Add XC0067/MAUIX2015 warning for duplicate implicit content property assignments in XAML

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/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);
 

src/Controls/src/SourceGen/MauiGResources.resx

@@ -240,4 +240,11 @@
     <value>Event handler '{0}' with correct signature not found in type '{1}'.</value>
     <comment>0 is handler name, 1 is type name</comment>
   </data>
+  <data name="DuplicatePropertyAssignmentTitle" xml:space="preserve">
+    <value>Property set multiple times</value>
+  </data>
+  <data name="DuplicatePropertyAssignmentMessage" xml:space="preserve">
+    <value>Property '{0}' is being set multiple times. Only the last value will be used.</value>
+    <comment>0 is a property name (e.g., "Border.Content", "Label.Text")</comment>
+  </data>
 </root>

src/Controls/src/SourceGen/SetPropertyHelpers.cs

@@ -534,7 +534,7 @@ static void SetValue(IndentedTextWriter writer, ILocalValue parentVar, IFieldSym
 		}
 	}
 
-	static bool CanGet(ILocalValue parentVar, string localName, SourceGenContext context, out ITypeSymbol? propertyType, out IPropertySymbol? propertySymbol)
+	internal static bool CanGet(ILocalValue parentVar, string localName, SourceGenContext context, out ITypeSymbol? propertyType, out IPropertySymbol? propertySymbol)
 	{
 		propertyType = null;
 		if ((propertySymbol = parentVar.Type.GetAllProperties(localName, context).FirstOrDefault()) == null)
@@ -546,7 +546,7 @@ static bool CanGet(ILocalValue parentVar, string localName, SourceGenContext con
 		return true;
 	}
 
-	static bool CanGetValue(ILocalValue parentVar, IFieldSymbol? bpFieldSymbol, bool attached, SourceGenContext context, out ITypeSymbol? propertyType)
+	internal static bool CanGetValue(ILocalValue parentVar, IFieldSymbol? bpFieldSymbol, bool attached, SourceGenContext context, out ITypeSymbol? propertyType)
 	{
 		propertyType = null;
 		if (bpFieldSymbol == null)

src/Controls/src/SourceGen/SourceGenContext.cs

@@ -38,10 +38,14 @@ public void ReportDiagnostic(Diagnostic diagnostic)
 		var noWarn = ProjectItem?.NoWarn;
 		if (!string.IsNullOrEmpty(noWarn))
 		{
-			var suppressedIds = noWarn!.Split(new[] { ',', ';' }, StringSplitOptions.RemoveEmptyEntries);
+			var suppressedIds = noWarn!.Split(new[] { ',', ';', ' ' }, StringSplitOptions.RemoveEmptyEntries);
 			foreach (var id in suppressedIds)
 			{
-				if (diagnostic.Id.Equals(id.Trim(), StringComparison.OrdinalIgnoreCase))
+				var code = id.Trim();
+				// Match full ID (e.g., "MAUIX2015") or bare numeric suffix (e.g., "2015")
+				if (code.Equals(diagnostic.Id, StringComparison.OrdinalIgnoreCase) ||
+					(diagnostic.Id.StartsWith("MAUIX", StringComparison.OrdinalIgnoreCase) &&
+					 code == diagnostic.Id.Substring("MAUIX".Length)))
 				{
 					return; // Suppress this diagnostic
 				}

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

@@ -1,3 +1,4 @@
+using System;
 using System.CodeDom.Compiler;
 using System.Collections.Generic;
 using System.Linq;
@@ -35,21 +36,72 @@ class SetPropertiesVisitor(SourceGenContext context, bool stopOnResourceDictiona
 	public bool SkipChildren(INode node, INode parentNode) => node is ElementNode en && en.IsLazyResource(parentNode, Context);
 	public bool IsResourceDictionary(ElementNode node) => node.IsResourceDictionary(Context);
 
+	// 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))
+		{
+			var propertyDisplayName = $"{parentNode.XmlType.Name}.{propertyName.LocalName}";
+			var location = LocationCreate(Context.ProjectItem.RelativePath!, lineInfo, propertyDisplayName);
+			context.ReportDiagnostic(Diagnostic.Create(Descriptors.DuplicatePropertyAssignment, location, propertyDisplayName));
+		}
+	}
+
+	/// <summary>
+	/// Resolves the type of an implicit content property and, when the property is a non-collection
+	/// non-Object type, calls <see cref="CheckForDuplicateProperty"/> to emit a diagnostic if it has
+	/// already been assigned.
+	/// </summary>
+	/// <param name="lookupName">XmlName used for the <c>GetBindableProperty</c> lookup (namespace may differ from <paramref name="trackingName"/>).</param>
+	/// <param name="trackingName">XmlName used as the deduplication key (always uses the parent element's namespace).</param>
+	void CheckImplicitContentForDuplicate(
+		ILocalValue parentVar,
+		ElementNode parentNode,
+		XmlName lookupName,
+		XmlName trackingName,
+		IXmlLineInfo lineInfo)
+	{
+		bool attached = false;
+		var localName = lookupName.LocalName;
+		var bpFieldSymbol = parentVar.Type.GetBindableProperty(lookupName.NamespaceURI, ref localName, out attached, context, lineInfo);
+		ITypeSymbol? propertyType = null;
+
+		bool hasProperty = (bpFieldSymbol != null && SetPropertyHelpers.CanGetValue(parentVar, bpFieldSymbol, attached, context, out propertyType))
+			|| SetPropertyHelpers.CanGet(parentVar, localName, context, out propertyType, out _);
+
+		bool isObject = propertyType != null && propertyType.SpecialType == SpecialType.System_Object;
+		if (hasProperty && propertyType != null && !isObject && !propertyType.CanAdd(context))
+			CheckForDuplicateProperty(parentNode, trackingName, lineInfo);
+	}
+
 	public void Visit(ValueNode node, INode parentNode)
 	{
 		//TODO support Label text as element
 
