[typemap] Validate exported Java fields

src/Microsoft.Android.Sdk.TrimmableTypeMap/ITrimmableTypeMapLogger.cs

@@ -22,6 +22,9 @@ void LogUnresolvableJavaPeerSkippedWarning (
 		string unresolvedAssemblyPath);
 	void LogJniAddNativeMethodRegistrationAttributeError (string managedTypeName);
 	void LogInvalidJavaNameError (string javaName, string invalidIdentifier);
+	void LogExportFieldWithParametersError ();
+	void LogExportFieldOnGenericTypeError ();
+	void LogExportFieldReturnsVoidError ();
 	void LogCustomJavaObjectError (string managedTypeName);
 	void LogCustomJavaObjectWarning (string managedTypeName);
 }

src/Microsoft.Android.Sdk.TrimmableTypeMap/Scanner/JavaPeerScanner.cs

@@ -667,11 +667,16 @@ static void SkipArrayShape (ref BlobReader reader)
 		var methods = new List<MarshalMethodInfo> ();
 		var fields = new List<JavaFieldInfo> ();
 		HashSet<string>? registeredMethodKeys = detectBaseOverrides ? new (StringComparer.Ordinal) : null;
+		bool isGenericType = typeDef.GetGenericParameters ().Count > 0;
 
 		// Pass 1: collect methods with [Register], [Export], or [ExportField] directly on them
 		foreach (var methodHandle in typeDef.GetMethods ()) {
 			var methodDef = index.Reader.GetMethodDefinition (methodHandle);
 
+			if (!ValidateExportField (methodDef, index, isGenericType)) {
+				continue;
+			}
+
 			// Check for [ExportField] — produces both a marshal method AND a field
 			CollectExportField (methodDef, index, fields);
 
@@ -735,6 +740,39 @@ static void SkipArrayShape (ref BlobReader reader)
 		return (methods, fields);
 	}
 
+	static bool IsExportFieldAttribute (CustomAttribute attribute, AssemblyIndex index)
+	{
+		return AssemblyIndex.IsCustomAttributeMatch (attribute, index.Reader, "Java.Interop", "ExportFieldAttribute");
+	}
+
+	bool ValidateExportField (MethodDefinition methodDef, AssemblyIndex index, bool isGenericType)
+	{
+		foreach (var caHandle in methodDef.GetCustomAttributes ()) {
+			var ca = index.Reader.GetCustomAttribute (caHandle);
+			if (!IsExportFieldAttribute (ca, index)) {
+				continue;
+			}
+
+			if (isGenericType) {
+				logger?.LogExportFieldOnGenericTypeError ();
+				return false;
+			}
+
+			var sig = methodDef.DecodeSignature (index.TypeRefSignatureProvider, index);
+			if (sig.ParameterTypes.Length != 0) {
+				logger?.LogExportFieldWithParametersError ();
+				return false;
+			}
+			if (sig.ReturnType.ManagedTypeName == "System.Void") {
+				logger?.LogExportFieldReturnsVoidError ();
+				return false;
+			}
+			return true;
+		}
+
+		return true;
+	}
+
 	static bool HasJniAddNativeMethodRegistrationAttribute (TypeDefinition typeDef, AssemblyIndex index)
 	{
 		const string JniAddNativeMethodRegistrationAttribute = "JniAddNativeMethodRegistrationAttribute";
@@ -1707,7 +1745,7 @@ bool TryGetMethodRegisterInfo (MethodDefinition methodDef, AssemblyIndex index,
 				return true;
 			}
 
-			if (attrName == "ExportFieldAttribute") {
+			if (IsExportFieldAttribute (ca, index)) {
 				(registerInfo, exportInfo) = ParseExportFieldAsMethod (ca, methodDef, index);
 				return true;
 			}
@@ -2611,9 +2649,8 @@ void CollectExportField (MethodDefinition methodDef, AssemblyIndex index, List<J
 	{
 		foreach (var caHandle in methodDef.GetCustomAttributes ()) {
 			var ca = index.Reader.GetCustomAttribute (caHandle);
-			var attrName = index.GetCustomAttributeName (ca);
 
