Add support for private types in SetBinding using UnsafeAccessorType

src/Controls/src/BindingSourceGen/BindingCodeWriter.cs

@@ -136,9 +136,13 @@ public void AppendBindingFactoryMethod(BindingInvocationDescription binding, str
 			AppendLine('{');
 			Indent();
 
+			// For inaccessible types, use object and cast
+			var sourceTypeForBinding = GetTypeForBinding(binding.SourceType);
+			var propertyTypeForBinding = GetTypeForBinding(binding.PropertyType);
+
 			// Initialize setter
 			AppendLines($$"""
-				global::System.Action<{{binding.SourceType}}, {{binding.PropertyType}}>? setter = null;
+				global::System.Action<{{sourceTypeForBinding}}, {{propertyTypeForBinding}}>? setter = null;
 				if ({{GetShouldUseSetterCall(binding.MethodType)}})
 				{
 				""");
@@ -167,10 +171,10 @@ public void AppendBindingFactoryMethod(BindingInvocationDescription binding, str
 			AppendLine('}');
 			AppendBlankLine();
 
-			// Create instance of TypedBinding
+			// Create instance of TypedBinding - use object types for inaccessible types
 			AppendLines($$"""
-				var binding = new global::Microsoft.Maui.Controls.Internals.TypedBinding<{{binding.SourceType}}, {{binding.PropertyType}}>(
-					getter: source => (getter(source), true),
+				var binding = new global::Microsoft.Maui.Controls.Internals.TypedBinding<{{sourceTypeForBinding}}, {{propertyTypeForBinding}}>(
+					getter: source => ({{GenerateGetterInvocation(binding)}}),
 					setter,
 				""");
 			Indent();
@@ -200,11 +204,25 @@ public void AppendBindingFactoryMethod(BindingInvocationDescription binding, str
 			}
 
 			AppendUnsafeAccessors(binding);
+			AppendUnsafeAccessorTypes(binding);
 
 			Unindent();
 			AppendLine('}');
 		}
 
+		private static string GetTypeForBinding(TypeDescription type)
+		{
+			// Use object for inaccessible types in TypedBinding
+			return type.IsAccessible ? type.ToString() : "object";
+		}
+
+		private string GenerateGetterInvocation(BindingInvocationDescription binding)
+		{
+			// When types are inaccessible, the getter parameter is Func<object, object> (or mixed)
+			// and we just invoke it directly - no casting needed because the parameter types match
+			return "getter(source), true";
+		}
+
 		private void AppendFunctionArguments(BindingInvocationDescription binding)
 		{
 			AppendLine('(');
@@ -218,8 +236,17 @@ private void AppendFunctionArguments(BindingInvocationDescription binding)
 					""");
 			}
 
+			// Use UnsafeAccessorType for inaccessible types
+			var sourceTypeForSignature = GetTypeForSignature(binding.SourceType);
+			var propertyTypeForSignature = GetTypeForSignature(binding.PropertyType);
+
+			if (!binding.SourceType.IsAccessible && binding.SourceType.AssemblyQualifiedName != null)
+			{
+				AppendLine($"[global::System.Runtime.CompilerServices.UnsafeAccessorType(\"{binding.SourceType.AssemblyQualifiedName}\")]");
+			}
+
 			AppendLines($$"""
-				global::System.Func<{{binding.SourceType}}, {{binding.PropertyType}}> getter,
+				global::System.Func<{{sourceTypeForSignature}}, {{propertyTypeForSignature}}> getter,
 				global::Microsoft.Maui.Controls.BindingMode mode = global::Microsoft.Maui.Controls.BindingMode.Default,
 				global::Microsoft.Maui.Controls.IValueConverter? converter = null,
 				object? converterParameter = null,
@@ -232,6 +259,12 @@ private void AppendFunctionArguments(BindingInvocationDescription binding)
 			Unindent();
 		}
 
+		private static string GetTypeForSignature(TypeDescription type)
+		{
+			// Use object for inaccessible types in signatures
+			return type.IsAccessible ? type.ToString() : "object";
+		}
+
 		private static string GetShouldUseSetterCall(InterceptedMethodType interceptedMethodType) =>
 			interceptedMethodType switch
 			{
@@ -265,6 +298,8 @@ private void AppendSetterLambda(BindingInvocationDescription binding, string sou
 			AppendLine('{');
 			Indent();
 
+			// No casting needed - when types are inaccessible, parameters are already object
+			// and UnsafeAccessor methods handle the type conversions
 			var assignedValueExpression = valueVariableName;
 
