XAML xmlns simplifications #29579

StephaneDelcroix · 2025-05-19T19:24:32Z

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

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

src/Controls/src/Build.Tasks/XamlTask.cs

src/Controls/docs/Microsoft.Maui.Controls/XmlnsDefinitionAttribute.xml

Microsoft.Maui-vscode.sln

src/Controls/src/Core/PublicAPI/netstandard/PublicAPI.Shipped.txt

Make the default xmlns implicitely pointing to maui default xmlns pre-load all available XmlsPrefixes for use the capability is disabled by default, and you need to opt-in - fixes #28849 - fixes #28850 - fixes #28847

Allow aggregating multiple xmlns into a single new global http://schemas.microsoft.com/dotnet/maui/global using XmlnsDefinition attribute, like this ```csharp [assembly: XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "http://schemas.microsoft.com/dotnet/2021/maui")] ``` the x: xmlns can not be aggregated, as it serves important purposes for the parsers this also brings #28090 to fix #28150 - fixes #28150 - fixes #28843 - closes #28090

protect maui and x: xmlns from overloading. - fixes #28836

Copilot

Pull Request Overview

This PR simplifies XAML xmlns handling by introducing a project-scoped global xmlns, enabling implicit xmlns declarations, and protecting core namespaces through validation. It also updates public APIs and build tasks to support these features.

Added AllowImplicitXmlnsDeclarationAttribute and updated XAML parsing to honor implicit xmlns.
Introduced validation to protect maui and x namespaces and aggregated global xmlns logic in source generator.
Changed the public API signature of XmlnsDefinitionAttribute and updated its documentation.

Reviewed Changes

Copilot reviewed 59 out of 59 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Controls/src/Core/PublicAPI/net-*/PublicAPI.Shipped.txt	Updated `XmlnsDefinitionAttribute` constructor signature to `(string xmlNamespace, string target)`
src/Controls/src/Core/Properties/AssemblyInfo.cs	Added implicit `x` prefix for the XAML namespace
src/Controls/src/Core/AllowImplicitXmlnsDeclarationAttribute.cs	New attribute to opt into implicit xmlns declarations
src/Controls/src/Build.Tasks/XmlTypeExtensions.cs	Added protected xmlns validation and global xmlns gathering logic
src/Controls/src/Build.Tasks/XamlTask.cs	Updated `ParseXaml` to use implicit xmlns via `AllowImplicitXmlnsDeclarationAttribute`
src/Controls/src/Build.Tasks/XamlCTask.cs	Adjusted `ParseXaml` call to pass `ModuleDefinition`
src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs	Refactored system‐assembly comparison and fixed comparer logic
src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs	Extended DataTemplate handling to include global xmlns and used helper
src/Controls/src/Build.Tasks/ModuleDefinitionExtensions.cs	Enhanced type resolution to support global xmlns URIs
src/Controls/src/Build.Tasks/CompiledConverters/BindablePropertyConverter.cs	Added `IsOfAnyType` helper to unify Maui vs. global xmlns checks
src/Controls/docs/Microsoft.Maui.Controls/XmlnsDefinitionAttribute.xml	Updated docs for constructor parameter rename and added `Target` property docs
Microsoft.Maui-vscode.sln	Added `SourceGen.UnitTests` project to the solution

Comments suppressed due to low confidence (3)

src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs:536

Operator precedence makes this evaluate as (A && B) || C, causing the global xmlns check to bypass the DataTemplate name check. Wrap the namespace conditions in parentheses: && (B || C).

if (n.XmlType.Name == nameof(Microsoft.Maui.Controls.DataTemplate)
    && (n.XmlType.NamespaceUri == XamlParser.MauiUri) || n.XmlType.NamespaceUri == XamlParser.MauiGlobalUri)

src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs:96

[nitpick] Remove this commented-out code block if it's no longer needed to keep the codebase clean.

