[XSG] Fix Setter.Value property element lookup to be namespace-agnostic

[StephaneDelcroix]

Note

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It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description

Fixes #34039

When <Setter.Value> is used as a property element, GetValueNode() in SetterValueProvider.cs looked up the Value property only by specific namespace URIs ("" and MauiUri). When the property element used a different namespace URI, the lookup returned null. Since PR #33681 changed the null-return behavior to a skip sentinel, this caused the Setter to be removed from Variables entirely, preventing the .Add() call from being generated.

Root Cause

GetValueNode() checked only two specific namespace URIs:

node.Properties.TryGetValue(new XmlName("", "Value"), out valueNode)
node.Properties.TryGetValue(new XmlName(XamlParser.MauiUri, "Value"), out valueNode)

Property elements like <Setter.Value> inherit their namespace URI from the XML reader, which varies depending on the xmlns declaration used. When neither URI matched, GetValueNode() returned null, triggering the skip sentinel introduced in #33681 (for the OnPlatform case), which removed the Setter from Variables and suppressed the Setters.Add() call.

Fix

Made GetValueNode() namespace-agnostic by iterating all properties and matching on LocalName == "Value":

foreach (var prop in node.Properties)
{
    if (prop.Key.LocalName == "Value")
        return prop.Value;
}

Testing

• Added SetterValueInTrigger unit test that reproduces the issue
• Test fails without fix, passes with fix
• Full SourceGen test suite: 189/192 pass (3 pre-existing AppThemeBinding failures unrelated to this change)

[Copilot]

Pull request overview

This PR fixes a .NET MAUI XAML Source Generator (XSG) regression where Setter instances using the <Setter.Value> property element could be dropped (and therefore not added to Trigger.Setters) when implicit/alternative xmlns declarations caused the Value property element’s namespace URI to differ from the previously hard-coded lookups.

Changes:

• Update SetterValueProvider.GetValueNode() to locate the Value property element by LocalName == "Value" regardless of namespace URI.
• Add a SourceGen unit test reproducing the issue with implicit xmlns and verifying the generated code contains the Setters.Add(...) call.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Controls/src/SourceGen/SetterValueProvider.cs	Makes <Setter.Value> lookup namespace-agnostic so XSG doesn’t drop setters when property-element namespaces vary.
src/Controls/tests/SourceGen.UnitTests/InitializeComponent/SetterValueInTrigger.cs	Adds a regression test covering <Setter.Value> inside a Trigger under implicit xmlns conditions.