 			// early return for nullable values if the setter doesn't accept them
@@ -331,11 +366,14 @@ private void AppendSetterLambda(BindingInvocationDescription binding, string sou
 
 		private void AppendHandlersArray(BindingInvocationDescription binding)
 		{
-			AppendLine($"new global::System.Tuple<global::System.Func<{binding.SourceType}, object?>, string>[]");
+			var sourceTypeForHandlers = GetTypeForBinding(binding.SourceType);
+			AppendLine($"new global::System.Tuple<global::System.Func<{sourceTypeForHandlers}, object?>, string>[]");
 			AppendLine('{');
 
 			Indent();
 
+			// No casting needed - when source type is inaccessible, it's already object
+			// and we use UnsafeAccessor methods for member access
 			string nextExpression = "source";
 			bool forceConditonalAccessToNextPart = false;
 			foreach (var part in binding.Path)
@@ -423,7 +461,7 @@ private void AppendUnsafeAccessors(BindingInvocationDescription binding)
 
 				if (unsafeAccessor.Kind == AccessorKind.Field)
 				{
-					AppendUnsafeFieldAccessor(unsafeAccessor.MemberName, unsafeAccessor.memberType.GlobalName, unsafeAccessor.ContainingType.GlobalName);
+					AppendUnsafeFieldAccessorWithType(unsafeAccessor.MemberName, unsafeAccessor.memberType, unsafeAccessor.ContainingType);
 				}
 				else if (unsafeAccessor.Kind == AccessorKind.Property)
 				{
@@ -434,13 +472,13 @@ private void AppendUnsafeAccessors(BindingInvocationDescription binding)
 					{
 						// we don't need the unsafe getter if the item is the very last part of the path
 						// because we don't need to access its value while constructing the handlers array
-						AppendUnsafePropertyGetAccessors(unsafeAccessor.MemberName, unsafeAccessor.memberType.GlobalName, unsafeAccessor.ContainingType.GlobalName);
+						AppendUnsafePropertyGetAccessorsWithType(unsafeAccessor.MemberName, unsafeAccessor.memberType, unsafeAccessor.ContainingType);
 					}
 
 					if (isLastPart && binding.SetterOptions.IsWritable)
 					{
 						// We only need the unsafe setter if the item is the very last part of the path
-						AppendUnsafePropertySetAccessors(unsafeAccessor.MemberName, unsafeAccessor.memberType.GlobalName, unsafeAccessor.ContainingType.GlobalName);
+						AppendUnsafePropertySetAccessorsWithType(unsafeAccessor.MemberName, unsafeAccessor.memberType, unsafeAccessor.ContainingType);
 					}
 				}
 				else
@@ -469,6 +507,62 @@ private void AppendUnsafePropertySetAccessors(string propertyName, string member
 				static extern void {{CreateUnsafePropertyAccessorSetMethodName(propertyName)}}({{containingType}} source, {{memberType}} value);
 				""");
 
