App.Config Transformation for projects which are not Web Projects in Visual Studio?

I tried several solutions and here is the simplest I personally found.
Dan pointed out in the comments that the original post belongs to Oleg Sych—thanks, Oleg!

Here are the instructions:

1. Add an XML file for each configuration to the project.

Typically you will have Debug and Release configurations so name your files App.Debug.config and App.Release.config. In my project, I created a configuration for each kind of environment, so you might want to experiment with that.

2. Unload project and open .csproj file for editing

Visual Studio allows you to edit .csproj files right in the editor—you just need to unload the project first. Then right-click on it and select Edit <ProjectName>.csproj.

3. Bind App.*.config files to main App.config

Find the project file section that contains all App.config and App.*.config references. You'll notice their build actions are set to None and that's okay:

<None Include="App.config" />
<None Include="App.Debug.config" />
<None Include="App.Release.config" />

Next, make all configuration-specific files dependant on the main App.config so Visual Studio groups them like it does designer and code-behind files.

Replace XML above with the one below:

<None Include="App.config" />
<None Include="App.Debug.config" >
  <DependentUpon>App.config</DependentUpon>
</None>
<None Include="App.Release.config" >
  <DependentUpon>App.config</DependentUpon>
</None>

4. Activate transformations magic (still necessary for Visual Studio versions such as VS2019)

In the end of file after

<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />

and before final

</Project>

insert the following XML -- please note there are two steps for the proper transformation to occur:

  <UsingTask TaskName="TransformXml" AssemblyFile="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\Web\Microsoft.Web.Publishing.Tasks.dll" />
  <Target Name="BeforeBuild" Condition="Exists('App.$(Configuration).config')">
    <!-- Generate transformed app config and replace it: will get the <runtime> node and assembly bindings properly populated -->
    <TransformXml Source="App.config" Destination="App.config" Transform="App.$(Configuration).config" />
  </Target>
  <Target Name="AfterBuild" Condition="Exists('App.$(Configuration).config')">
    <!-- Generate transformed app config in the intermediate directory: this will transform sections such as appSettings -->
    <TransformXml Source="App.config" Destination="$(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>

Now you can reload the project, build it and enjoy App.config transformations!

FYI

Make sure that your App.*.config files have the right setup like this:

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
     <!--magic transformations here-->
</configuration>

This works now with the Visual Studio AddIn treated in this article: SlowCheetah - Web.config Transformation Syntax now generalized for any XML configuration file.

You can right-click on your web.config and click "Add Config Transforms." When you do this, you'll get a web.debug.config and a web.release.config. You can make a web.whatever.config if you like, as long as the name lines up with a configuration profile. These files are just the changes you want made, not a complete copy of your web.config.

You might think you'd want to use XSLT to transform a web.config, but while they feels intuitively right it's actually very verbose.

Here's two transforms, one using XSLT and the same one using the XML Document Transform syntax/namespace. As with all things there's multiple ways in XSLT to do this, but you get the general idea. XSLT is a generalized tree transformation language, while this deployment one is optimized for a specific subset of common scenarios. But, the cool part is that each XDT transform is a .NET plugin, so you can make your own.
<?xml version="1.0" ?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:template match="@*|node()">
  <xsl:copy>           
    <xsl:apply-templates select="@*|node()"/>
  </xsl:copy>
</xsl:template>
<xsl:template match="/configuration/appSettings">
  <xsl:copy>
    <xsl:apply-templates select="node()|@*"/>
    <xsl:element name="add">
      <xsl:attribute name="key">NewSetting</xsl:attribute>
      <xsl:attribute name="value">New Setting Value</xsl:attribute>
    </xsl:element>
  </xsl:copy>
</xsl:template>
</xsl:stylesheet>
Or the same thing via the deployment transform:
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
   <appSettings>
      <add name="NewSetting" value="New Setting Value" xdt:Transform="Insert"/>
   </appSettings>
</configuration>

Another solution I've found is NOT to use the transformations but just have a separate config file, e.g. app.Release.config. Then add this line to your csproj file.

  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|x86' ">
    <AppConfig>App.Release.config</AppConfig>
  </PropertyGroup>

This will not only generate the right myprogram.exe.config file but if you're using Setup and Deployment Project in Visual Studio to generate MSI, it'll force the deployment project to use the correct config file when packaging.

Inspired by Oleg and others in this question, I took the solution https://stackoverflow.com/a/5109530/2286801 a step further to enable the following.

Works with ClickOnce
Works with Setup and Deployment projects in VS 2010
Works with VS2010, 2013, 2015 (didn't test 2012 although should work as well).
Works with Team Build. (You must install either A) Visual Studio or B) Microsoft.Web.Publishing.targets and Microsoft.Web.Publishing.Tasks.dll)

This solution works by performing the app.config transformation before the app.config is referenced for the first time in the MSBuild process. It uses an external targets file for easier management across multiple projects.

Instructions:

Similar steps to the other solution. I've quoted what remains the same and included it for completeness and easier comparison.

0. Add a new file to your project called AppConfigTransformation.targets

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <!-- Transform the app config per project configuration.-->
  <PropertyGroup>
    <!-- This ensures compatibility across multiple versions of Visual Studio when using a solution file.
         However, when using MSBuild directly you may need to override this property to 11.0 or 12.0 
         accordingly as part of the MSBuild script, ie /p:VisualStudioVersion=11.0;
         See http://blogs.msdn.com/b/webdev/archive/2012/08/22/visual-studio-project-compatability-and-visualstudioversion.aspx -->
    <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">10.0</VisualStudioVersion>
  </PropertyGroup>

  <Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v$(VisualStudioVersion)\Web\Microsoft.Web.Publishing.targets" />

  <Target Name="SetTransformAppConfigDestination" BeforeTargets="PrepareForBuild" 
          Condition="exists('app.$(Configuration).config')">
    <PropertyGroup>
      <!-- Force build process to use the transformed configuration file from now on. -->
      <AppConfig>$(IntermediateOutputPath)$(TargetFileName).config</AppConfig>
    </PropertyGroup>
    <Message Text="AppConfig transformation destination: = $(AppConfig)" />
  </Target>

  <!-- Transform the app.config after the prepare for build completes. -->
  <Target Name="TransformAppConfig" AfterTargets="PrepareForBuild" Condition="exists('app.$(Configuration).config')">
    <!-- Generate transformed app config in the intermediate directory -->
    <TransformXml Source="app.config" Destination="$(AppConfig)" Transform="app.$(Configuration).config" />
  </Target>

</Project>

1. Add an XML file for each configuration to the project.

Typically you will have Debug and Release configurations so name your files App.Debug.config and App.Release.config. In my project, I created a configuration for each kind of enironment so you might want to experiment with that.

2. Unload project and open .csproj file for editing

Visual Studio allows you to edit .csproj right in the editor—you just need to unload the project first. Then right-click on it and select Edit .csproj.

3. Bind App.*.config files to main App.config

Find the project file section that contains all App.config and App.*.config references and replace as follows. You'll notice we use None instead of Content.

<ItemGroup>
  <None Include="app.config"/>
  <None Include="app.Production.config">
    <DependentUpon>app.config</DependentUpon>
  </None>
  <None Include="app.QA.config">
    <DependentUpon>app.config</DependentUpon>
  </None>
  <None Include="app.Development.config">
    <DependentUpon>app.config</DependentUpon>
  </None>
</ItemGroup>

4. Activate transformations magic

In the end of file after
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
and before final
</Project>

insert the following XML:

<Import Project="AppConfigTransformation.targets" />

Done!

App.Config Transformation for projects which are not Web Projects in Visual Studio?

Instructions:

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