C#. Event handlers gets garbage collected while capturing still on, leading to crash

[wwwMADwww]

Hello. I'm using library in .NET windows application through provided in src_csharp .NET wrappers.

On tag 17.1.429 everything was fine. Today i tried pre-release 17.1.1327 and started getting random unrecoverable crashes. I looked in the windows Event Logs, sometimes it says:

Application: vgarender.exe
CoreCLR Version: 7.0.523.17405
.NET Version: 7.0.5
Description: The application requested process termination through System.Environment.FailFast.
Message: A callback was made on a garbage collected delegate of type 'screen_capture_lite_csharp!SCL.ScreenCaptureCallbackWithContext::Invoke'.
Stack:
(no stack info here, empty line)

It seems that when we setting callback for events like OnNewFrame, etc., the delegate is passed to the unmanaged code but for some reason nothing is holding it in the managed code, so it's get garbage collected on next GC run and no longer can be used by unmanaged code too.

How to reproduce:

• Use version of code on commit 12babfcad42ba408a49f72fa9e31a6f8901546a3 or later. Here was done some refactoring on C# wrappers side.
• Run Example_CSharp and just wait. To speed things up better add GC.Collect(); in the end of SCL_CreateMonitorCaptureManager method before return statement.

Result:

• Application will crash eventually, if you add GC.Collect(); then application crashes immediately (on the next event i presume), with messages like An unhandled exception of type 'System.ExecutionEngineException' occurred in Unknown Module. or Fatal error. Internal CLR error. (0x80131506)

[wwwMADwww]

I googled and tinker around a bit. Consider this code from MonitorCaptureConfiguration.cs :

Lets see using dnSpy what it translated to after compilation:

This new ScreenCaptureCallbackWithContext(MonitorCaptureConfiguration.OnNewFrame) is successfully passed to unmanaged library but it's, as i assumed in OP, indeed used nowhere else and held by no one in managed code and thus can be GC'ed while some unmanaged code still use it. So this object needs to be placed in field or something, that's what i did in PR #157