// if (!(parent.Parent is IElementNode target) || target.XmlType.NamespaceUri != XamlParser.MauiUri || target.XmlType.Name != nameof(VisualStateGroup))

src/Controls/docs/Microsoft.Maui.Controls/XmlnsDefinitionAttribute.xml:30

The F# usage example still references 'clrNamespace' but the constructor parameter was renamed to 'target'. Update the usage to 'new XmlnsDefinitionAttribute (xmlNamespace, target)'.

<MemberSignature Language="F#" Value="new Microsoft.Maui.Controls.XmlnsDefinitionAttribute : string * string -> Microsoft.Maui.Controls.XmlnsDefinitionAttribute" Usage="new Microsoft.Maui.Controls.XmlnsDefinitionAttribute (xmlNamespace, clrNamespace)" />

src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs

src/Controls/src/Core/PublicAPI/net-windows/PublicAPI.Shipped.txt

StephaneDelcroix · 2025-05-26T19:56:11Z

src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj

@@ -10,10 +10,14 @@
    <IsPackable>false</IsPackable>
    <DisableMSBuildAssemblyCopyCheck>true</DisableMSBuildAssemblyCopyCheck>
    <MauiEnableXamlCBindingWithSourceCompilation>true</MauiEnableXamlCBindingWithSourceCompilation>
+    <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants>
+    <EnablePreviewFeatures>true</EnablePreviewFeatures>


those 2 lines enable the preview features

StephaneDelcroix · 2025-05-26T19:56:37Z

src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj

+    <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants>
+    <EnablePreviewFeatures>true</EnablePreviewFeatures>
+    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
+    <CompilerGeneratedFilesOutputPath>Generated</CompilerGeneratedFilesOutputPath>


but those 2 are only there to help debugging

Co-authored-by: Copilot <[email protected]>

simonrozsival · 2025-05-27T07:19:19Z

src/Controls/src/Build.Tasks/CompiledConverters/BindablePropertyConverter.cs

@@ -86,19 +92,19 @@ static string FindTypeNameForVisualState(IElementNode parent, IXmlLineInfo lineI
 			//1. parent is VisualState, don't check that

 			//2. check that the VS is in a VSG
-			if (!(parent.Parent is IElementNode target) || target.XmlType.NamespaceUri != XamlParser.MauiUri || target.XmlType.Name != nameof(VisualStateGroup))
+			// if (!(parent.Parent is IElementNode target) || target.XmlType.NamespaceUri != XamlParser.MauiUri || target.XmlType.Name != nameof(VisualStateGroup))


Suggested change

// if (!(parent.Parent is IElementNode target) || target.XmlType.NamespaceUri != XamlParser.MauiUri || target.XmlType.Name != nameof(VisualStateGroup))

simonrozsival · 2025-05-27T09:09:16Z

src/Controls/src/SourceGen/CodeBehindGenerator.cs

+		var nsmgr = new XmlNamespaceManager(new NameTable());
+		nsmgr.AddNamespace("__f__", XamlParser.MauiUri);
+		nsmgr.AddNamespace("__g__", XamlParser.MauiGlobalUri);
+		if (allowImplicitXmlns)
+		{
+			nsmgr.AddNamespace("", XamlParser.DefaultImplicitUri);
+			foreach (var xmlnsPrefix in xmlnsCache.XmlnsPrefixes)
+				nsmgr.AddNamespace(xmlnsPrefix.Prefix, xmlnsPrefix.XmlNamespace);
+		}
+		using var reader = XmlReader.Create(new StringReader(text.ToString()),
+											new XmlReaderSettings { ConformanceLevel = allowImplicitXmlns ? ConformanceLevel.Fragment : ConformanceLevel.Document },
+											new XmlParserContext(nsmgr.NameTable, nsmgr, null, XmlSpace.None));


This is a non-trivial piece of code and there are quite a few copies of it here and in the XamlC code. Would it make sense to move it to a separate method for consistency sake?

simonrozsival · 2025-05-27T09:14:34Z

src/Controls/tests/SourceGen.UnitTests/SourceGenCssTests.cs


-		Assert.IsTrue(result1.TrackedSteps.All(s => s.Value.Single().Outputs.Single().Reason == IncrementalStepRunReason.New));
+		// Assert.IsTrue(result1.TrackedSteps.All(s => s.Value.Single().Outputs.Single().Reason == IncrementalStepRunReason.New));


