MSBuild: Output properties from imported projects - msbuild

Question

Let's say I have a build.proj like this:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0"
DefaultTargets="AfterBuild"
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<CustomAfterMicrosoftCSharpTargets>$(MSBuildThisFileDirectory)Common.Build.targets</CustomAfterMicrosoftCSharpTargets>
<Configuration>Release</Configuration>
<Platform>Any CPU</Platform>
<ProjectProperties>
Configuration=$(Configuration);
Platform=$(Platform);
CustomAfterMicrosoftCSharpTargets=$(CustomAfterMicrosoftCSharpTargets);
</ProjectProperties>
</PropertyGroup>
<ItemGroup>
<ProjectToBuild Include="$(MSBuildThisFileDirectory)src\Proj\MyApp.csproj" />
</ItemGroup>
<Target Name="Build">
<MSBuild Targets="Build"
Projects="#(ProjectToBuild)"
Properties="$(ProjectProperties)" />
</Target>
<Target Name="AfterBuild" DependsOn="Build">
<Message Text="ChildProperty: $(ChildProperty)" />
</Target>
</Project>
In Common.Build.targets, I have a Target that creates a property:
<Target Name="DoSomethingUseful">
<!-- Do something useful -->
<CreateProperty Value="SomeComputedThingy">
<Output TaskParameter="Value" PropertyName="ChildProperty"/>
</CreateProperty>
</Target>
Now if I build build.proj, I do not see the value of ChildProperty in the message. The output is blank: ChildProperty:.
I was under the impression that any output for a target is merged back to global context after its execution. But it seems that it only applies to anything within that target file.
How do I make ChildProperty bubble up to the parent build.proj?

Answer 1

When you are calling <MSBuild> task on dependent projects, read TargetOutputs output parameter of the task. See example from MSDN:
<Target Name="BuildOtherProjects">
<MSBuild
Projects="#(ProjectReferences)"
Targets="Build">
<Output
TaskParameter="TargetOutputs"
ItemName="AssembliesBuiltByChildProjects" />
</MSBuild>
</Target>
You will also need to ensure the target you are calling in dependent projects correctly populates Returns or Output parameter (Returns takes precedence if used). E.g.:
<Target Name="MyTarget" Inputs="..." Outputs="..." Returns="$(MyOutputValue)">
<PropertyGroup>
<MyOutputValue>set it here</MyOutputValue>
</PropertyGroup>
</Target>