Can I cache ItemGroup with metadata? - msbuild

Question

Assume I have a ItemGroup with metadata that takes some times to build (10 seconds):
<Target Name="BuildItemGroup">
<ItemGroup>
<File Include="5">
<Value>5a</Value>
</File>
<File Include="4">
<Value>4a</Value>
</File>
...
</ItemGroup>
<Message Text="Wait 10 seconds..." Importance="High" />
</Target>
And I am going to use the same ItemGroup few times in a recursive MSBuild task:
<Target Name="Recursive" DependsOnTargets="BuildItemGroup" Condition="$(Value) > 0" >
<PropertyGroup>
<Value>$([MSBuild]::Subtract($(Value), 1))</Value>
</PropertyGroup>
<MSBuild Projects="$(MSBuildProjectFile)" Targets="Recursive" Properties="Value=$(Value)" />
</Target>
<PropertyGroup>
<Value>5</Value>
</PropertyGroup>
<Target Name="Default" DependsOnTargets="Recursive" />
It will takes extra 40 seconds finish the Recursive task for 4 loops. Is there a way to cache the ItemGroup in such manner for loop usage?

Answer 1

No, calling MSBuild task will create new context which will re-evaluate all the properties and items.
The only way you can use to pass data between parent\child contexts is properties (and environment variables which become global properties).
Depending on how big this list is - you can try to pass it as a property (loosing metadata) and re-create itemgroup in your child msbuild context.
Or you can switch to inline tasks using C# and d whatever you want with full C# power - e.g. save data to disk and read it back from the file.
UPDATE: Finally I've done such inline task. Actually it's a pair of tasks one to save and another one - to load persisted items from disk. A bit ugly code - I have to create a serializable copy for Item and metadata classes. But it's good enough as a PoC. It's not guaranteed that it will safely work in multi-threaded environment, especially in your case of 200+ projects.
See the code in this repository