[XSG] Fix invalid code generation for Setter with OnPlatform without Default

src/Controls/src/SourceGen/NodeSGExtensions.cs

@@ -548,6 +548,15 @@ public static bool TryProvideValue(this ElementNode node, IndentedTextWriter wri
 		if (GetKnownValueProviders(context).TryGetValue(variable.Type, out var valueProvider)
 			&& valueProvider.TryProvideValue(node, writer, context, getNodeValue, out returnType0, out value))
 		{
+			// Check for "skip this node" sentinel: returnType is null and value is empty
+			// This happens when a Setter has no value (e.g., OnPlatform with no matching platform)
+			if (returnType0 is null && string.IsNullOrEmpty(value))
+			{
+				// Remove from Variables so it won't be added to any collection
+				context.Variables.Remove(node);
+				return true;
+			}
+
 			var variableName = NamingHelpers.CreateUniqueVariableName(context, returnType0 ?? context.Compilation.ObjectType);
 			context.Writer.WriteLine($"var {variableName} = {value};");
 			context.Variables[node] = new LocalVariable(returnType0 ?? context.Compilation.ObjectType, variableName);

src/Controls/src/SourceGen/SetPropertyHelpers.cs

@@ -71,6 +71,10 @@ public static void SetPropertyValue(IndentedTextWriter writer, ILocalValue paren
 			return;
 		}
 
+		// If the node was removed from Variables (e.g., Setter with no value due to OnPlatform), skip silently
+		if (valueNode is ElementNode en && !context.Variables.ContainsKey(en))
+			return;
+
 		var location = LocationCreate(context.ProjectItem.RelativePath!, (IXmlLineInfo)valueNode, localName);
 		context.ReportDiagnostic(Diagnostic.Create(Descriptors.MemberResolution, location, localName));
 	}

src/Controls/src/SourceGen/SetterValueProvider.cs

@@ -16,6 +16,11 @@ public bool CanProvideValue(ElementNode node, SourceGenContext context)
 		// Get the value node (shared logic with TryProvideValue)
 		var valueNode = GetValueNode(node);
 
+		// Must have a value node to provide a value
+		// This can be null when OnPlatform removes the Value property (no matching platform, no Default)
+		if (valueNode == null)
+			return false;
+
 		// Value must be a simple ValueNode (not a MarkupNode or ElementNode)
 		if (valueNode is MarkupNode or ElementNode)
 			return false;
@@ -38,8 +43,11 @@ public bool TryProvideValue(ElementNode node, IndentedTextWriter writer, SourceG
 		var valueNode = GetValueNode(node);
 		if (valueNode == null)
 		{
+			// The value was removed (e.g., OnPlatform with no matching platform and no Default)
+			// Signal to skip this Setter entirely by returning true with null returnType and empty value
+			returnType = null;
 			value = string.Empty;
-			return false;
+			return true;
 		}
 
 		var bpNode = (ValueNode)node.Properties[new XmlName("", "Property")];

src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs

@@ -148,7 +148,9 @@ public void Visit(ElementNode node, INode parentNode)
 			}
 			else if (parentVar.Type.CanAdd(context))
 			{
-				Writer.WriteLine($"{parentVar.ValueAccessor}.Add({Context.Variables[node].ValueAccessor});");
+				// Skip if the node was removed from Variables (e.g., Setter with no value due to OnPlatform)
+				if (Context.Variables.TryGetValue(node, out var nodeVar))
+					Writer.WriteLine($"{parentVar.ValueAccessor}.Add({nodeVar.ValueAccessor});");
 			}
 			else
 			{
@@ -196,8 +198,9 @@ public void Visit(ElementNode node, INode parentNode)
 
 			if (propertyType.CanAdd(context))
 			{
-				Writer.WriteLine($"{variable.ValueAccessor}.Add({Context.Variables[node].ValueAccessor});");
-
+				// Skip if the node was removed from Variables (e.g., Setter with no value due to OnPlatform)
+				if (Context.Variables.TryGetValue(node, out var nodeVar))
+					Writer.WriteLine($"{variable.ValueAccessor}.Add({nodeVar.ValueAccessor});");
 			}
 			else
 			//report diagnostic: not a collection

