Can Universal Windows Platform be used to develop windows software and not app
UWP apps do not have to be distributed via public Microsoft Store. You can distribute the apps via sideloading and alternatively via Microsoft Store for Business which is a company-specific view into Microsoft Store, where you can publish internal apps.
Furthermore, you can package existing Desktop apps as UWP apps including their installers with the Desktop Bridge, which is useful when your app includes functionality, which is unavailable in pure UWP apps.
The main advantage of using UWP especially in conjunction with Microsoft Store for Business is simpler deployment and easier install and update lifecycle, however it might not suit you in complex scenarios.
How do I real time communicate a Universal Windows App with a older .Net 4.5 app in the same solution. IPC?
If I understand correctly, you were developing a app which will can automate the DOM operation through Cortana.
Do you need to submit this app to App Store?
If no, as Scott said, you can use WCF to communicate between UWP process and .NET 4.5 desktop app process.
If yes, as Andrew pointed out, you need an intermediate remote server hosting the service.
System.Diagnostics.Process not available in Universal Windows App
You don't have access to other processes.
UWP applications (and their predecessors WinRT applications) pretty much run in a sandbox. For the sake of system stability they usually don't have access to each other. OEM and carriers have access to capability declarations in some cases that allow deeper access into the system but the general application developer does not.
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