Proposal: Make WinUI 3 usable from third party host applications (and non-WinUI 3 first party assemblies)

[myth384]

Proposal: Make WinUI 3 usable from third party host applications (and non-WinUI 3 first party assemblies)

Summary

WinUI 3 needs an effortless way to instantiate new WinUI 3 Windows, both modal and modeless, from plug-in type applications for third party host applications and non-WinUI 3 first party assemblies.

Rationale

• WinUI 3 is the way forward for desktop development, but that should be for every use case.
• We shouldn't have to rely on other frameworks (WPF/Uno Platform) to be able to show WinUI 3 controls inside XAML islands.
• Multiple plug-ins using WinUI 3 should not conflict with each other in any way.
• This proposal also covers other use cases, like using WinUI 3 from first party assemblies using other UI frameworks.

Scope

Capability	Priority
WinUI 3 Application object can be isolated inside an AssemblyLoadContext, if isolation is not provided by the host application	Must
References and Resources from XAML respect the path defined in the AssemblyLoadContext	Must
Multiple isolated Application objects can coexist inside an Appdomain	Must
The Applications objects can be started, without blocking the host application	Must
New WinUI 3 Windows can be opened modal and modeless	Must
Xaml Islands can coexist with the proposed solution	Should

Important Notes

Currently there is no easy way to implement WinUI 3 in plug-in type applications for a third party host application, like multiple Autodesk and Microsoft Office products. I tried so many ways, but most of the time I had to conclude I entered a cul-de-sac. My current workaround is to use a WinUI3 UserControl with all contents, hosted by a single UnoXamlHost placed inside a WPF Window.
Typically, these plug-ins are (COM interop) class libraries instantiated by the host application and need direct access to objects exposed by host API. I think it is safe to assume most host applications don't use AssemblyLoadContext yet, if ever. Isolation could still be achieved by implementing AssemblyLoadContext at the plug-in side.

Open Questions

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[myth384]

FYI: Avalonis UI's AppBuilderBase<TAppBuilder> class has the method SetupWithoutStarting() for the discribed use case. After calling this method, new Windows can be shown without blocking the host application.

[jozefizso]

This would help us create Office COM add-ins with modern user interface.

The COM add-ins are hosted in the Office apps and we need a way to initialize the WinUI in a host app outside of our control.