Wix Toolset - upgrade and do not overwrite config file - wix

Question

I have an WPF app that connects to some web service on some URL. I have made an installation and it works like a charm, it even asks about URL and changes .config file as instructed. Now, I want to upgrade the app, but leave the .config file intact.
I have tried some solutions on web, it doesn't want to upgrade, it seems that only does a clean install every time.
<Package InstallerVersion="400" Compressed="yes" InstallScope="perMachine" Platform="x64" />
<Icon Id="icon.ico" SourceFile="$(var.ProjectDir)Icon.ico" />
<Property Id="ARPPRODUCTICON" Value="icon.ico" />
<WixVariable Id="WixUIBannerBmp" Value="Images\installer_top-banner.bmp" />
<WixVariable Id="WixUIDialogBmp" Value="Images\installer_background.bmp" />
<WixVariable Id="WixUILicenseRtf" Value="$(var.ProjectDir)\license.rtf" />
<Property Id="ARPURLINFOABOUT" Value="http://www.Company.com" />
<Property Id="WIXUI_INSTALLDIR" Value="INSTALLFOLDER" />
<Property Id="FULLURL" Value="http://demo.Company.com/WcfFullPortal.svc" Secure="yes" />
<Property Id="AUTH" Value="0" Secure="yes" />
<Property Id="AUTHVALUE" Value="$(var.httpValue)" Secure="yes" />
<Property Id="PREVIOUSVERSIONSINSTALLED" Secure="yes" />
<Upgrade Id="1d96517a-8fc5-4150-b1cb-c3adf479a57d">
<UpgradeVersion Minimum="1.0.0.0" Maximum="99.0.0.0" Property="PREVIOUSVERSIONSINSTALLED" IncludeMinimum="yes" IncludeMaximum="no" />
</Upgrade>
<UIRef Id="SetupDialogUI" />
<Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOXTEXT" Value="Launch MyApp" />
<Property Id="WixShellExecTarget" Value="[#MyApp.UI.WPF.exe]" />
<CustomAction Id="LaunchApplication" BinaryKey="WixCA" DllEntry="WixShellExec" Impersonate="yes" />
<MajorUpgrade DowngradeErrorMessage="A newer version of [ProductName] is already installed." AllowSameVersionUpgrades="yes" Schedule="afterInstallExecute" />
<MediaTemplate EmbedCab="yes" />
<!-- 32-bit / 64-bit variables -->
<?if $(var.Platform) = x64 ?>
<?define bitness = "(64 bit)" ?>
<?define Win64 = "yes" ?>
<?define PlatformProgramFilesFolder = "ProgramFiles64Folder" ?>
<?else?>
<?define bitness = "(32 bit)" ?>
<?define Win64 = "no" ?>
<?define PlatformProgramFilesFolder = "ProgramFilesFolder" ?>
<?endif?>
<Condition Message="Minimum supported OS is Windows 7."><![CDATA[Installed OR (VersionNT >= 600)]]></Condition>
<!--
<?if $(var.Platform) = x64 ?>
<Condition Message="You need to install the 32-bit version of this product on 32-bit Windows.">
<![CDATA[VersionNT64]]>
</Condition>
<?endif?>
<?if $(var.Platform) = x86 ?>
<Condition Message="You need to install the 64-bit version of this product on 64-bit Windows.">
<![CDATA[NOT VersionNT64]]>
</Condition>
<?endif?> -->
</Product>
...

Answer 1

Two approaches:
Finish the work in the installer. MSI doesn't persist (remember) properties on subsequent insallations. You have to harvest the value from the config file back into the property so it can be shown in the UI again and reapplied during the installation.
http://robmensching.com/blog/posts/2010/5/2/the-wix-toolsets-remember-property-pattern/
Move the complexity out of the installer and into the application. Design the application so it asks for the setting on first run. Use the app.config appSettings element file attribute to specify a second file to contain the override setting. Have the installer create the app.config but not the second file. Have the application save the first run obtained setting to the second file. The installer doesn't know about this so it'll never touch the file during subsequent installs.
https://learn.microsoft.com/en-us/dotnet/framework/configure-apps/file-schema/appsettings/appsettings-element-for-configuration
I'm a WiX expert and can handle approach 1 easily. For people new to WiX it can be a challenge. I generally go with approach 1 when it's just a handful of settings and the program is non interactive like a windows service or say a web application where the inputs are pool identity. Otherwise I find it's easier for my customers to support if I move the complexity to code they better understand.