App config transform xml

<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  <!-- To modify your build process, add your task inside one of the targets below and uncomment it.
       Other similar extension points exist, see Microsoft.Common.targets.
  <Target Name="BeforeBuild">
  </Target>
  <Target Name="AfterBuild">
  </Target>
  -->
  <UsingTask TaskName="TransformXml" AssemblyFile="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v11.0\Web\Microsoft.Web.Publishing.Tasks.dll" />
  <Target Name="CustomTarget" BeforeTargets="BeforeBuild">
    <Message Text="Transforming: App.$(Configuration).config" Importance="high" />
    <TransformXml Source="App.config" Transform="App.$(Configuration).config" Destination="App.config" />
  </Target>