-			if (attrName != "ExportFieldAttribute") {
+			if (!IsExportFieldAttribute (ca, index)) {
 				continue;
 			}
 

src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs

@@ -59,6 +59,12 @@ public void LogJniAddNativeMethodRegistrationAttributeError (string managedTypeN
 			log.LogCodedError ("XA4251", Properties.Resources.XA4251, managedTypeName);
 		public void LogInvalidJavaNameError (string javaName, string invalidIdentifier) =>
 			log.LogCodedError ("XA4258", Properties.Resources.XA4258, javaName, invalidIdentifier);
+		public void LogExportFieldWithParametersError () =>
+			log.LogCodedError ("XA4205", Java.Interop.Localization.Resources.JavaCallableWrappers_XA4205);
+		public void LogExportFieldOnGenericTypeError () =>
+			log.LogCodedError ("XA4207", Java.Interop.Localization.Resources.JavaCallableWrappers_XA4207);
+		public void LogExportFieldReturnsVoidError () =>
+			log.LogCodedError ("XA4208", Java.Interop.Localization.Resources.JavaCallableWrappers_XA4208);
 		public void LogCustomJavaObjectError (string managedTypeName) =>
 			log.LogError ("{0}", $"XA4212: {string.Format (Properties.Resources.XA4212, managedTypeName)}");
 		public void LogCustomJavaObjectWarning (string managedTypeName) =>

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/TrimmableTypeMapBuildTests.cs

@@ -36,6 +36,98 @@ public void Build_WithTrimmableTypeMap_Succeeds ([Values] bool isRelease, [Value
 			AssertTrimmableTypeMapOutputs (intermediateDir);
 		}
 
+		[TestCase ("llvm-ir", AndroidRuntime.CoreCLR, "parameters", "XA4205")]
+		[TestCase ("trimmable", AndroidRuntime.CoreCLR, "parameters", "XA4205")]
+		[TestCase ("trimmable", AndroidRuntime.NativeAOT, "parameters", "XA4205")]
+		[TestCase ("llvm-ir", AndroidRuntime.CoreCLR, "void", "XA4208")]
+		[TestCase ("trimmable", AndroidRuntime.CoreCLR, "void", "XA4208")]
+		[TestCase ("trimmable", AndroidRuntime.NativeAOT, "void", "XA4208")]
+		[TestCase ("llvm-ir", AndroidRuntime.CoreCLR, "generic", "XA4207")]
+		[TestCase ("trimmable", AndroidRuntime.CoreCLR, "generic", "XA4207")]
+		[TestCase ("trimmable", AndroidRuntime.NativeAOT, "generic", "XA4207")]
+		[TestCase ("llvm-ir", AndroidRuntime.CoreCLR, "parameters-and-void", "XA4205")]
+		[TestCase ("trimmable", AndroidRuntime.CoreCLR, "parameters-and-void", "XA4205")]
+		[TestCase ("trimmable", AndroidRuntime.NativeAOT, "parameters-and-void", "XA4205")]
+		[TestCase ("llvm-ir", AndroidRuntime.CoreCLR, "generic-parameters-and-void", "XA4207")]
+		[TestCase ("trimmable", AndroidRuntime.CoreCLR, "generic-parameters-and-void", "XA4207")]
+		[TestCase ("trimmable", AndroidRuntime.NativeAOT, "generic-parameters-and-void", "XA4207")]
+		public void Build_InvalidExportField_ReportsLegacyDiagnostic (
+			string typeMapImplementation,
+			AndroidRuntime runtime,
+			string invalidShape,
+			string expectedCode)
+		{
+			bool isRelease = runtime == AndroidRuntime.NativeAOT;
+			if (IgnoreUnsupportedConfiguration (runtime, release: isRelease)) {
+				return;
+			}
+
+			var initializer = invalidShape switch {
+				"parameters" => "public int InitialValue (int value) => value;",
+				"void" => "public void InitialValue () { }",
+				"generic" => "public int InitialValue () => 42;",
+				"parameters-and-void" => "public void InitialValue (int value) { }",
+				"generic-parameters-and-void" => "public void InitialValue (int value) { }",
+				_ => throw new InvalidOperationException ($"Unknown invalid [ExportField] shape '{invalidShape}'."),
+			};
+			var proj = CreateExportFieldValidationProject (runtime, typeMapImplementation, $"""
+						[ExportField ("VALUE")]
+						{initializer}
+				""", genericType: invalidShape.StartsWith ("generic", StringComparison.Ordinal));
+
+			using var builder = CreateApkBuilder ();
+			builder.ThrowOnBuildFailure = false;
+			Assert.IsFalse (builder.Build (proj), $"{runtime}/{typeMapImplementation} should reject {invalidShape} [ExportField] initializers.");
+			StringAssertEx.Contains ($"error {expectedCode}", builder.LastBuildOutput, $"The build should report {expectedCode}.");
+			if (invalidShape == "parameters-and-void") {
+				Assert.IsFalse (
+					builder.LastBuildOutput.Any (line => line.Contains ("error XA4208", StringComparison.Ordinal)),
+					"XA4205 should take precedence over XA4208, matching LLVM-IR."
+				);
+			} else if (invalidShape == "generic-parameters-and-void") {
+				Assert.IsFalse (
+					builder.LastBuildOutput.Any (line =>
+						line.Contains ("error XA4205", StringComparison.Ordinal) ||
+						line.Contains ("error XA4208", StringComparison.Ordinal)),
+					"XA4207 should take precedence over initializer signature diagnostics, matching LLVM-IR."
+				);
+			}
+		}
+
+		static XamarinAndroidApplicationProject CreateExportFieldValidationProject (
+			AndroidRuntime runtime,
+			string typeMapImplementation,
+			string members,
+			bool genericType = false)
+		{
+			var typeParameters = genericType ? "<T>" : "";
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = runtime == AndroidRuntime.NativeAOT,
+				References = {
+					new BuildItem.Reference ("Mono.Android.Export"),
+				},
+			};
+			proj.SetRuntime (runtime);
+			proj.SetProperty ("AndroidTypeMapImplementation", typeMapImplementation);
+			proj.Sources.Add (new BuildItem.Source ("ExportFieldValidation.cs") {
+				TextContent = () => $$"""
+					using Android.Runtime;
+					using Java.Interop;
+
+					namespace ExportFieldValidation {
+						[Register ("com/example/exportfields/ValidationPeer")]
+						class ValidationPeer{{typeParameters}} : Java.Lang.Object {
+							public ValidationPeer () {
+							}
+
+					{{members}}
+						}
+					}
+					""",
+			});
+			return proj;
+		}
+
 		[Test]
 		public void Build_PublishAotProject_UsesTrimmableTypeMapForCoreClrDebug ()
 		{

tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TrimmableTypeMapGeneratorTests.cs

@@ -53,6 +53,12 @@ public void LogJniAddNativeMethodRegistrationAttributeError (string managedTypeN
 			logMessages.Add ($"XA4251: Type '{managedTypeName}' uses [JniAddNativeMethodRegistrationAttribute], which is not supported by the trimmable type map.");
 		public void LogInvalidJavaNameError (string javaName, string invalidIdentifier) =>
 			logMessages.Add ($"XA4258: Java name '{javaName}' contains reserved Java identifier '{invalidIdentifier}'.");
+		public void LogExportFieldWithParametersError () =>
+			logMessages.Add ("XA4205: [ExportField] can only be used on methods with 0 parameters.");
+		public void LogExportFieldReturnsVoidError () =>
+			logMessages.Add ("XA4208: [ExportField] cannot be used on a method returning 'void'.");
+		public void LogExportFieldOnGenericTypeError () =>
+			logMessages.Add ("XA4207: [ExportField] cannot be used on a generic type.");
 		public void LogCustomJavaObjectError (string managedTypeName) =>
 			logMessages.Add ($"XA4212: Type `{managedTypeName}` implements `Android.Runtime.IJavaObject` but does not inherit `Java.Lang.Object` or `Java.Lang.Throwable`. This is not supported.");
 		public void LogCustomJavaObjectWarning (string managedTypeName) =>

tests/Mono.Android-Tests/Mono.Android-Tests/Java.Interop/ExportTests.cs

@@ -305,6 +305,16 @@ public void ExportField_StaticObject_InitializerVisibleFromJava ()
 				Assert.AreEqual (43, value.IntValue ());
 			}
 		}
+
+		[Test, Category ("Export")]
+		public void ExportField_InstancePrimitive_InitializerVisibleFromJava ()
+		{
+			using var value = new ExportInstanceField ();
+			using var klass = Java.Lang.Class.FromType (typeof (ExportInstanceField));
+			var field = JNIEnv.GetFieldID (klass.Handle, "ANSWER", "I");
+			Assert.AreNotEqual (IntPtr.Zero, field, "JNI field id for ANSWER not found");
+			Assert.AreEqual (42, JNIEnv.GetIntField (value.Handle, field));
+		}
 	}
 
 	// ---------------------------------------------------------------
@@ -341,6 +351,12 @@ class ExportStaticField : Java.Lang.Object
 		public static Java.Lang.Integer GetObjectAnswer () => new Java.Lang.Integer (43);
 	}
 
+	class ExportInstanceField : Java.Lang.Object
+	{
+		[ExportField ("ANSWER")]
+		public int GetAnswer () => 42;
+	}
+
 	class ExportString : Java.Lang.Object
 	{
 		[Export]