XAML xmlns simplifications

[StephaneDelcroix]

Description of Change

This PR includes multiple changes related to xmlns, as tracked by #28836.

New global xmlns

http://schemas.microsoft.com/dotnet/maui/global is a new xmlns that you can use to aggregate multiple xmlns together.

The scope of it is local to the project.

By default, it contains the maui xmlns (sourcegenerated) YourNamespace and YourNamespace.Pages (enabled by the updated template.

You can extend this in e.g. GlobalXmlns.cs added by the template (#29203), or adding assembly level XmlnsDefinition anywhere.

You can ad CLR namespaces as usual, but also other xmlns

[assembly:XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "http://mycompany.com/schema/myxmlns")]

That global xmlns also have a XmlnsPrefix "global" associated to it.

IMPORTANT NOTE: you can not aggregate the x xmlns into the global one. It plays a special role for the parsers and inflators.

This change is implemented using a source generator, so there's no requirement to change any other tool consuming xaml making use of this

Make the xmlns delcarations in XAML implicit

The XAML inflators uses the global xmlns as default, and preloads all other xmlns declared by a XmlnsDefinition that also have a XmlnsPrefix attribute. e.g. the x: prefix is there to be used without the need of being declared.

<ContentPage x:Class="MyNamespace.MyPage" Title="no default xmlns">
    <VerticalStackLayout>
        <Label x:Name="label" Text="No xmlns:x declaration, but x: usage anyway" />
    </VerticalStackLayout>
</ContentPage>

This part is opt-in, you can enable it by setting

  <PropertyGroup>
    <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants>
    <EnablePreviewFeatures>true</EnablePreviewFeatures>
  </PropertyGroup>

in your .csproj

Protect some xmlns

with a fix for #28150, it becomes necessary to avoid the overload of the maui, and x, xmlns namespaces.

Issues Fixed

• fixes make the default xmlns declaration optional #28849

• fixes pre-defined xmlns prefixes #28850

• fixes Implicit global xmlns #28847

• fixes XAML xmlns simplifications #28836

• fixes XmlnsDefinitonAttribute is not supported for "this" assembly #28150

• fixes xmlns aggregation #28843

• fixes protect maui xmlns from overloading #28839

• closes Support XmlnsDefinitionAttribute in "this" assembly #28090

• closes [X] implicit xmlns declarations #29181

• closes [X] global xmlns #28969

• closes [X] Protect some xmlns #28909

[Copilot]

Pull Request Overview

Adds a new global XAML xmlns, enables implicit xmlns declarations via a preview feature flag, protects built-in xmlns entries, and updates related source generators and metadata.

• Introduces AllowImplicitXmlnsDeclarationAttribute and wiring in build tasks to support implicit/default xmlns usage.
• Changes XmlnsDefinitionAttribute constructor parameter name and surfaces a new Target property.
• Updates public API shipped/unshipped files, documentation, and solution to expose new types and behaviors.

Reviewed Changes

Copilot reviewed 42 out of 42 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
PublicAPI/netstandard/PublicAPI.Shipped.txt	Updated XmlnsDefinitionAttribute ctor signature
PublicAPI/net/*.Unshipped.txt & *.Shipped.txt (all platforms)	Added AllowImplicitXmlnsDeclarationAttribute and Target API
Properties/AssemblyInfo.cs	Re-added x: xmlns prefix for XAML
AllowImplicitXmlnsDeclarationAttribute.cs	New attribute to opt into implicit xmlns support
Build.Tasks/XmlTypeExtensions.cs	Extended XML namespace gathering and added prefix extraction
Build.Tasks/XamlTask.cs	Wired implicit xmlns logic into XAML parsing
Build.Tasks/XamlCTask.cs	Updated XAML compilation to pass module context
docs/XmlnsDefinitionAttribute.xml	Updated ctor sig in docs and added Target member
Microsoft.Maui-vscode.sln	Included new SourceGen unit test project

Comments suppressed due to low confidence (1)

src/Controls/src/Build.Tasks/XmlTypeExtensions.cs:45

• The new logic protecting built-in xmlns definitions and throwing BuildException on invalid entries lacks corresponding unit tests. Please add tests covering both allowed and prohibited XmlnsDefinitionAttribute scenarios to validate this behavior.

if (   attribute.XmlNamespace != XamlParser.MauiGlobal