src/Controls/tests/Xaml.UnitTests/Issues/Maui33676.xaml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Maui33676">
+    <ContentPage.Resources>
+        <!-- This should work when building for iOS -->
+        <Style TargetType="Label" x:Key="iOSOnlyStyle">
+            <Setter Property="Margin" Value="{OnPlatform iOS='0, 4, 0, 0'}" />
+        </Style>
+        <!-- This should also work when building for Android (no matching platform, no Default) -->
+    </ContentPage.Resources>
+    <Label x:Name="label" Style="{StaticResource iOSOnlyStyle}"/>
+</ContentPage>

src/Controls/tests/Xaml.UnitTests/Issues/Maui33676.xaml.cs

@@ -0,0 +1,96 @@
+using System;
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Devices;
+using Xunit;
+
+using static Microsoft.Maui.Controls.Xaml.UnitTests.MockSourceGenerator;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui33676 : ContentPage
+{
+	public Maui33676() => InitializeComponent();
+
+	[Collection("Issue")]
+	public class Tests : IDisposable
+	{
+		MockDeviceInfo mockDeviceInfo;
+
+		public Tests() => DeviceInfo.SetCurrent(mockDeviceInfo = new MockDeviceInfo());
+
+		public void Dispose() => DeviceInfo.SetCurrent(null);
+
+		[Theory]
+		[XamlInflatorData]
+		// BUG: When a Setter uses OnPlatform without a Default value and the target platform 
+		// doesn't match any of the specified platforms, SourceGen generates invalid code:
+		// ((IValueProvider)).ProvideValue(...) - trying to call a method on a type instead of an instance
+		// 
+		// Expected behavior: The Setter should be skipped entirely or use a default value
+		internal void SetterWithOnPlatformWithoutDefaultShouldNotGenerateInvalidCode(XamlInflator inflator)
+		{
+			// Test compiling for Android when OnPlatform only specifies iOS
+			if (inflator == XamlInflator.SourceGen)
+			{
+				var result = CreateMauiCompilation()
+					.WithAdditionalSource(
+"""
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui33676 : ContentPage
+{
+	public Maui33676() => InitializeComponent();
+}
+""")
+					.RunMauiSourceGenerator(typeof(Maui33676), targetFramework: "net10.0-android");
+
+				var generated = result.GeneratedInitializeComponent();
+
+				// BUG: Generated code contains invalid C# like:
+				// ((global::Microsoft.Maui.Controls.Xaml.IValueProvider)).ProvideValue(...)
+				// This causes: CS0119 'IValueProvider' is a type, which is not valid in the given context
+				// This should NOT happen - when OnPlatform has no matching value, the Setter should be omitted
+				Assert.DoesNotContain("((global::Microsoft.Maui.Controls.Xaml.IValueProvider)).ProvideValue", generated, StringComparison.Ordinal);
+
+				Assert.Empty(result.Diagnostics);
+			}
+			else
+			{
+				mockDeviceInfo.Platform = DevicePlatform.Android;
+				var page = new Maui33676(inflator);
+				Assert.NotNull(page);
+			}
+		}
+
+		[Theory]
+		[XamlInflatorData]
+		// When the platform DOES match, the Setter should work correctly
+		internal void SetterWithOnPlatformMatchingPlatform(XamlInflator inflator)
+		{
+			if (inflator == XamlInflator.SourceGen)
+			{
+				var result = CreateMauiCompilation()
+					.WithAdditionalSource(
+"""
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui33676 : ContentPage
+{
+	public Maui33676() => InitializeComponent();
+}
+""")
+					.RunMauiSourceGenerator(typeof(Maui33676), targetFramework: "net10.0-ios");
+
+				Assert.Empty(result.Diagnostics);
+				var generated = result.GeneratedInitializeComponent();
+				Assert.Contains("0, 4, 0, 0", generated, StringComparison.Ordinal);
+			}
+			else
+			{
+				mockDeviceInfo.Platform = DevicePlatform.iOS;
+				var page = new Maui33676(inflator);
+				Assert.NotNull(page);
+			}
+		}
+	}
+}

