Example MSBuild Traversal

dirs.proj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <SerializeProjects>true</SerializeProjects>
  </PropertyGroup>
  <ItemGroup>
    <Project Include="Samples.sln" />
    <Project Include="Samples.sln">
      <AdditionalProperties>Platform=x64</AdditionalProperties>
    </Project>
    <Project Include="pkg\dirs.proj" />
  </ItemGroup>
  <Import Project="lib\Traversal.targets" />
</Project>

Traversal.targets

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="Build">
  <PropertyGroup>
    <BuildInParallel Condition="'$(BuildInParallel)' == ''">true</BuildInParallel>
  </PropertyGroup>

  <ItemDefinitionGroup>
    <Project>
      <AdditionalPropeties />
      <BuildInParallel>$(BuildInParallel)</BuildInParallel>
    </Project>
  </ItemDefinitionGroup>

  <Target Name="BuildAllProjects">
    <!-- To Serialize we use msbuild's batching functionality '%' to force it to batch all similar projects with the same identity
         however since the project names are unique it will essentially force each to run in its own batch -->
    <MSBuild Targets="Build" Projects="@(Project)" Condition="'$(SerializeProjects)'=='true'" BuildInParallel="%(BuildInParallel)" Properties="Dummy=%(Identity)" />
    <MSBuild Targets="Build" Projects="@(Project)" Condition="'$(SerializeProjects)'!='true'" BuildInParallel="$(BuildInParallel)" />
  </Target>

  <Target Name="CleanAllProjects">
    <!-- To Serialize we use msbuild's batching functionality '%' to force it to batch all similar projects with the same identity
         however since the project names are unique it will essentially force each to run in its own batch -->
    <MSBuild Targets="Clean" Projects="@(Project)" Condition="'$(SerializeProjects)'=='true'" BuildInParallel="%(BuildInParallel)" Properties="Dummy=%(Identity)" />
    <MSBuild Targets="Clean" Projects="@(Project)" Condition="'$(SerializeProjects)'!='true'" BuildInParallel="$(BuildInParallel)" />
  </Target>

  <PropertyGroup>
    <TraversalBuildDependsOn>
      BuildAllProjects;
      $(TraversalBuildDependsOn);
    </TraversalBuildDependsOn>
    <TraversalCleanDependsOn>
      CleanAllProjects;
      $(TraversalCleanDependsOn);
    </TraversalCleanDependsOn>
  </PropertyGroup>

  <Target Name="Build" DependsOnTargets="$(TraversalBuildDependsOn)" />
  <Target Name="Clean" DependsOnTargets="$(TraversalCleanDependsOn)" />
  <Target Name="Rebuild" DependsOnTargets="Clean;Build" />
</Project>