Fix net.android.init.gradle.kts for Gradle 9.x compatibility

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Microsoft.Android.Sdk.Bindings.Gradle.targets

@@ -14,7 +14,8 @@ This file contains MSBuild targets that support building and operating on Androi
   <PropertyGroup>
     <_AGPOutDirAbs>$(IntermediateOutputPath)gradle/</_AGPOutDirAbs>
     <_AGPOutDirAbs Condition=" !$([System.IO.Path]::IsPathRooted('$(_AGPOutDirAbs)')) ">$(MSBuildProjectDirectory)/$(_AGPOutDirAbs)</_AGPOutDirAbs>
-    <_AGPInitScriptPath>$(_AGPOutDirAbs)net.android.init.gradle.kts</_AGPInitScriptPath>
+    <!-- Users can override _AGPInitScriptPath to point to their own init.gradle.kts file as an escape hatch -->
+    <_AGPInitScriptPath Condition=" '$(_AGPInitScriptPath)' == '' ">$(_AGPOutDirAbs)net.android.init.gradle.kts</_AGPInitScriptPath>
     <_BuildAndroidGradleProjectsStamp>$(_AndroidStampDirectory)_BuildAndroidGradleProjects.stamp</_BuildAndroidGradleProjectsStamp>
   </PropertyGroup>
 
@@ -74,7 +75,9 @@ This file contains MSBuild targets that support building and operating on Androi
     <RemoveDir Directories="@(AndroidGradleProject->'%(OutputPath)outputs')" />
 
     <!-- Create the net.android.init.gradle.kts script used to override the Gradle project output directory -->
+    <!-- Skip CopyResource if user has overridden _AGPInitScriptPath to point to their own file -->
     <CopyResource
+        Condition=" $([System.String]::Copy('$(_AGPInitScriptPath)').StartsWith('$(_AGPOutDirAbs)')) "
         ResourceName="net.android.init.gradle.kts"
         OutputPath="$(_AGPInitScriptPath)"
     />

src/Xamarin.Android.Build.Tasks/Resources/net.android.init.gradle.kts

@@ -5,10 +5,10 @@
  *    https://docs.gradle.org/current/kotlin-dsl/gradle/org.gradle.api.invocation/-gradle/projects-loaded.html
  */
 gradle.projectsLoaded {
-	if (gradle.startParameter.projectProperties.containsKey("netAndroidBuildDirOverride")) {
+	gradle.startParameter.projectProperties["netAndroidBuildDirOverride"]?.let { buildDir ->
 		rootProject.allprojects {
 			afterEvaluate {
-				layout.buildDirectory.set(file(gradle.startParameter.projectProperties["netAndroidBuildDirOverride"]))
+				layout.buildDirectory.set(file(buildDir))
 			}
 		}
 	}

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

@@ -574,5 +574,61 @@ public void TestFacebook () {{
 			FileAssert.Exists (Path.Combine (Root, builder.ProjectDirectory, proj.OutputPath, $"{moduleName}-release.aar"));
 		}
 
+		/// <summary>
+		/// Test case data for AGP/Gradle version combinations.
+		/// Gradle 9.x has stricter Kotlin type checking for null safety.
+		/// See: https://github.com/dotnet/android/issues/9818
+		/// </summary>
+		static IEnumerable<object[]> GetAgpGradleVersionTestData ()
+		{
+			// AGP 8.5.0 with Gradle 8.7 (minimum required for AGP 8.5)
+			yield return new object[] { "8.5.0", "8.7" };
+			// AGP 8.7.0 with Gradle 8.9 (minimum required for AGP 8.7)
+			yield return new object[] { "8.7.0", "8.9" };
+			// AGP 8.8.0 with Gradle 8.10 (minimum required for AGP 8.8)
+			yield return new object[] { "8.8.0", "8.10.2" };
+			// AGP 8.9.0 with Gradle 8.11 (minimum required for AGP 8.9)
+			yield return new object[] { "8.9.0", "8.11.1" };
+			// AGP 9.0.0 with Gradle 9.1 (minimum required for AGP 9.0)
+			// This tests the Gradle 9.x stricter Kotlin type checking fix
+			yield return new object[] { "9.0.0", "9.1" };
+		}
+
+		/// <summary>
+		/// Verifies that .NET Android binding projects work with various AGP and Gradle versions.
+		/// This test ensures compatibility with Gradle's evolving Kotlin type checking behavior,
+		/// particularly the stricter null safety checks introduced in Gradle 9.x.
+		/// </summary>
+		[Test]
+		[TestCaseSource (nameof (GetAgpGradleVersionTestData))]
+		public void BindLibraryWithMultipleGradleVersions (string agpVersion, string gradleVersion)
+		{
+			var gradleProject = AndroidGradleProject.CreateDefault (GradleTestProjectDir, agpVersion, gradleVersion);
+			var moduleName = gradleProject.Modules.First ().Name;
+
+			var proj = new XamarinAndroidBindingProject {
+				Jars = {
+					new BuildItem (KnownProperties.AndroidGradleProject, gradleProject.BuildFilePath) {
+						Metadata = {
+							{ "ModuleName", moduleName },
+							{ "Bind", "true" },
+							{ "Configuration", "Release" },
+						},
+					},
+				},
+				Sources = {
+					new BuildItem.Source ("Foo.cs") {
+						TextContent = () => @$"public class Foo {{ public Foo () {{ System.Console.WriteLine (GradleTest.{moduleName}Class.GetString(""TestString"")); }} }}"
+					},
+				},
+				MetadataXml = $@"<metadata><attr path=""/api/package[@name='{gradleProject.Modules.First ().PackageName}']"" name=""managedName"">GradleTest</attr></metadata>",
+			};
+
+			using var builder = CreateDllBuilder ();
+			builder.Verbosity = LoggerVerbosity.Detailed;
+			Assert.IsTrue (builder.Build (proj), $"Build with AGP {agpVersion} and Gradle {gradleVersion} should have succeeded.");
+			FileAssert.Exists (Path.Combine (Root, builder.ProjectDirectory, proj.OutputPath, $"{moduleName}-release.aar"));
+		}
+
 	}
 }

