SIGABRT: Failed to marshal the Objective-C object 0x282cce680 (type: CoreBluetooth_CBPeripheral__CBPeripheralDelegate). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'CoreBluetooth.CBPeripheral+_CBPeripheralDelegate' does not have a constructor that takes one IntPtr argument).

[zwells]

We have an app in production that reads BLE ad packets, and it has been running fine for the past year until iOS 17 came out, and now we are getting this crash:

SIGABRT: Failed to marshal the Objective-C object 0x282cce680 (type: CoreBluetooth_CBPeripheral__CBPeripheralDelegate). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'CoreBluetooth.CBPeripheral+_CBPeripheralDelegate' does not have a constructor that takes one IntPtr argument).

Stack Trace

Runtime.MissingCtor (System.IntPtr ptr, System.IntPtr klass, System.Type type, ObjCRuntime.Runtime+MissingCtorResolution resolution)
Runtime.ConstructNSObject[T] (System.IntPtr ptr, System.Type type, ObjCRuntime.Runtime+MissingCtorResolution missingCtorResolution)
Runtime.ConstructNSObject (System.IntPtr ptr, System.IntPtr klass, ObjCRuntime.Runtime+MissingCtorResolution missingCtorResolution)
Runtime.GetNSObject (System.IntPtr ptr, ObjCRuntime.Runtime+MissingCtorResolution missingCtorResolution, System.Boolean evenInFinalizerQueue)
Runtime.GetNSObject (System.IntPtr ptr)
CBPeripheral.get_WeakDelegate ()
CBPeripheral.EnsureCBPeripheralDelegate ()
CBPeripheral.remove_DiscoveredService (System.EventHandler`1[TEventArgs] value)
Plugin.Bluetooth.Device..ctor (CoreBluetooth.CBPeripheral peripheral) <0x106e5e770 + 0x00153> in <a820575b929f4a13bfbda461a7222989#05679fad2e06f8641a6c40ed4f50ad68>:0
BluetoothScanner.OnDiscoveredPeripheral (System.Object sender, CoreBluetooth.CBDiscoveredPeripheralEventArgs eventArgs)
CBCentralManager+_CBCentralManagerDelegate.DiscoveredPeripheral (CoreBluetooth.CBCentralManager central, CoreBluetooth.CBPeripheral peripheral, Foundation.NSDictionary advertisementData, Foundation.NSNumber RSSI)
(wrapper managed-to-native) UIKit.UIApplication.xamarin_UIApplicationMain(int,intptr,intptr,intptr,intptr&)
UIApplication.UIApplicationMain (System.Int32 argc, System.String[] argv, System.IntPtr principalClassName, System.IntPtr delegateClassName)
UIApplication.Main (System.String[] args, System.Type principalClass, System.Type delegateClass)
Application.Main (System.String[] args)

Steps to Reproduce

1. Run an app using bluetooth api on iOS 17 in a high volume environment

Expected Behavior

Processing of BLE advertisements with no crashing

Actual Behavior

Random crashing after a few seconds of running the application (only happening on iOS 17)

Environment

Xcode 15.0.1 (also tried Xcode 15.1)
iOS 17 (and all subsequent versions)

Version information

Visual Studio Community 2022 for Mac
Version 17.6.7 (build 417)
Installation UUID: 297f9b93-4d59-458a-882b-6528b4fdd966

Runtime
.NET 7.0.3 (64-bit)
Architecture: Arm64
Microsoft.macOS.Sdk 13.1.1007; git-rev-head:8afca776a0a96613dfb7200e0917bb57f9ed5583; git-branch:release/7.0.1xx-xcode14.2

Roslyn (Language Service)
4.6.0-3.23180.6+99e956e42697a6dd886d1e12478ea2b27cceacfa

NuGet
Version: 6.4.0.117

.NET SDK (Arm64)
SDK: /usr/local/share/dotnet/sdk/7.0.311/Sdks
SDK Versions:
	7.0.311
	7.0.310
	7.0.309
	7.0.308
	7.0.307
	7.0.306
	7.0.304
	7.0.302
	7.0.203
	7.0.202
	7.0.201
	7.0.200
	7.0.102
	7.0.101
	7.0.100
	6.0.417
	6.0.416
	6.0.415
	6.0.414
	6.0.413
	6.0.412
	6.0.410
	6.0.408
	6.0.407
	6.0.406
	6.0.405
	6.0.404
	6.0.403
	6.0.401
MSBuild SDKs: /Applications/Visual Studio.app/Contents/MonoBundle/MSBuild/Current/bin/Sdks

.NET SDK (x64)
SDK Versions:
	3.1.426
	3.1.425
	3.1.423

.NET Runtime (Arm64)
Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	7.0.14
	7.0.13
	7.0.12
	7.0.11
	7.0.10
	7.0.9
	6.0.25
	6.0.24
	6.0.23
	6.0.22
	6.0.21

.NET Runtime (x64)
Runtime: /usr/local/share/dotnet/x64/dotnet
Runtime Versions:
	3.1.32
	3.1.31
	3.1.29

Xamarin.Profiler
Version: 1.8.0.49
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

