It is possible to do this kind of conversion with msbuild - msbuild

Question

It is possible to do this kind of conversion with msbuild? Transforming metadata into items?
This:
<ItemGroup>
<Group Include="G1">
<A>1</A>
<B>1</B>
</Group>
<Group Include="G2">
<A>2</A>
<B>2</B>
</Group>
</ItemGroup>
To this:
<ItemGroup>
<A>1</A>
<A>2</A>
<B>1</B>
<B>2</B>
</ItemGroup>

Answer 1

You can use batching:
This creates the new item groups A abd B based on Group. The new item groups don't have to use the same name as the metadata. Set ItemName in CreateItem/Output to use a different name.
<?xml version="1.0" encoding="utf-8"?>
<Project
xmlns="http://schemas.microsoft.com/developer/msbuild/2003"
ToolsVersion="4.0"
DefaultTargets="Default">
<ItemGroup>
<Group Include="G1">
<A>1</A>
<B>1</B>
</Group>
<Group Include="G2">
<A>2</A>
<B>2</B>
</Group>
</ItemGroup>
<Target Name="Default">
<Message Text="#(Group)" Importance="High" />
<CreateItem Include="%(Group.A)">
<Output TaskParameter="Include" ItemName="A" />
</CreateItem>
<CreateItem Include="%(Group.B)" AdditionalMetadata="From=%(Group.Identity)">
<Output TaskParameter="Include" ItemName="B" />
</CreateItem>
<Message Text="A=#(A)" Importance="High" />
<Message Text="B=#(B):%(B.From)" Importance="High" />
</Target>
</Project>
The new "B" group also defines a metadata item called From that gives each item the original item group name it was copied from.
Update: With msbuild 3.5 or newer you can also use this instead of CreateItem:
<ItemGroup>
<A Include="%(Group.A)" />
<B Include="%(Group.B)">
<From>%(Group.Identity)</From>
</B>
</ItemGroup>