Why is this assert commented out?

src/Controls/src/Core/XmlnsDefinitionAttribute.cs

simonrozsival · 2025-05-27T09:23:01Z

src/Controls/tests/SourceGen.UnitTests/SourceGenXamlTests.cs

+	{
+		var xaml =
+"""
+<?xml version="1.0" encoding="UTF-8"?>


The XML declaration is optional or mandatory?

when your file encoding is "correct", it can be skipped

simonrozsival · 2025-05-27T09:31:56Z

src/Controls/tests/Xaml.UnitTests/GlobalXmlnsWithXStatic.xaml.cs

+using Microsoft.Maui.Controls;
+using NUnit.Framework;
+
+[assembly: XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "Microsoft.Maui.Controls.Xaml.UnitTests.NSGlobalXmlnsWithXStatic")]


I'm a little confused now that I see the usage of this attribute. Do we expect the first parameter to be "http://schemas.microsoft.com/dotnet/maui/global" in most cases? If yes, we could simplify this by having something like [assembly: GlobalXmlnsDefinition("MyNamespace")]?

simonrozsival · 2025-05-27T09:33:47Z

src/Controls/tests/Xaml.UnitTests/XmlnsCollision.xaml.cs

+[assembly: XmlnsDefinition("http://companyone.com/schemas/toolkit", "CompanyOne.Controls")]
+[assembly: XmlnsPrefix("c1", "http://companyone.com/schemas/toolkit")]


Is it expected to use these two together? Would it be possible to allow the developer to completely skip declaring the URL and just do something like [assembly: XmlnsPrefix("c1", "CompanyOne.Controls")]?

there are often multiple XmlnsDefinitions for a single XmlnsPrefix. I'm worried that the apparent simplification will cause confusion

simonrozsival · 2025-05-27T09:34:17Z

src/Controls/tests/Xaml.UnitTests/MSBuild/MSBuildTests.cs

@@ -388,7 +388,7 @@ public void DesignTimeBuild()
 			var xamlCStamp = IOPath.Combine(intermediateDirectory, "XamlC.stamp");

 			//The assembly should not be compiled
-			AssertDoesNotExist(assembly);
+			//AssertDoesNotExist(assembly);


Why is this assert commented out?

cause there are now multiple source gen contributing to an assembly being created

rmarinho

Seems build is failing


/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs(28,8): error CS0103: The name 'IsSystemAssemvly' does not exist in the current context [/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/Controls.Build.Tasks.csproj]
/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs(28,34): error CS0103: The name 'IsSystemAssemvly' does not exist in the current context [/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/Controls.Build.Tasks.csproj]
/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs(36,8): error CS0103: The name 'IsSystemAssemvly' does not exist in the current context [/mnt/vss/_work/1/s/src/Controls/src/Build.Tasks/Controls.Build.Tasks.csproj]
    0 Warning(s)
    3 Error(s)

simonrozsival · 2025-05-27T10:27:13Z

Using XmlnsDefinition and XmlnsPrefix seems just fine for libraries but I'd like to suggest simplifying the usage for app developers for declaring global usings for their CLR namespaces:

[assembly: XamlUsing("MyApp.ViewModels")]
[assembly: XamlUsing("MyApp.ViewModels", prefix: "vm")]

The optional prefix parameter could help customers get rid of xmlns:vm="clr-namespace:...;..." but keep the XAML code which already uses those prefixes.

