[XSG] Pre-compute x:DataType with PropagateDataTypeVisitor

src/Controls/src/Build.Tasks/BuildException.cs

@@ -81,6 +81,7 @@ class BuildExceptionCode
 		public static BuildExceptionCode NamescopeDuplicate = new BuildExceptionCode("XC", 0064, nameof(NamescopeDuplicate), "");
 		public static BuildExceptionCode ContentPropertyAttributeMissing = new BuildExceptionCode("XC", 0065, nameof(ContentPropertyAttributeMissing), "");
 		public static BuildExceptionCode InvalidXaml = new BuildExceptionCode("XC", 0066, nameof(InvalidXaml), "");
+		public static BuildExceptionCode DuplicatePropertyAssignment = new BuildExceptionCode("XC", 0067, nameof(DuplicatePropertyAssignment), ""); //warning
 
 
 		//Extensions

src/Controls/src/Build.Tasks/ErrorMessages.resx

@@ -270,8 +270,12 @@
   </data>
   <data name="StaticResourceSyntax" xml:space="preserve">
     <value>A key is required in {StaticResource}.</value>
-  </data>  
+  </data>
   <data name="XSharedNotSupported" xml:space="preserve">
     <value>x:Shared is only supported with SourceGen inflator. Remove the attribute or switch to SourceGen.</value>
   </data>
+  <data name="DuplicatePropertyAssignment" xml:space="preserve">
+    <value>Property '{0}' is being set multiple times. Only the last value will be used.</value>
+    <comment>0 is property name (e.g. "Border.Content" or "Label.Text")</comment>
+  </data>
 </root>

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

@@ -43,6 +43,31 @@ public bool IsResourceDictionary(ElementNode node)
 
 		ModuleDefinition Module { get; } = context.Body.Method.Module;
 
+		// Track properties that have been set to detect duplicates
+		readonly Dictionary<ElementNode, HashSet<XmlName>> setProperties = new Dictionary<ElementNode, HashSet<XmlName>>();
+
+		void CheckForDuplicateProperty(ElementNode parentNode, XmlName propertyName, IXmlLineInfo lineInfo)
+		{
+			if (!setProperties.TryGetValue(parentNode, out var props))
+			{
+				props = new HashSet<XmlName>();
+				setProperties[parentNode] = props;
+			}
+
+			if (!props.Add(propertyName))
+			{
+				// Property is being set multiple times
+				Context.LoggingHelper.LogWarningOrError(
+					DuplicatePropertyAssignment,
+					Context.XamlFilePath,
+					lineInfo.LineNumber,
+					lineInfo.LinePosition,
+					0,
+					0,
+					$"{parentNode.XmlType.Name}.{propertyName.LocalName}");
+			}
+		}
+
 		public void Visit(ValueNode node, INode parentNode)
 		{
 			//TODO support Label text as element
@@ -69,6 +94,12 @@ public void Visit(ValueNode node, INode parentNode)
 				return;
 			if (propertyName.Equals(XmlName.mcIgnorable))
 				return;
+
+			// TODO: Add duplicate implicit content property checking here to match SourceGen behavior
+			// (SourceGen Visit(ValueNode) emits MAUIX2015 when a ValueNode assigns the same implicit
+			// content property that was already assigned by an ElementNode or another ValueNode).
+			// Without this check, <Border>text<Label/></Border> produces MAUIX2015 under SourceGen
+			// but no warning under XamlC.
 			Context.IL.Append(SetPropertyValue(Context.Variables[(ElementNode)parentNode], propertyName, node, Context, node));
 		}
 
