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If I create a Packager Project containing the UWP and the helper app, then the helper gets packaged, thus runs in a sandbox and can no longer access the stdin/stdout
If you have created desktop extension for UWP app, even the extension's process was managed with UWP main process, it also has permission to access the folder with specific path, it will not be effected by Sandbox. You can do anything that desktop app can do. For more please refer Stefan's blog UWP with Desktop Extension
I've tried to ship the helper as an asset file of the UWP app so it can be executed freely but the helper's .exe does not make it to the final package.
Before windows application package Project template, we use above way to implement full trust launcher that copy the exe file in to the project and add the specific execution path in the package manifest content.
Maybe this .exe could be downloaded at runtime ? (Is that fine with Microsoft Store policies and with anti-viruses ?)
Even you could download exe file, but you also need to add exe file execution path in manifest content, but it does not allow other path except app's installation folder.
Approach 2: localhost server (with loopback ??)
localhost server is approved way to make them commination, for enable loopback please refer this document
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