Root [Export] callback delegates to prevent GC on CoreCLR

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -425,6 +425,11 @@ static Delegate CreateDynamicCallback (MethodInfo method)
 			return (Delegate)dynamic_callback_gen.Invoke (null, new object [] { method })!;
 		}
 
+		// [Export] callback delegates are created dynamically via DynamicCallbackCodeGenerator and are not
+		// cached in static fields (unlike non-[Export] connector delegates). Without rooting them here,
+		// CoreCLR's GC can collect them between JNI registration and first invocation, causing a crash.
+		static readonly Lock prevent_delegate_gc_lock = new Lock ();
+		static readonly List<Delegate> prevent_delegate_gc = new List<Delegate> ();
 		static List<JniNativeMethodRegistration> sharedRegistrations = new List<JniNativeMethodRegistration> ();
 
 		static bool FastRegisterNativeMembers (JniType nativeClass, Type type, ReadOnlySpan<char> methods)
@@ -562,6 +567,9 @@ public override void RegisterNativeMembers (
 							if (minfo == null)
 								throw new InvalidOperationException (FormattableString.Invariant ($"Specified managed method '{mname.ToString ()}' was not found. Signature: {signature.ToString ()}"));
 							callback = CreateDynamicCallback (minfo);
+							lock (prevent_delegate_gc_lock) {
+								prevent_delegate_gc.Add (callback);
+							}
 							needToRegisterNatives = true;
 						} else {
 							Type callbackDeclaringType = type;

tests/MSBuildDeviceIntegration/Tests/MonoAndroidExportTest.cs

@@ -106,5 +106,94 @@ protected override void OnCreate (Bundle bundle)
 				Assert.True (b.Uninstall (proj), "Project should have uninstalled.");
 			}
 		}
+
+		[Test]
+		public void ExportedMembersSurviveGarbageCollection (
+			[Values] bool isRelease,
+			[Values] AndroidRuntime runtime)
+		{
+			if (runtime == AndroidRuntime.NativeAOT) {
+				Assert.Ignore ("NativeAOT does not support Mono.Android.Export");
+			}
+			if (IgnoreUnsupportedConfiguration (runtime, release: isRelease)) {
+				return;
+			}
+
+			AssertCommercialBuild ();
+			var proj = new XamarinAndroidApplicationProject (packageName: PackageUtils.MakePackageName (runtime)) {
+				IsRelease = isRelease,
+				References = {
+					new BuildItem.Reference ("Mono.Android.Export"),
+				},
+			};
+			proj.SetRuntime (runtime);
+			proj.Sources.Add (new BuildItem.Source ("ContainsExportedMethods.cs") {
+				TextContent = () => @"using System;
+using Java.Interop;
+
+namespace UnnamedProject {
+	class ContainsExportedMethods : Java.Lang.Object {
+		[Export]
+		public void Exported ()
+		{
+			Console.WriteLine (""# ExportedCallbackInvoked"");
+		}
+	}
+}
+",
+			});
+			proj.MainActivity = @"using System;
+using Android.App;
+using Android.OS;
+using Android.Runtime;
+
+namespace UnnamedProject
+{
+	[Activity (Label = ""UnnamedProject"", MainLauncher = true, Icon = ""@drawable/icon"")]
+	public class MainActivity : Activity {
+		protected override void OnCreate (Bundle bundle)
+		{
+			base.OnCreate (bundle);
+
+			var foo = new ContainsExportedMethods ();
+
+			// Force GC to collect any unrooted delegates
+			for (int i = 0; i < 10; i++) {
+				GC.Collect ();
+				GC.WaitForPendingFinalizers ();
+			}
+
+			// Invoke the [Export] method through JNI (Java -> native delegate -> C#)
+			// This path crashes with SIGABRT if the delegate was garbage collected
+			IntPtr klass = JNIEnv.GetObjectClass (foo.Handle);
+			IntPtr methodId = JNIEnv.GetMethodID (klass, ""Exported"", ""()V"");
+			JNIEnv.CallVoidMethod (foo.Handle, methodId);
+
+			Console.WriteLine (""# ExportCallbackSurvivedGC"");
+		}
+	}
+}";
+			proj.SetAndroidSupportedAbis (DeviceAbi);
+			proj.SetDefaultTargetDevice ();
+			using (var b = CreateApkBuilder ()) {
+				b.LatestTargetFrameworkVersion (out string apiLevel);
+				proj.SupportedOSPlatformVersion = "24.0";
+				proj.AndroidManifest = $@"<?xml version=""1.0"" encoding=""utf-8""?>
+<manifest xmlns:android=""http://schemas.android.com/apk/res/android"" android:versionCode=""1"" android:versionName=""1.0"" package=""{proj.PackageName}"">
+	<uses-sdk android:targetSdkVersion=""{apiLevel}"" />
+	<application android:label=""${{PROJECT_NAME}}"">
+	</application >
+</manifest>";
+				Assert.True (b.Install (proj), "Project should have installed.");
+				RunProjectAndAssert (proj, b, doNotCleanupOnUpdate: true);
+				Assert.True (WaitForActivityToStart (proj.PackageName, "MainActivity",
+					Path.Combine (Root, b.ProjectDirectory, "logcat.log"), 30), "Activity should have started.");
+				string expectedLogcatOutput = "ExportCallbackSurvivedGC";
+				Assert.IsTrue (MonitorAdbLogcat ((line) => {
+					return line.Contains (expectedLogcatOutput);
+				}, Path.Combine (Root, b.ProjectDirectory, "startup-logcat.log"), 45), $"Output did not contain {expectedLogcatOutput}!");
+				Assert.True (b.Uninstall (proj), "Project should have uninstalled.");
+			}
+		}
 	}
 }