It is possible to do this kind of conversion with msbuild

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>

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>