@@ -140,8 +171,32 @@ public void Visit(ElementNode node, INode parentNode)
 					var name = new XmlName(node.NamespaceURI, contentProperty);
 					if (skips.Contains(name))
 						return;
-					if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(propertyName))
+					if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(name))
 						return;
+
+					// Resolve the content property type to determine if it is a collection.
+					// Use CanGet (CLR property resolution) rather than GetBindablePropertyReference+GetValue/Get
+					// to avoid side effects: GetBindablePropertyReference can emit ObsoleteProperty diagnostics
+					// and GetValue/Get generate IL instructions; both would be duplicated by SetPropertyValue.
+					// CanGet covers all content properties because every BP-backed property has a CLR wrapper.
+					var propLocalName = name.LocalName;
+					bool canResolveProperty = CanGet(parentVar, propLocalName, Context, out TypeReference contentPropType);
+
+					// Skip duplicate check when the property cannot be resolved, is System.Object (unresolved
+					// generics would produce false positives), or is a collection type. Matches SourceGen behavior.
+					if (canResolveProperty && contentPropType != null)
+					{
+						bool isObject = contentPropType.FullName == "System.Object";
+						bool isCollection = !isObject
+							&& contentPropType.ImplementsInterface(Context.Cache, Module.ImportReference(Context.Cache, ("mscorlib", "System.Collections", "IEnumerable")))
+							&& contentPropType.GetMethods(Context.Cache, md => md.Name == "Add" && md.Parameters.Count == 1, Module).Any();
+
+						// Use parent namespace for duplicate tracking to match how explicit property assignments are keyed
+						var contentPropertyName = new XmlName(((ElementNode)parentNode).NamespaceURI, contentProperty);
+						if (!isCollection && !isObject)
+							CheckForDuplicateProperty((ElementNode)parentNode, contentPropertyName, node);
+					}
+
 					Context.IL.Append(SetPropertyValue(Context.Variables[(ElementNode)parentNode], name, node, Context, node));
 				}
 				// Collection element, implicit content, or implicit collection element.
@@ -539,7 +594,7 @@ static bool TryCompileBindingPath(ElementNode node, ILContext context, VariableD
 				// continue compilation
 			}
 
-			if (   dataTypeNode is ElementNode enode
+			if (dataTypeNode is ElementNode enode
 				&& enode.XmlType.NamespaceUri == XamlParser.X2009Uri
 				&& enode.XmlType.Name == nameof(Xaml.NullExtension))
 			{

src/Controls/src/SourceGen/AnalyzerReleases.Unshipped.md

@@ -30,3 +30,4 @@ MAUIX2010 | XamlParsing | Info | ExpressionNotSettable
 MAUIX2011 | XamlParsing | Warning | AmbiguousMemberWithStaticType
 MAUIX2012 | XamlParsing | Error | CSharpExpressionsRequirePreviewFeatures
 MAUIX2013 | XamlParsing | Error | AsyncLambdaNotSupported
+MAUIX2015 | XamlInflation | Warning | DuplicatePropertyAssignment

src/Controls/src/SourceGen/BindingContextDataType.cs

@@ -0,0 +1,36 @@
+using Microsoft.CodeAnalysis;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Microsoft.Maui.Controls.SourceGen;
+
+/// <summary>
+/// Distinguishes between different states of x:DataType resolution on a node.
+/// </summary>
+enum BindingContextDataTypeKind
+{
+	/// <summary>x:DataType resolved to a known type.</summary>
+	Resolved,
+
+	/// <summary>x:DataType="{x:Null}" — explicit opt-out of compiled bindings for this scope.</summary>
+	ExplicitNull,
+
+	/// <summary>x:DataType is present but couldn't be resolved (diagnostic already reported).</summary>
+	Unresolved,
+}
+
+/// <summary>
+/// Represents the resolved x:DataType for a node in the XAML tree.
+/// Pre-computed by <see cref="PropagateDataTypeVisitor"/> and stored in
+/// <see cref="SourceGenContext.BindingContextDataTypes"/>.
+/// </summary>
+/// <param name="Kind">The resolution state.</param>
+/// <param name="Symbol">The resolved type symbol. Non-null only when <paramref name="Kind"/> is <see cref="BindingContextDataTypeKind.Resolved"/>.</param>
+/// <param name="Origin">The node where x:DataType was declared (for diagnostics). Null when inherited from a parent context.</param>
+/// <param name="IsInherited">True if the data type was inherited from an ancestor, false if declared on this node.</param>
+/// <param name="CrossedTemplateBoundary">True if the data type was inherited across a DataTemplate boundary (likely a bug).</param>
+record BindingContextDataType(
+	BindingContextDataTypeKind Kind,
+	ITypeSymbol? Symbol,
+	INode? Origin,
+	bool IsInherited,
+	bool CrossedTemplateBoundary);

src/Controls/src/SourceGen/CompiledBindingMarkup.cs

@@ -29,15 +29,17 @@ public CompiledBindingMarkup(ElementNode node, string path, ILocalValue bindingE
 	private Location GetLocation(INode node, string? context = null)
 		=> LocationHelpers.LocationCreate(_context.ProjectItem.RelativePath!, (IXmlLineInfo)node, context ?? "Binding");
 
