How to use AddOptionalPackageAsync?

Tanaka_Jimha Thanks! I tried all your suggestions, but no luck so far. Here are some more details:

I don't want to install all the optional dependencies at the same time when the main app is installed, so I don't think I should add all of them to the <OptionalPackages> element in the appinstaller file, right? I only want one of them installed with the main app, so I add that optional package to the <OptionalPackages> element, and then all the other optional packages I want to be able to install via an in-app option, so I add all of those packages to the <RelatedPackages> element in the appinstaller file. You can see the details of how I do this in the code that I posted with the original message.

I think https://docs.microsoft.com/en-us/uwp/api/windows.management.deployment.packagemanager.addpackagebyappinstallerfileasync?view=winrt-19041 for the same reason is also not the right tool to implement my in-app feature add thing, right? As far as I can tell, it doesn't provide an option to install a specific optional package of the app, right? Or did I miss an option there somewhere?

I did add <rescap:Capability Name="packageManagement" /> to my main app now, and it shows up in the UI when I install that, but it doesn't change the error message I get.

When I install all the optional packages on my system, and then run the following ps code:

> $x = Get-AppxPackage Julia*
> $x.Dependencies

I get this output:

Name              : Julia-1.4.1
Publisher         : CN=Julialang
Architecture      : X64
ResourceId        :
Version           : 1.0.0.0
PackageFullName   : Julia-1.4.1_1.0.0.0_x64__87dh6scs60qg2
InstallLocation   : C:\Program Files\WindowsApps\Julia-1.4.1_1.0.0.0_x64__87dh6scs60qg2
IsFramework       : False
PackageFamilyName : Julia-1.4.1_87dh6scs60qg2
PublisherId       : 87dh6scs60qg2
IsResourcePackage : False
IsBundle          : False
IsDevelopmentMode : False
NonRemovable      : False
IsPartiallyStaged : False
SignatureKind     : Developer
Status            : Ok

Name              : Julia-1.4.2
Publisher         : CN=Julialang
Architecture      : X64
ResourceId        :
Version           : 1.0.0.0
PackageFullName   : Julia-1.4.2_1.0.0.0_x64__87dh6scs60qg2
InstallLocation   : C:\Program Files\WindowsApps\Julia-1.4.2_1.0.0.0_x64__87dh6scs60qg2
IsFramework       : False
PackageFamilyName : Julia-1.4.2_87dh6scs60qg2
PublisherId       : 87dh6scs60qg2
IsResourcePackage : False
IsBundle          : False
IsDevelopmentMode : False
NonRemovable      : False
IsPartiallyStaged : False
SignatureKind     : Developer
Status            : Ok

So from this I take it that I should use "Julia-1.4.2_87dh6scs60qg2" as the string that I pass to AddOptionalPackageAsync? I'm doing that now, but I still get the same error message "Access is denied".

I do think everything is in one related set, I'm creating all the packages with a PackagingLayout.xml file that looks like this:

<PackagingLayout xmlns="http://schemas.microsoft.com/appx/makeappx/2017">
  <!-- Main game -->
  <PackageFamily ID="Julia" FlatBundle="true" ManifestPath="appxmanifest.xml" ResourceManager="false">
    
    <!-- x64 code package-->
    <Package ID="Julia-x64" ProcessorArchitecture="x64">
      <Files>
        <File DestinationPath="Julia\Julia.exe" SourcePath="C:\Users\david\source\repos\WinJulia\Release\Julia.exe"/>
        <File DestinationPath="Images\*.png" SourcePath="Images\*.png"/>
      </Files>
    </Package>

    
  </PackageFamily>

  <PackageFamily ID="Julia-1.4.2" Optional="true" ManifestPath="juliaversions\julia-1.4.2-appxmanifest.xml" ResourceManager="false">
    <Package ID="Julia-1.4.2.x64" ProcessorArchitecture="x64">
      <Files>
        <File DestinationPath="Julia\**" SourcePath="C:\Users\david\source\repos\WinJulia\optionalpackages\julia-1.4.2\Julia\**"/>
        <File DestinationPath="Images\*.png" SourcePath="Images\*.png"/>
      </Files>
    </Package>
  </PackageFamily>

  <PackageFamily ID="Julia-1.4.1" Optional="true" ManifestPath="juliaversions\julia-1.4.1-appxmanifest.xml" ResourceManager="false">
    <Package ID="Julia-1.4.1.x64" ProcessorArchitecture="x64">
      <Files>
        <File DestinationPath="Julia\**" SourcePath="C:\Users\david\source\repos\WinJulia\optionalpackages\julia-1.4.1\Julia\**"/>
        <File DestinationPath="Images\*.png" SourcePath="Images\*.png"/>
      </Files>
    </Package>
  </PackageFamily>

  <PackageFamily ID="Julia-1.4.0" Optional="true" ManifestPath="juliaversions\julia-1.4.0-appxmanifest.xml" ResourceManager="false">
    <Package ID="Julia-1.4.0.x64" ProcessorArchitecture="x64">
      <Files>
        <File DestinationPath="Julia\**" SourcePath="C:\Users\david\source\repos\WinJulia\optionalpackages\julia-1.4.0\Julia\**"/>
        <File DestinationPath="Images\*.png" SourcePath="Images\*.png"/>
      </Files>
    </Package>
  </PackageFamily>
</PackagingLayout>

And I believe that should create all packages as being in one related set, right?