Support Global / Custom Xmlns for Uno targets

[dansiegel]

While it is great that Uno has historically tried to stay close to the Microsoft API first with UWP and now with WinUI, there are times where a break is needed to provide a better experience.

For years the UWP and WinUI team have failed to reintroduce the XmlnsDefinitionAttribute which is part of the public API for WPF, Silverlight, Xamarin.Forms and .NET MAUI, and is part of the private API used within WinUI. This results in XAML that is more verbose than what is really needed. For example the entirety of the Uno Toolkit and Themes could just instantly work as part of the out of box experience if the Uno team decided to add the XmlnsDefinition for these libraries to the core schema: http://schemas.microsoft.com/winfx/2006/xaml/presentation.

With the introduction of the .NET 10 preview 5, the .NET MAUI team has now taken XAML a step further with support for Global and Implicit XmlnsDefinitions, further reducing the noise within XAML to something that could simply be:

<Page x:Class="MyApp.MyView">
  <ExtendedSplashScreen>

It is true that there isn't much the Uno team can do about forcing the WinUI team to adopt these changes, however as many Uno apps don't actually rely on WinUI and I can still target Windows devices with the desktop target, I think it would be good for Uno to support a break in behavior here to provide a better developer and library author experience.

[agneszitte]

Related Uno issue created: #20991