Feature: alternative "using:" syntax to include namespace to XAML file

Currently Avalonia supports WPF’style namespaces in XAML files, using clr-namespace with assembly. With that we need to provide long path to assemblies, if namespace isn’t contained in current one. UWP and XF supports another syntax, using “using:” keyword (for UWP it is only one available syntax). With this we don’t need to provide assembly, instead it will internally search for namespace in referenced projects and packages.

Proposal syntax:

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="using:System"
        xmlns:controls="using:MyNamespaceInAnotherAssembly">
  <Style Selector="controls|MyControl" />
</Window>

UWP docs: https://docs.microsoft.com/en-us/windows/uwp/xaml-platform/xaml-namespaces-and-namespace-mapping#mapping-custom-types-to-xaml-namespaces-and-prefixes

Xamarin Forms docs: https://docs.microsoft.com/en-us/xamarin/xamarin-forms/xaml/namespaces#declaring-namespaces-for-types

[jkoritzinsky]

I added this to the WinUI prototype Xaml compiler based on XamlX, and this is pretty easy to add to the XamlX library.

[FoggyFinder]

“using:” is well known UWP and XF syntax not only for C#, but also for F#/VB/C++ projects as well.

Just because UWP doesn’t care about anything except C#. To use it properly (release mode = .net native) you have to edit gatekeeper config manually. At least for F# for sure. I doubt the situation is better for VB (which is literally dead for Microsoft) and C++/CLI.

Also not all developers who use Avalonia have previous UWP/XF/WPF background, besides there are features that specific only to Avalonia already (at least I think so).

From the start Avalonia has better F# support than the rest of .net - ui - frameworks. Providing one alternative synonym for keyword doesn’t sound like a big deal but it allows write more idiomatic code.