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidGradleProject.cs

@@ -13,6 +13,16 @@ public class AndroidGradleProject
 
 		public string BuildFilePath => Path.Combine (ProjectDirectory, "build.gradle.kts");
 
+		/// <summary>
+		/// Android Gradle Plugin version (e.g., "8.5.0", "9.0.0")
+		/// </summary>
+		public string AgpVersion { get; set; } = "8.5.0";
+
+		/// <summary>
+		/// Gradle wrapper version to use (e.g., "8.12", "9.0"). If null, uses system default.
+		/// </summary>
+		public string? GradleVersion { get; set; }
+
 		GradleCLI gradleCLI = new GradleCLI ();
 
 		public AndroidGradleProject (string directory)
@@ -26,12 +36,27 @@ public void Create ()
 			gradleCLI.Init (ProjectDirectory);
 			var settingsFile = Path.Combine (ProjectDirectory, "settings.gradle.kts");
 			File.WriteAllText (settingsFile, settings_gradle_kts_content);
-			File.WriteAllText (BuildFilePath, build_gradle_kts_content);
+			File.WriteAllText (BuildFilePath, GetBuildGradleKtsContent ());
 			foreach (var module in Modules) {
 				module.Create ();
 				File.AppendAllText (settingsFile, $"{Environment.NewLine}include(\":{module.Name}\")");
 			}
 			File.AppendAllText (Path.Combine (ProjectDirectory, "gradle.properties"), "android.useAndroidX=true");
+
+			// Update Gradle wrapper version if specified
+			if (!string.IsNullOrEmpty (GradleVersion)) {
+				var wrapperPropertiesPath = Path.Combine (ProjectDirectory, "gradle", "wrapper", "gradle-wrapper.properties");
+				if (File.Exists (wrapperPropertiesPath)) {
+					var content = File.ReadAllText (wrapperPropertiesPath);
+					// Replace the distribution URL with the specified Gradle version
+					content = System.Text.RegularExpressions.Regex.Replace (
+						content,
+						@"distributionUrl=.*",
+						$@"distributionUrl=https\://services.gradle.org/distributions/gradle-{GradleVersion}-bin.zip"
+					);
+					File.WriteAllText (wrapperPropertiesPath, content);
+				}
+			}
 		}
 
 		public static AndroidGradleProject CreateDefault (string projectDir, bool isApplication = false)
@@ -47,12 +72,30 @@ public static AndroidGradleProject CreateDefault (string projectDir, bool isAppl
 			return proj;
 		}
 
-		const string build_gradle_kts_content =
-@"
-plugins {
-    id(""com.android.application"") version ""8.5.0"" apply false
-    id(""com.android.library"") version ""8.5.0"" apply false
-}
+		/// <summary>
+		/// Creates a default Gradle project with specified AGP and Gradle versions.
+		/// </summary>
+		public static AndroidGradleProject CreateDefault (string projectDir, string agpVersion, string? gradleVersion, bool isApplication = false)
+		{
+			var proj = new AndroidGradleProject (projectDir) {
+				AgpVersion = agpVersion,
+				GradleVersion = gradleVersion,
+				Modules = {
+					new AndroidGradleModule (Path.Combine (projectDir, "TestModule")) {
+						IsApplication = isApplication,
+					},
+				},
+			};
+			proj.Create ();
+			return proj;
+		}
+
+		string GetBuildGradleKtsContent () =>
+$@"
+plugins {{
+    id(""com.android.application"") version ""{AgpVersion}"" apply false
+    id(""com.android.library"") version ""{AgpVersion}"" apply false
+}}
 ";
 		const string settings_gradle_kts_content =
 @"