Example.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup>
    <Content Include="App.config">
      <SubType>Designer</SubType>
    </Content>
    <Content Include="App.Base.config">
      <DependentUpon>App.config</DependentUpon>
    </Content>
    <Content Include="App.Debug.config">
      <DependentUpon>App.config</DependentUpon>
    </Content>
    <Content Include="App.Release.config">
      <DependentUpon>App.config</DependentUpon>
    </Content>
  </ItemGroup>
  <Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v10.0\WebApplications\Microsoft.WebApplication.targets" />
  <Target Name="BeforeBuild">
    <TransformXml Source="App.Base.config" Transform="App.$(Configuration).config" Destination="App.config" />
  </Target>
</Project>