ConfigurationTransformAdd.csproj

  <UsingTask TaskName="TransformXml" AssemblyFile="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\Web\Microsoft.Web.Publishing.Tasks.dll" />
  <Target Name="AfterCompile" Condition="Exists('App.$(Configuration).config')">
    <!--Generate transformed app config in the intermediate directory-->
    <TransformXml Source="App.config" Destination="$(IntermediateOutputPath)$(TargetFileName).config" Transform="App.$(Configuration).config" />
    <!--Force build process to use the transformed configuration file from now on.-->
    <ItemGroup>
      <AppConfigWithTargetPath Remove="App.config" />
      <AppConfigWithTargetPath Include="$(IntermediateOutputPath)$(TargetFileName).config">
        <TargetPath>$(TargetFileName).config</TargetPath>
      </AppConfigWithTargetPath>
    </ItemGroup>
  </Target>
  <!--Override After Publish to support ClickOnce AfterPublish. Target replaces the untransformed config file copied to the deployment directory with the transformed one.-->
  <Target Name="AfterPublish">
    <PropertyGroup>
      <DeployedConfig>$(_DeploymentApplicationDir)$(TargetName)$(TargetExt).config$(_DeploymentFileMappingExtension)</DeployedConfig>
    </PropertyGroup>
    <!--Publish copies the untransformed App.config to deployment directory so overwrite it-->
    <Copy Condition="Exists('$(DeployedConfig)')" SourceFiles="$(IntermediateOutputPath)$(TargetFileName).config" DestinationFiles="$(DeployedConfig)" />
  </Target>