an item list into a new item list by substituting the string on the right-hand side using metadata of the original items.

The ->'^%(Identity)' magic is partially described in the task batching documentation. The Identity metadata is just the value itself, so that transformation just prepends "^". It also causes batching because of the % sigil, so the whole WriteLinesToFile task gets executed for each CustomBuildStep item.