How to extract directory from property? - msbuild

Question

I have a property GroupProj storing a full path name. How can I extract the directory of the property?
I have the following code, but it doesn't work as expected:
<PropertyGroup>
<GroupProj>C:\development\project\default.groupproj</GroupProj>
</PropertyGroup>
<Target Name="Default">
<Message Text="Echo: $(GroupProj->'%(RootDir)')" />
</Target>
I will describe my actual intention of doing so. Perhaps there is a way to do the job that I am not aware of.
I have a Delphi groupproj (MSBuild project) file, C:\development\project\default.groupproj:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<Projects Include="project1.dproj">
<Dependencies/>
</Projects>
<Projects Include="project2.dproj">
<Dependencies/>
</Projects>
<Projects Include="project3.dproj">
<Dependencies/>
</Projects>
</ItemGroup>
...
</Project>
There are other 3 MSBuild files (project1.dproj, project2.dproj and project3.dproj) stored in same folder as default.groupproj.
I create a MSBuild project file (c:\test.targets):
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build" ToolsVersion="3.5">
<Import Project="$(GroupProj)" />
<Target Name="Build">
<MSBuild BuildInParallel="True" Projects="project1.dproj;project2.dproj;project3.dproj"/>
</Target>
</Project>
And execute as:
c:\> msbuild /p:GroupProj="C:\development\project\default.groupproj" test.targets
The execution shall fail as MSBuild can't find projectN.dproj file. The issue shall be the working directory isn't set to default.groupproj.
One straight solution come into my mind is to extract directory of $(GroupProj) and concat to there projectN.dproj file.
That's the whole story of my question.

Answer 1

Try something like this:
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<GroupProj>C:\development\project\default.groupproj</GroupProj>
</PropertyGroup>
<Target Name="Build">
<CreateItem Include="$(GroupProj)">
<Output TaskParameter="Include" ItemName="ItemFromProp"/>
</CreateItem>
<Message Text="1. #(ItemFromProp -> '%(RootDir)%(Directory)')"/>
<Message Text="2. %(ItemFromProp.RootDir)%(ItemFromProp.Directory)"/>
<Message Text="3. %(ItemFromProp.Identity)"/>
<Message Text="4. %(ItemFromProp.FullPath)"/>
<Message Text="5. %(ItemFromProp.FileName)"/>
<Message Text="6. %(ItemFromProp.Extension)"/>
</Target>
</Project>
EDIT:
To build the projects in parallel try this:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Target Name="GetGroupProjPath">
<ItemGroup>
<GroupProj Include="$(GroupProj)" />
<GroupProjPath Include="#(GroupProj->'%(Directory)')" />
</ItemGroup>
<PropertyGroup>
<GroupProjPath>#(GroupProjPath->'%(RootDir)%(Identity)')</GroupProjPath>
</PropertyGroup>
</Target>
<Import Project="$(GroupProj)" />
<Target Name="GetDProjs" DependsOnTargets="GetGroupProjPath">
<ItemGroup>
<DProjs Include="#(Projects->'$(GroupProjPath)%(FileName)%(Extension)')" />
</ItemGroup>
</Target>
<Target Name="Build" DependsOnTargets="GetDProjs">
<Message Text="#(DProjs)" />
</Target>
</Project>