 		// if it's implicit content property, get the content property name
+		bool isImplicitContentProperty = false;
+		ILocalValue? parentVar = null;
 		if (!node.TryGetPropertyName(parentNode, out XmlName propertyName))
 		{
 			if (!parentNode.IsCollectionItem(node))
 				return;
 			string? contentProperty;
 			if (!Context.Variables.ContainsKey((ElementNode)parentNode))
 				return;
-			var parentVariable = Context.Variables[(ElementNode)parentNode];
-			if ((contentProperty = parentVariable.Type.GetContentPropertyName(context)) != null)
+			parentVar = Context.Variables[(ElementNode)parentNode];
+			if ((contentProperty = parentVar.Type.GetContentPropertyName(context)) != null)
+			{
 				propertyName = new XmlName(((ElementNode)parentNode).NamespaceURI, contentProperty);
+				isImplicitContentProperty = true;
+			}
 			else
 				return;
 		}
@@ -63,13 +115,12 @@ public void Visit(ValueNode node, INode parentNode)
 		if (propertyName.Equals(XmlName.mcIgnorable))
 			return;
 
-		// Check that parent has a variable before accessing
-		var parentElement = (ElementNode)parentNode;
-		if (!Context.Variables.TryGetValue(parentElement, out var parentVar))
-		{
-			var parentKey = parentElement.Properties.TryGetValue(XmlName.xKey, out var keyNode) && keyNode is ValueNode vn ? vn.Value?.ToString() : "(none)";
-			throw new System.InvalidOperationException($"SetPropertiesVisitor.Visit(ValueNode): Parent '{parentElement.XmlType.Name}' x:Key='{parentKey}' not in Variables for propertyName='{propertyName}'");
-		}
+		parentVar ??= Context.Variables.TryGetValue((ElementNode)parentNode, out var pv) ? pv : null;
+		// Parent element not yet in Variables — can occur for markup extensions or nodes
+		// processed before their parent is fully initialized. Silently skip; the assignment
+		// will be handled through another visitor pass.
+		if (parentVar == null)
+			return;
 
 		// Try to set runtime name (for x:Name with RuntimeNameProperty attribute)
 		if (TrySetRuntimeName(propertyName, parentVar, node))
@@ -79,6 +130,10 @@ public void Visit(ValueNode node, INode parentNode)
 		if (propertyName == XmlName.xName)
 			return;
 
+		// Check for duplicate property assignment for implicit content properties
+		if (isImplicitContentProperty)
+			CheckImplicitContentForDuplicate(parentVar, (ElementNode)parentNode, propertyName, propertyName, node);
+
 		SetPropertyHelpers.SetPropertyValue(Writer, parentVar, propertyName, node, Context);
 	}
 
@@ -207,6 +262,7 @@ public void Visit(ElementNode node, INode parentNode)
 				var key1 = ((ElementNode)parentNode).Properties.TryGetValue(XmlName.xKey, out var kn1) && kn1 is ValueNode vn1 ? vn1.Value?.ToString() : "(none)";
 				throw new System.InvalidOperationException($"SetPropertiesVisitor.Visit(ElementNode) propertyName != Empty: Parent '{((ElementNode)parentNode).XmlType.Name}' x:Key='{key1}' not in Variables");
 			}
+
 			SetPropertyHelpers.SetPropertyValue(Writer, pVar1, propertyName, node, Context);
 		}
 		else if (parentNode.IsCollectionItem(node) && parentNode is ElementNode)
@@ -225,8 +281,14 @@ public void Visit(ElementNode node, INode parentNode)
 				var name = new XmlName(node.NamespaceURI, contentProperty);
 				if (skips.Contains(name))
 					return;
-				if (parentNode is ElementNode node1 && node1.SkipProperties.Contains(propertyName))
+				if (parentNode is ElementNode node1 && node1.SkipProperties.Contains(name))
 					return;
+
+				// Only check for duplicate property assignment if the property is not a collection
+				// Use parent namespace for duplicate tracking to match how explicit property assignments are keyed
+				var contentPropertyName = new XmlName(((ElementNode)parentNode).NamespaceURI, contentProperty);
+				CheckImplicitContentForDuplicate(parentVar, (ElementNode)parentNode, name, contentPropertyName, node);
+
 				SetPropertyHelpers.SetPropertyValue(Writer, parentVar, name, node, Context);
 			}
 			else if (parentVar.Type.CanAdd(context))