Update appsettings.json from a WiX installer

.NET applications keep their settings in appsettings.json, but WiX only ships util:XmlFile for XML. The usual workaround is a C# custom action. That means scheduling it as a deferred action after InstallFiles, passing paths in through CustomActionData, running it elevated, and writing your own rollback.

WixJsonFileExtension does all of that for you. Each change is one JsonFile element.

1. Add the package

<!-- MyInstaller.wixproj -->
<ItemGroup>
  <PackageReference Include="WixJsonFileExtension" Version="7.1.0" />
</ItemGroup>

2. Edit the file

<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs"
     xmlns:Json="http://schemas.hegsie.com/wix/JsonExtension">
  <Package Name="MyApp" Manufacturer="Me" Version="1.0.0" UpgradeCode="PUT-GUID-HERE">
    <Property Id="DB_SERVER" Value="localhost" />
    <Property Id="API_URL" Value="https://api.example.com" />
    <Property Id="PORT" Value="5000" />

    <StandardDirectory Id="ProgramFiles64Folder">
      <Directory Id="INSTALLFOLDER" Name="MyApp">
        <Component Id="AppConfig">
          <File Id="AppSettings" Name="appsettings.json" Source="appsettings.json" />

          <!-- A string, with an installer property substituted in -->
          <Json:JsonFile Id="SetConnection" File="[#AppSettings]"
                         ElementPath="$.ConnectionStrings.DefaultConnection"
                         Value="Server=[DB_SERVER];Database=MyApp;Trusted_Connection=True"
                         Action="setValue" />

          <!-- A nested value -->
          <Json:JsonFile Id="SetApiUrl" File="[#AppSettings]"
                         ElementPath="$.Api.BaseUrl" Value="[API_URL]" Action="setValue" />

          <!-- Written as a JSON number, not a string -->
          <Json:JsonFile Id="SetPort" File="[#AppSettings]"
                         ElementPath="$.Kestrel.Port" Value="[PORT]" ValueType="number" />
        </Component>
      </Directory>
    </StandardDirectory>
  </Package>
</Wix>

Set the properties on the command line, or from your UI:

msiexec /i MyApp.msi DB_SERVER=sql01 API_URL=https://api.contoso.com PORT=8080

How it runs

• Changes are applied after InstallFiles, in the deferred, elevated part of the install. Files under Program Files can be written.
• If the install fails or is cancelled, every change is rolled back.
• Key order, indentation and line endings in the file are preserved, so diffs stay small.
• Add OnlyIfExists="yes" to change a value only when it is already present.

Next steps

• Keep user settings across upgrades
• Cookbook for logging, feature flags, environment-specific settings and arrays
• Full attribute reference