Keep user settings in appsettings.json across upgrades

A major upgrade replaces appsettings.json with the new version’s copy, so values changed after the first install are lost. To keep particular settings, read them from the installed file before it is replaced, then write them into the new one.

<Component Id="AppConfig">
  <File Id="AppSettings" Name="appsettings.json" Source="appsettings.json" />

  <!-- readValue runs right after CostFinalize, before the previous version is removed or the
       new file is installed, so it reads the copy already on disk. On a first install the file
       doesn't exist and DefaultValue is used. -->
  <Json:JsonFile Id="ReadApiUrl" File="[INSTALLFOLDER]appsettings.json"
                 ElementPath="$.Api.BaseUrl" Action="readValue"
                 Property="KEEP_API_URL" DefaultValue="https://localhost:5001" />

  <!-- Runs after InstallFiles and writes the value into the new file. -->
  <Json:JsonFile Id="WriteApiUrl" File="[#AppSettings]"
                 ElementPath="$.Api.BaseUrl" Value="[KEEP_API_URL]" Action="setValue" />
</Component>

Repeat the pair for each setting you want to keep. This carries over chosen keys; it isn’t a general merge:

Make the DefaultValue match the value in the file you ship, so a first install writes the same default back.

Alternatives

If the application can layer configuration, the cleanest design is often to never edit the shipped file. Keep user overrides in a separate file, for example under ProgramData, and add it with ConfigurationBuilder. Use the pattern above when the settings have to live in appsettings.json itself.

See also