src/Controls/tests/Xaml.UnitTests/Issues/Maui33676_VisualState.xaml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Maui33676_VisualState">
+    <ContentPage.Resources>
+        <Color x:Key="Gray950">#0D0D0D</Color>
+        <Color x:Key="Gray200">#E0E0E0</Color>
+        <Style TargetType="Button" x:Key="ButtonWithVisualState">
+            <Setter Property="VisualStateManager.VisualStateGroups">
+                <VisualStateGroupList>
+                    <VisualStateGroup x:Name="CommonStates">
+                        <VisualState x:Name="Normal" />
+                        <VisualState x:Name="Disabled">
+                            <VisualState.Setters>
+                                <Setter Property="TextColor" Value="{AppThemeBinding Light={StaticResource Gray950}, Dark={StaticResource Gray200}}" />
+                                <Setter Property="BackgroundColor" Value="{OnPlatform Android='Red'}" />
+                            </VisualState.Setters>
+                        </VisualState>
+                    </VisualStateGroup>
+                </VisualStateGroupList>
+            </Setter>
+        </Style>
+    </ContentPage.Resources>
+    <Button x:Name="button" Style="{StaticResource ButtonWithVisualState}"/>
+</ContentPage>

src/Controls/tests/Xaml.UnitTests/Issues/Maui33676_VisualState.xaml.cs

@@ -0,0 +1,61 @@
+using System;
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Devices;
+using Xunit;
+
+using static Microsoft.Maui.Controls.Xaml.UnitTests.MockSourceGenerator;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui33676_VisualState : ContentPage
+{
+	public Maui33676_VisualState() => InitializeComponent();
+
+	[Collection("Issue")]
+	public class Tests : IDisposable
+	{
+		MockDeviceInfo mockDeviceInfo;
+
+		public Tests() => DeviceInfo.SetCurrent(mockDeviceInfo = new MockDeviceInfo());
+
+		public void Dispose() => DeviceInfo.SetCurrent(null);
+
+		[Theory]
+		[XamlInflatorData]
+		// When a Setter inside VisualState uses OnPlatform without a Default value and the target platform 
+		// doesn't match, SourceGen should handle it gracefully
+		internal void SetterInVisualStateWithOnPlatformWithoutDefaultShouldNotGenerateInvalidCode(XamlInflator inflator)
+		{
+			// Test compiling for MacCatalyst when OnPlatform only specifies Android
+			if (inflator == XamlInflator.SourceGen)
+			{
+				var result = CreateMauiCompilation()
+					.WithAdditionalSource(
+"""
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui33676_VisualState : ContentPage
+{
+	public Maui33676_VisualState() => InitializeComponent();
+}
+""")
+					.RunMauiSourceGenerator(typeof(Maui33676_VisualState), targetFramework: "net10.0-maccatalyst");
+
+				var generated = result.GeneratedInitializeComponent();
+
+				// Check that we're NOT generating code that tries to add object to IList<Setter>
+				// This would happen if IValueProvider.ProvideValue() is called on a Setter with no value
+				Assert.DoesNotContain("((global::Microsoft.Maui.Controls.Xaml.IValueProvider)).ProvideValue", generated, StringComparison.Ordinal);
+
+				// Should not have any compilation errors
+				Assert.Empty(result.Diagnostics);
+			}
+			else
+			{
+				mockDeviceInfo.Platform = DevicePlatform.macOS;
+				var page = new Maui33676_VisualState(inflator);
+				Assert.NotNull(page);
+			}
+		}
+	}
+}