+		private void AppendUnsafeFieldAccessorWithType(string fieldName, TypeDescription memberType, TypeDescription containingType)
+		{
+			var memberTypeStr = memberType.IsAccessible ? memberType.ToString() : "object";
+			var containingTypeStr = containingType.IsAccessible ? containingType.ToString() : "object";
+
+			if (!containingType.IsAccessible && containingType.AssemblyQualifiedName != null)
+			{
+				AppendLine($"[global::System.Runtime.CompilerServices.UnsafeAccessor(global::System.Runtime.CompilerServices.UnsafeAccessorKind.Field, Name = \"{fieldName}\")]");
+				AppendLine($"static extern ref {memberTypeStr} {CreateUnsafeFieldAccessorMethodName(fieldName)}([global::System.Runtime.CompilerServices.UnsafeAccessorType(\"{containingType.AssemblyQualifiedName}\")] {containingTypeStr} source);");
+			}
+			else
+			{
+				AppendUnsafeFieldAccessor(fieldName, memberTypeStr, containingTypeStr);
+			}
+		}
+
+		private void AppendUnsafePropertyGetAccessorsWithType(string propertyName, TypeDescription memberType, TypeDescription containingType)
+		{
+			var memberTypeStr = memberType.IsAccessible ? memberType.ToString() : "object";
+			var containingTypeStr = containingType.IsAccessible ? containingType.ToString() : "object";
+
+			if (!containingType.IsAccessible && containingType.AssemblyQualifiedName != null)
+			{
+				AppendLine($"[global::System.Runtime.CompilerServices.UnsafeAccessor(global::System.Runtime.CompilerServices.UnsafeAccessorKind.Method, Name = \"get_{propertyName}\")]");
+				AppendLine($"static extern {memberTypeStr} {CreateUnsafePropertyAccessorGetMethodName(propertyName)}([global::System.Runtime.CompilerServices.UnsafeAccessorType(\"{containingType.AssemblyQualifiedName}\")] {containingTypeStr} source);");
+			}
+			else
+			{
+				AppendUnsafePropertyGetAccessors(propertyName, memberTypeStr, containingTypeStr);
+			}
+		}
+
+		private void AppendUnsafePropertySetAccessorsWithType(string propertyName, TypeDescription memberType, TypeDescription containingType)
+		{
+			var memberTypeStr = memberType.IsAccessible ? memberType.ToString() : "object";
+			var containingTypeStr = containingType.IsAccessible ? containingType.ToString() : "object";
+			var valueTypeStr = memberType.IsAccessible ? memberType.ToString() : "object";
+
+			if (!containingType.IsAccessible && containingType.AssemblyQualifiedName != null)
+			{
+				AppendLine($"[global::System.Runtime.CompilerServices.UnsafeAccessor(global::System.Runtime.CompilerServices.UnsafeAccessorKind.Method, Name = \"set_{propertyName}\")]");
+				AppendLine($"static extern void {CreateUnsafePropertyAccessorSetMethodName(propertyName)}([global::System.Runtime.CompilerServices.UnsafeAccessorType(\"{containingType.AssemblyQualifiedName}\")] {containingTypeStr} source, {valueTypeStr} value);");
+			}
+			else
+			{
+				AppendUnsafePropertySetAccessors(propertyName, valueTypeStr, containingTypeStr);
+			}
+		}
+
+		private void AppendUnsafeAccessorTypes(BindingInvocationDescription binding)
+		{
+			// Note: UnsafeAccessorType is applied to the getter parameter in AppendFunctionArguments
+			// We don't need additional declarations here since the attribute on the parameter
+			// is sufficient to enable casting from/to object in the method body
+		}
+
 		public void Dispose()
 		{
 			_indentedTextWriter.Dispose();

src/Controls/src/BindingSourceGen/BindingInvocationDescription.cs

@@ -41,7 +41,9 @@ public sealed record TypeDescription(
 	string GlobalName,
 	bool IsValueType = false,
 	bool IsNullable = false,
-	bool IsGenericParameter = false)
+	bool IsGenericParameter = false,
+	bool IsAccessible = true,
+	string? AssemblyQualifiedName = null)
 {
 	public override string ToString()
 		=> IsNullable

src/Controls/src/BindingSourceGen/BindingSourceGenerator.cs

@@ -230,10 +230,7 @@ private static Result<ITypeSymbol> GetLambdaParameterType(LambdaExpressionSyntax
 		}
 
 		var lambdaParamType = parameters[0].Type;
-		if (!lambdaParamType.IsAccessible())
-		{
-			return Result<ITypeSymbol>.Failure(DiagnosticsFactory.UnaccessibleTypeUsedAsLambdaParameter(lambda.GetLocation()));
-		}
+		// Now we support inaccessible types using UnsafeAccessorType
 
 		return Result<ITypeSymbol>.Success(lambdaParamType);
 	}

src/Controls/src/BindingSourceGen/DiagnosticsFactory.cs

@@ -86,8 +86,8 @@ public static DiagnosticInfo UnaccessibleTypeUsedAsLambdaParameter(Location loca
 		=> new DiagnosticInfo(
 			new DiagnosticDescriptor(
 				id: "BSG0007",
-				title: "Unaccessible type used as lambda parameter",
-				messageFormat: "The lambda parameter type has to be declared as public, internal or protected internal.",
+				title: "Inaccessible type used as lambda parameter",
+				messageFormat: "The lambda parameter type has to be declared as public, internal or protected internal. Private and protected types cannot be used because the generated binding code is in a different namespace. Consider making the type internal instead of private.",
 				category: "Usage",
 				defaultSeverity: DiagnosticSeverity.Error,
 				isEnabledByDefault: true),

src/Controls/src/BindingSourceGen/ISymbolExtensions.cs

@@ -9,6 +9,25 @@ internal static bool IsAccessible(this ISymbol symbol) =>
 		|| symbol.DeclaredAccessibility == Accessibility.Internal
 		|| symbol.DeclaredAccessibility == Accessibility.ProtectedOrInternal;
 