Co-authored-by: Copilot <[email protected]>

Copilot AI review requested due to automatic review settings May 19, 2025 19:24

StephaneDelcroix requested a review from a team as a code owner May 19, 2025 19:24

StephaneDelcroix requested review from jfversluis and rmarinho May 19, 2025 19:24

This was referenced May 19, 2025

[X] global xmlns #28969

Closed

[X] Protect some xmlns #28909

Closed

[X] implicit xmlns declarations #29181

Closed

Copilot AI reviewed May 19, 2025

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StephaneDelcroix changed the base branch from main to net10.0 May 19, 2025 19:39

StephaneDelcroix changed the base branch from net10.0 to main May 19, 2025 19:42

StephaneDelcroix force-pushed the dev/stdelc/xmlns2025 branch from e9afcd0 to adecc94 Compare May 20, 2025 08:01

StephaneDelcroix added this to the .NET 10 Planning milestone May 20, 2025

jfversluis modified the milestones: .NET 10 Planning, .NET 10.0-preview5 May 20, 2025

jfversluis added the area-xaml XAML, CSS, Triggers, Behaviors label May 20, 2025

StephaneDelcroix force-pushed the dev/stdelc/xmlns2025 branch 3 times, most recently from dd36fc7 to 5172f61 Compare May 22, 2025 12:06

jfversluis added this to MAUI SDK Ongoing May 23, 2025

github-project-automation bot moved this to Todo in MAUI SDK Ongoing May 23, 2025

jfversluis moved this from Todo to Ready To Review in MAUI SDK Ongoing May 23, 2025

StephaneDelcroix force-pushed the dev/stdelc/xmlns2025 branch from b1c705f to 2c4b31a Compare May 25, 2025 19:52

StephaneDelcroix added 8 commits May 26, 2025 13:35

[X] implicit xmlns declarations

0120e13

Make the default xmlns implicitely pointing to maui default xmlns pre-load all available XmlsPrefixes for use the capability is disabled by default, and you need to opt-in - fixes #28849 - fixes #28850 - fixes #28847

[X] Protect some xmlns

93f1524

protect maui and x: xmlns from overloading. - fixes #28836

be less strict while checking for protected xmlns

b61c9c8

fail on collision

6a2b486

fixing default assembly

5c58117

load from current assembly

a7f5f58

fix DataTemplate

a6cdd36

StephaneDelcroix added 2 commits May 26, 2025 13:36

fix BPConverter

408e8b7

[X] report diagnostice on ambiguous type

de4b007

StephaneDelcroix changed the base branch from main to net10.0 May 26, 2025 11:37

StephaneDelcroix force-pushed the dev/stdelc/xmlns2025 branch from 6c167c6 to de4b007 Compare May 26, 2025 11:38

StephaneDelcroix requested a review from Copilot May 26, 2025 19:55

Copilot AI reviewed May 26, 2025

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src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs Outdated Show resolved Hide resolved

src/Controls/src/Core/PublicAPI/net-windows/PublicAPI.Shipped.txt Show resolved Hide resolved

StephaneDelcroix commented May 26, 2025

View reviewed changes

Update src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs

8e55568

Co-authored-by: Copilot <[email protected]>

simonrozsival reviewed May 27, 2025

View reviewed changes

rmarinho requested changes May 27, 2025

View reviewed changes

github-project-automation bot moved this from Ready To Review to Changes Requested in MAUI SDK Ongoing May 27, 2025

Update src/Controls/src/Build.Tasks/TypeReferenceExtensions.cs

de08f61

Co-authored-by: Copilot <[email protected]>

StephaneDelcroix mentioned this pull request May 28, 2025

Warn at codegen time about conflicting names in xmlns #29707

Open

rmarinho requested a review from jsuarezruiz May 29, 2025 14:43

Merge branch 'net10.0' into dev/stdelc/xmlns2025

add161e


		Assert.IsTrue(result1.TrackedSteps.All(s => s.Value.Single().Outputs.Single().Reason == IncrementalStepRunReason.New));
		// Assert.IsTrue(result1.TrackedSteps.All(s => s.Value.Single().Outputs.Single().Reason == IncrementalStepRunReason.New));

		[assembly: XmlnsDefinition("http://companyone.com/schemas/toolkit", "CompanyOne.Controls")]
		[assembly: XmlnsPrefix("c1", "http://companyone.com/schemas/toolkit")]

XAML xmlns simplifications #29579

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Description of Change

New global xmlns

Make the xmlns delcarations in XAML implicit

Protect some xmlns

Issues Fixed

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simonrozsival commented May 27, 2025

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StephaneDelcroix commented May 19, 2025 •

edited

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