Updater
Version: 11

Xamarin.Android
Version: 13.2.2.0 (Visual Studio Community)
Commit: xamarin-android/d17-5/45b0e14
Android SDK: /Users/zwells/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		6.0  (API level 23)
		12.0 (API level 31)
		11.0 (API level 30)
		10.0 (API level 29)
		7.1  (API level 25)
		9.0  (API level 28)
		13.0 (API level 33)
		7.0  (API level 24)

SDK Command-line Tools Version: 7.0
SDK Platform Tools Version: 34.0.3
SDK Build Tools Version: 33.0.0

Build Information: 
Mono: d9a6e87
Java.Interop: xamarin/java.interop/d17-5@149d70fe
SQLite: xamarin/sqlite/3.40.1@68c69d8
Xamarin.Android Tools: xamarin/xamarin-android-tools/d17-5@ca1552d

Microsoft Build of OpenJDK
Java SDK: /Library/Java/JavaVirtualMachines/microsoft-11.jdk
11.0.16.1
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

Eclipse Temurin JDK
Java SDK: /Library/Java/JavaVirtualMachines/temurin-8.jdk
1.8.0.302
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

Android SDK Manager
Version: 17.6.0.50
Hash: a715dca
Branch: HEAD
Build date: 2023-11-10 12:19:52 UTC

Android Device Manager
Version: 0.0.0.1309
Hash: 06e3e77
Branch: HEAD
Build date: 2023-11-10 12:19:52 UTC

Xamarin Designer
Version: 17.6.3.9
Hash: 2648399ae8
Branch: remotes/origin/d17-6
Build date: 2023-11-10 12:19:46 UTC

Apple Developer Tools
Xcode: 15.1 22502
Build: 15C5059c

Xamarin.Mac
Not Installed

Xamarin.iOS
Version: 16.4.0.23 Visual Studio Community
Hash: 9defd91b3
Branch: xcode14.3
Build date: 2023-10-23 16:15:00-0400

Build Information
Release ID: 1706070417
Git revision: 179aa48a5702754e41a8218d0403ddc3be46706f
Build date: 2023-11-10 12:18:09+00
Build branch: release-17.6
Build lane: release-17.6

Operating System
Mac OS X 14.1.1
Darwin 23.1.0 Darwin Kernel Version 23.1.0
    Mon Oct  9 21:28:31 PDT 2023
    root:xnu-10002.41.9~6/RELEASE_ARM64_T8112 arm64

Enabled user installed extensions
NuGet Package Explorer 0.12
NuGet Package Management Extensions 0.34

Build Logs

SAV.iOS_Release_iPhone_Build_2023-12-05T16_34_30.5478590Z.msbuild.log.zip

[rolfbjarne]

This looks like this issue: https://web.archive.org/web/20170630214134/https://bugzilla.xamarin.com/show_bug.cgi?id=34242

Can you try the workaround at the end of the comment here: https://web.archive.org/web/20170630214134/https://bugzilla.xamarin.com/show_bug.cgi?id=34242#c9 and see if that works for you?

---

Copying the comment here:

This is what happens:

1. OnDiscoveredPeripheral is called, a CBDiscoveredPeripheralEventArgs instance containing a CBPeripheral instance is passed in.
2. OnDiscoveredPeripheral adds an event handler to the CBPeripheral instance. This will create an instance of an internal delegate (_CBPeripheralDelegate) that handles the event, and store this object as a field on the CBPeripheral instance. In this process the CBPeripheral instance is "marked dirty", which means that the GC will only free it if native code does not reference the instance anymore.
3. Native code creates a weak reference to the CBPeripheral instance.
4. The GC runs, and sees that native code does not retain the CBPeripheral instance (there's only a weak reference, which is invisible to the GC), and thus the CBPeripheral instance is scheduled for collection. By extension the _CBPeripheralDelegate instance is also scheduled for collection, and subsequently collected. Apparently only the _CBPeripheralDelegate instance is collected, and not the CBPeripheral instance (the GC is allowed to not collect everything; this is not a bug).
5. Native code reads the weak reference to the CBPeripheral instance, and calls OnDiscoveredPeripheral again.
6. OnDiscoveredPeripheral tries to add an event handler again, but runs into the ObjC marshalling exception because the internal delegate instance has been collected.

The workaround is to keep the CBPeripheral instances you care about visible to the GC in managed code (put in them in a list for instance). This way step 4 above doesn't happen.

[zwells]

We implemented this previously, and that had resolved this issue in the past until now.

[rolfbjarne]

Are you putting the CBPeripheral instances you get in the OnDiscoveredPeripheral event handler in that list?

[zwells]

The DiscoveredDevicesRegistry stores an instance of IDevice, which contains a reference to the CBPeripheral instance. I can change it to just store the CBPeripheral instance if we think that will make a difference.

[rolfbjarne]

The DiscoveredDevicesRegistry stores an instance of IDevice, which contains a reference to the CBPeripheral instance. I can change it to just store the CBPeripheral instance if we think that will make a difference.

That sounds like it should be OK.

However, that also means I'll need a test project I can use to reproduce this in order to track it down then, I still remember the similar issue from 2015 and the test project back then was instrumental in figuring out what was happening.

[zwells]

We changed it to just storing the CBPeripheral and it appears to have resolved! Thanks

[rolfbjarne]

Great! OK, I'll close this then, feel free to reopen if you run into this issue again.