-	public bool TryCompileBinding(ITypeSymbol sourceType, bool isTemplateBinding, out string? newBindingExpression)
+	public bool TryCompileBinding(ITypeSymbol sourceType, bool isTemplateBinding, out string? newBindingExpression, out Diagnostic? propertyNotFoundDiagnostic)
 	{
 		newBindingExpression = null;
+		propertyNotFoundDiagnostic = null;
 
 		if (!TryParsePath(
 			sourceType,
 			out ITypeSymbol? propertyType,
 			out SetterOptions? setterOptions,
-			out EquatableArray<IPathPart>? parsedPath)
+			out EquatableArray<IPathPart>? parsedPath,
+			out propertyNotFoundDiagnostic)
 			|| propertyType is null
 			|| setterOptions is null
 			|| !parsedPath.HasValue)
@@ -261,11 +263,13 @@ bool TryParsePath(
 		ITypeSymbol sourceType,
 		out ITypeSymbol? propertyType,
 		out SetterOptions? setterOptions,
-		out EquatableArray<IPathPart>? bindingPath)
+		out EquatableArray<IPathPart>? bindingPath,
+		out Diagnostic? propertyNotFoundDiagnostic)
 	{
 		propertyType = sourceType;
 		setterOptions = null;
 		bindingPath = default;
+		propertyNotFoundDiagnostic = null;
 
 		var isNullable = false;
 		var path = _path.Trim('.', ' ');
@@ -310,10 +314,9 @@ bool TryParsePath(
 				if (!previousPartType.TryGetProperty(p, _context, out var property, out var currentPropertyType, out var isAssumedWritable) 
 					|| currentPropertyType is null)
 				{
-					// Report as WARNING (not Error) because the property might be generated by another source generator
-					// that runs after the MAUI XAML source generator. The binding will fall back to reflection at runtime.
-					// This allows scenarios like CommunityToolkit.Mvvm's [ObservableProperty] to work correctly.
-					_context.ReportDiagnostic(Diagnostic.Create(Descriptors.BindingPropertyNotFound, GetLocation(_node), p, previousPartType.ToFQDisplayString()));
+					// Don't emit directly — let the caller decide whether to report this.
+					// For x:Reference bindings, the caller silently falls back to runtime.
+					propertyNotFoundDiagnostic = Diagnostic.Create(Descriptors.BindingPropertyNotFound, GetLocation(_node), p, previousPartType.ToFQDisplayString());
 					return false;
 				}
 

src/Controls/src/SourceGen/Descriptors.cs

@@ -325,6 +325,14 @@ public static class Descriptors
 			defaultSeverity: DiagnosticSeverity.Error,
 			isEnabledByDefault: true);
 
+		public static DiagnosticDescriptor DuplicatePropertyAssignment = new DiagnosticDescriptor(
+			id: "MAUIX2015",
+			title: new LocalizableResourceString(nameof(MauiGResources.DuplicatePropertyAssignmentTitle), MauiGResources.ResourceManager, typeof(MauiGResources)),
+			messageFormat: new LocalizableResourceString(nameof(MauiGResources.DuplicatePropertyAssignmentMessage), MauiGResources.ResourceManager, typeof(MauiGResources)),
+			category: "XamlInflation",
+			defaultSeverity: DiagnosticSeverity.Warning,
+			isEnabledByDefault: true);
+
 		// public static BuildExceptionCode TypeResolution = new BuildExceptionCode("XC", 0000, nameof(TypeResolution), "");
 		// public static BuildExceptionCode PropertyResolution = new BuildExceptionCode("XC", 0001, nameof(PropertyResolution), "");
 		// public static BuildExceptionCode MissingEventHandler = new BuildExceptionCode("XC", 0002, nameof(MissingEventHandler), "");

src/Controls/src/SourceGen/InitializeComponentCodeWriter.cs

@@ -130,7 +130,8 @@ PrePost newblock() =>
 			return;
 		}
 
-""");
+"""
+);
 					}
 					Visit(root, sgcontext);
 
@@ -176,6 +177,7 @@ static void Visit(RootNode rootnode, SourceGenContext visitorContext, bool useDe
 		rootnode.Accept(new CreateValuesVisitor(visitorContext), null);
 		rootnode.Accept(new SetNamescopesAndRegisterNamesVisitor(visitorContext), null); //set namescopes for {x:Reference} and FindByName
 		rootnode.Accept(new SetFieldsForXNamesVisitor(visitorContext), null);
+		rootnode.Accept(new PropagateDataTypeVisitor(visitorContext), null);
 		// rootnode.Accept(new FillResourceDictionariesVisitor(visitorContext), null);
 		rootnode.Accept(new SetResourcesVisitor(visitorContext), null);
 		rootnode.Accept(new SetPropertiesVisitor(visitorContext, true), null);