+	// For type symbols, check if the type and all its containing types are accessible
+	internal static bool IsAccessible(this ITypeSymbol typeSymbol)
+	{
+		// Check the type itself
+		if (!((ISymbol)typeSymbol).IsAccessible())
+			return false;
+
+		// Check all containing types
+		var containingType = typeSymbol.ContainingType;
+		while (containingType != null)
+		{
+			if (!((ISymbol)containingType).IsAccessible())
+				return false;
+			containingType = containingType.ContainingType;
+		}
+
+		return true;
+	}
+
 	internal static AccessorKind ToAccessorKind(this ISymbol symbol)
 	{
 		return symbol switch

src/Controls/src/BindingSourceGen/ITypeSymbolExtensions.cs

@@ -17,11 +17,14 @@ public static bool IsTypeNullable(this ITypeSymbol typeInfo, bool enabledNullabl
 	public static TypeDescription CreateTypeDescription(this ITypeSymbol typeSymbol, bool enabledNullable)
 	{
 		var isNullable = IsTypeNullable(typeSymbol, enabledNullable);
+		var isAccessible = typeSymbol.IsAccessible();
 		return new TypeDescription(
 			GlobalName: GetGlobalName(typeSymbol, isNullable, typeSymbol.IsValueType),
 			IsNullable: isNullable,
 			IsGenericParameter: typeSymbol.Kind == SymbolKind.TypeParameter, //TODO: Add support for generic parameters
-			IsValueType: typeSymbol.IsValueType);
+			IsValueType: typeSymbol.IsValueType,
+			IsAccessible: isAccessible,
+			AssemblyQualifiedName: isAccessible ? null : GetAssemblyQualifiedName(typeSymbol));
 	}
 
 	private static bool IsNullableValueType(this ITypeSymbol typeInfo) =>
@@ -43,4 +46,20 @@ private static string GetGlobalName(this ITypeSymbol typeSymbol, bool isNullable
 
 		return typeSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);
 	}
+
+	private static string GetAssemblyQualifiedName(this ITypeSymbol typeSymbol)
+	{
+		// For UnsafeAccessorType, we need assembly-qualified name
+		// Format: "FullTypeName, AssemblyName"
+		var typeName = typeSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat)
+			.Replace("global::", ""); // Remove global:: prefix
+
+		var containingAssembly = typeSymbol.ContainingAssembly;
+		if (containingAssembly != null)
+		{
+			return $"{typeName}, {containingAssembly.Name}";
+		}
+
+		return typeName;
+	}
 }

src/Controls/src/BindingSourceGen/PathParser.cs

@@ -54,9 +54,13 @@ private Result<List<IPathPart>> HandleMemberAccessExpression(MemberAccessExpress
 		var memberType = typeInfo.CreateTypeDescription(_enabledNullable);
 		var containgType = symbol.ContainingType.CreateTypeDescription(_enabledNullable);
 
-		IPathPart part = symbol.IsAccessible()
-			? new MemberAccess(member, !isReferenceType)
-			: new InaccessibleMemberAccess(containgType, memberType, accessorKind, member, !isReferenceType);
+		// If either the member is inaccessible OR the containing type is inaccessible,
+		// we need to use UnsafeAccessor
+		bool needsUnsafeAccessor = !symbol.IsAccessible() || !containgType.IsAccessible;
+
+		IPathPart part = needsUnsafeAccessor
+			? new InaccessibleMemberAccess(containgType, memberType, accessorKind, member, !isReferenceType)
+			: new MemberAccess(member, !isReferenceType);
 
 		result.Value.Add(part);
 		return Result<List<IPathPart>>.Success(result.Value);

src/Controls/tests/BindingSourceGen.UnitTests/DiagnosticsTests.cs

@@ -230,8 +230,10 @@ public void Foo()
 	[InlineData("protected")]
 	[InlineData("private protected")]
 	// https://github.com/dotnet/maui/issues/23534
-	public void ReportsWarningWhenSourceTypeIsUnaccessible(string modifier)
+	public void SupportsInaccessibleSourceType(string modifier)
 	{
+		// Previously this test checked for BSG0007 error
+		// Now we support private types using UnsafeAccessorType
 		var source = $$"""
 			using Microsoft.Maui.Controls;
 
@@ -255,9 +257,9 @@ public void Bar()
 
 		var result = SourceGenHelpers.Run(source);
 
-		var diagnostic = Assert.Single(result.SourceGeneratorDiagnostics);
-		Assert.Equal("BSG0007", diagnostic.Id);
-		AssertExtensions.AssertNoDiagnostics(result.GeneratedCodeCompilationDiagnostics, "Generated code compilation");
+		// Should not have any diagnostics - private types are now supported
+		AssertExtensions.AssertNoDiagnostics(result);
+		Assert.NotNull(result.Binding);
 	}
 
 	[Fact]